Questions Q1. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross In a chemical reaction, . is directly proportional to (Total for question = 1 mark) (Q03 WCH15/01, June 2023) Q2. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . The equation for a redox reaction is shown. 2V2+(aq) + Cu2+(aq) → 2V3+(aq) + Cu(s) What is the cell diagram for this reaction? A B C D V(s) | V3+(aq), V2+(aq) | | Cu(s) | Cu2+(aq) V(s) | V2+(aq), V3+(aq) | | Cu2+(aq) | Cu(s) Pt(s) | V3+(aq), V2+(aq) | | Cu(s) | Cu2+(aq) Pt(s) | V2+(aq), V3+(aq) | | Cu2+(aq) | Cu(s) (Total for question = 1 mark) (Q01 WCH15/01, June 2023) Q3. Answer the questions with a cross in the boxes you think are correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . This question is about alkaline hydrogen-oxygen fuel cells. (a) What is the half-equation at the negative electrode? A B C D H2(g) + 2OH−(aq) → 2H2O(l) + 2e− 2H2O(l) + 2e− → H2(g) + 2OH−(aq) O2(g) + 2H2O(l) + 4e− → 4OH−(aq) 4OH−(aq) → O2(g) + 2H2O(l) + 4e− (b) Which statement is correct for an alkaline hydrogen-oxygen fuel cell when compared with an acidic hydrogen-oxygen fuel cell? (1) Use this space for rough working. Anything you write in this space will gain no credit. (Total for question = 2 marks) (Q02 WCH15/01, June 2022) Q4. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross An electrochemical cell is set up: a half-cell is made from a piece of zinc and a solution of zinc chloride, ZnCl2 a second half-cell is made from a piece of metal G and a solution of its chloride, GCl2 the two half-cells are connected and a current allowed to pass for some time. The zinc electrode increased in mass by 1.635 g. The electrode of metal G decreased in mass by 0.6075 g. What is metal G? A B C copper iron magnesium . D manganese (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q11 WCH15/01, Oct 2023) Q5. The hydride of arsenic, arsine, is a toxic gas used in the production of semiconductors. (a) Draw a dot-and-cross diagram for arsine, AsH3 . (1) (b) Arsine is a reducing agent and reacts with cerium(IV) sulfate solution, forming arsenic. The data from an experiment are shown. Volume of arsine gas = 350 cm3 at 115 000 Pa and 20 °C Volume of cerium(IV) sulfate solution = 488 cm3 Concentration of cerium(IV) sulfate solution = 0.102 mol dm−3 (i) Complete the half-equation. (1) AsH3 → As (ii) Calculate the final oxidation state of the cerium ion formed in the reaction. (6) (Total for question = 8 marks) (Q13 WCH15/01, Jan 2022) Q6. For each question, select one answer from A to D and put a cross in the box . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . A group of students carry out an experiment to find the concentration of chlorine, Cl2(aq), in a solution. Excess potassium iodide solution is added to a 10.0 cm3 sample of the chlorine solution. Cl2(aq) + 2I−(aq) → 2Cl−(aq) + I2(aq) The iodine produced is titrated with a solution of thiosulfate ions of known concentration, using starch indicator. The concentration of the Cl2(aq) is between 0.038 and 0.042 mol dm−3 . (a) What concentration of thiosulfate ions, in mol dm−3 , is required to give a titre of approximately 20 cm3 ? (1) A 0.010 B C D 0.020 0.040 0.080 (b) What is the most suitable volume of 0.1 mol dm−3 potassium iodide solution, in cm3 , to add to the 10.0 cm3 of chlorine solution? (1) A B C D 7.6 8.0 8.4 10.0 (c) What is the colour change at the end-point of the titration? (1) A B C D colourless to pale yellow pale yellow to colourless colourless to blue-black blue-black to colourless (Total for question = 3 marks) (Q08 WCH15/01, Jan 2022) Q7. The amount of calcium ethanedioate, CaC2O4, present in a sample of spinach is determined by redox titration. 11.4 g of spinach leaves are stirred in 50.0 cm3 of a warm acidified solution of 0.0100 mol dm−3 potassium manganate(VII), KMnO4, oxidising all the ethanedioate ions, The excess manganate(VII) ions, iron(II) sulfate, FeSO4. . , are then titrated with an acidified solution of 0.0500 mol dm−3 25.95 cm3 of iron(II) sulfate solution was needed for complete reaction. Calculate the percentage by mass of CaC2O4 present in the spinach leaves. Give your answer to an appropriate number of significant figures. (Total for question = 6 marks) (Q22 WCH15/01, June 2023) Q8. Iron is generally prescribed when a person is suffering from anaemia. Many people find it easier to take medicine in liquid form rather than solid tablets. An iron solution, advertised for this purpose, contains iron as Fe2+. An adult is advised to take 90.0 mg of iron daily. A 25.0 cm3 sample of the solution was made up to 100.0 cm3 using sulfuric acid and deionised water. 25.0 cm3 samples of the diluted iron(II) solution were titrated with a 0.0125 mol dm−3 potassium manganate(VII) solution. The mean titre was 16.20 cm3 . The reaction is shown. MnO4− + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O Calculate the volume of iron solution required to obtain a 90.0 mg dose of iron. Give your answer to an appropriate number of significant figures. Volume required by an adult to obtain a 90.0 mg dose ........................................................... cm 3 (Total for question = 6 marks) (Q16 WCH15/01, Oct 2024) Q9. Answer the question with a cross in the box you think is correct ( ). If you change your mind about an answer, put a line through the box ( ) and then mark your new answer with a cross ( ). Which is the diagram for the cell made up of the nickel and zinc half-cells that gives a positive value for ? Use your Data Booklet. (Total for question = 1 mark) (Q05 WCH15/01, June 2024) Q10. This question is about cell reactions involving chromium. Use your Data Booklet when answering this question. (a) (i) Name the type of the forward reaction that is shown in the right-hand electrode systems in the Data Booklet. (1) ............................................................................................................................................. (ii) Name the series formed when the right-hand electrode systems are placed in order, most negative first. (1) ............................................................................................................................................. (b) A student set up the following apparatus to measure the standard electrode potential for right-hand electrode system 8 in your Data Booklet. (i) Identify X and Y. (2) X: ............................................................................................................................................. Y: ............................................................................................................................................. (ii) Give two reasons why the initial reading on the voltmeter may differ from the stated value given in the Data Booklet. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (iii) The voltmeter is removed and the cell is allowed to run for one hour. Explain the changes that would occur in the right-hand half-cell during this time. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (c) (i) Explain, by calculating to reduce Cr(VI) to Cr(II). values, why Fe(II) is used to reduce Cr(VI) to Cr(III) but zinc is used (4) Numbers chosen of the right-hand electrode systems from the Data Booklet ............................................................................................................................................. Explanation ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) State the essential condition required for these reactions to occur. (1) ............................................................................................................................................. (Total for question = 13 marks) (Q14 WCH15/01, June 2024) Q11. This question is about coloured transition metal complex ions. (a) The [Fe(H2O)6]2+ complex ion is green and the [Fe(H2O)6]3+ complex ion is yellow. (i) Explain why the [Fe(H2O)6]2+ complex ion is coloured. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Explain why the two iron complex ions are different colours. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) The thiocyanate ion, SCN−, forms a 'blood-red' coloured complex ion with iron(III) ions. (i) The dot-and-cross diagram for the thiocyanate ion can be drawn in more than one way. For example, the negative charge can be on the nitrogen atom as shown. Complete the alternative dot-and-cross diagram of the thiocyanate ion where the negative charge is on the sulfur atom. Use (•) for the nitrogen electrons, (×) for the carbon electrons, (*) for the sulfur electrons and (Δ) for the extra electron. (2) (ii) 12.8 cm3 of 0.05 mol dm−3 iron(III) chloride solution reacted with 8.0 cm3 of 0.08 mol dm−3 ammonium thiocyanate to form a complex ion. Deduce the formula of the octahedral complex ion that iron(III) ions form with thiocyanate ions in aqueous solution. You must show your working. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. Formula ............................................................................................................................................. (c) Colorimetry was used to determine the concentration of a solution of copper(II) sulfate. Six solutions of copper(II) sulfate of known concentration were prepared and their absorbance at a wavelength of 635 nm measured. The results are shown. (i) Plot a graph of absorbance against concentration using the data in the table. (3) (ii) 50.0 cm3 of a solution of copper(II) sulfate of unknown concentration was pipetted into a 250.0 cm3 volumetric flask and the volume made up to the mark with deionised water. The absorbance at a wavelength of 635 nm for this diluted copper(II) sulfate solution was 0.72. Determine the concentration of the original copper(II) sulfate solution. You must show your working on the graph. (2) (iii) Give one possible reason why any concentrations of copper(II) sulfate determined from the graph of values over 0.50 mol dm−3 are uncertain. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (d) The complex ion formed from cobalt(III) ions and ethane-1,2-diamine is a yellow-orange colour. The equation for the formation of this complex ion is shown. [Co(H2O)6]3+ + 3H2NCH2CH2NH2 → [Co(H2NCH2CH2NH2)3]3+ + 6H2O (i) Give a reason, by referring to the equation, why the formation of the diamine complex from the aqueous complex ion is thermodynamically favoured. (1) ............................................................................................................................................. ............................................................................................................................................. (ii) Explain, by reference to the structure of ethane-1,2-diamine, why it is a bidentate ligand. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (iii) Explain how the H—N—H bond angles would be expected to change in ethane-1,2-diamine when the complex ion is formed. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (Total for question = 22 marks) (Q18 WCH15/01, Oct 2024) Q12. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . What is the minimum volume, in cm3, of oxygen needed for the complete combustion of a mixture of 50 cm3 of propene and 50 cm3 of but-1-ene gases? All gas volumes are measured at the same temperature and pressure. A B C D 225 300 525 700 (Total for question = 1 mark) (Q09 WCH15/01, Jan 2023) Q13. This question is about compounds and complex ions of iron. (a) A compound of iron contains, by mass, 39.5% potassium, 28.2% iron and 32.3% oxygen. Calculate the empirical formula of this compound. (2) (b) The reaction between iodide ions and persulfate ions is thermodynamically feasible under standard conditions. The reaction is very slow but is catalysed by the presence of Fe2+ ions. (i) Give a reason why the activation energy for this reaction is high in the absence of a catalyst. (1) ............................................................................................................................................. ............................................................................................................................................. (ii) Write the ionic equations for the reactions that occur when the catalyst of Fe2+ ions is added. State symbols are not required. (2) (c) In aqueous solution, iron(II) exists as the complex ion [Fe(H2O)6]2+. (i) Explain how water acts as a monodentate ligand. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Explain the shape of the [Fe(H2O)6]2+ complex ion. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (d) Haemoglobin is an iron(II) complex which carries oxygen around the body. Part of the structure of oxyhaemoglobin is shown. The four nitrogen atoms are part of a multidentate ligand in the haem group. Explain, in terms of the iron(II) complex, why carbon monoxide is toxic. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (e) Ethanedioate ligands react with iron(II) ions in aqueous solution. Explain, in terms of entropy, why this reaction is feasible. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (f) Potassium dichromate(VI) oxidises iron(II) ions to iron(III) ions in acid solution. Iron(III) ions do not react with acidified potassium dichromate(VI). A solution Y contains 6.28 g dm−3 of iron as a mixture of iron(II) ions and iron(III) ions. 50 cm3 of dilute sulfuric acid is added to a 25.0 cm3 portion of Y and it is titrated with a solution containing 2.56 g dm−3 of dichromate(VI) ions, . The titration is repeated until concordant results are obtained. The mean titre is 22.55 cm3. Calculate the percentage of iron present as iron(III) ions in Y. (5) (Total for question = 18 marks) (Q19 WCH15/01, Oct 2023) Q14. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . For the reaction shown, 4V3+(aq) + O2(g) + 2H2O(l) → 4VO2+(aq) + 4H+(aq) Which statement can be deduced from this information only? A B C D the reaction will not occur under any conditions the reactants are kinetically stable with respect to the products the reactants are thermodynamically unstable with respect to the products an aqueous solution of V3+ will oxidise rapidly on standing in air (Total for question = 1 mark) (Q04 WCH15/01, June 2023) Q15. For each question, select one answer from A to D and put a cross in the box . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . An electrochemical cell is made from the electrode systems shown by these half-equations. The apparatus used to measure the value for under standard conditions is shown. (a) Which silver compound could be used as the electrolyte in the left-hand half-cell? (1) A B C D silver nitrate silver hydroxide silver chloride silver carbonate (b) The electrolyte in the right-hand half-cell is prepared using equal volumes of (1) (c) Which is the equation for the overall cell reaction under standard conditions? (1) (d) Which is the value of in volts? (1) A B C D −1.80 −0.20 +0.20 +1.80 (e) Which is the cell diagram for this cell, using the conventional representation of half-cells? (1) (Total for question = 5 marks) (Q03 WCH15/01, Jan 2022) Q16. This question is about electrochemical cells and redox reactions. (a) Draw a labelled diagram of the apparatus you would use to measure the standard electrode potential of a Cu2+(aq)|Cu(s) electrode with a standard hydrogen electrode. Include essential conditions. (5) (b) The standard electrode potentials for two half-cells are shown. (i) Explain, in terms of electrode potentials, why acidified dichromate(VI) ions react with concentrated hydrochloric acid to form chlorine, even though . (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Write the cell diagram, using the conventional representation of half-cells, for the reaction to produce chlorine. (2) (c) When 25.0 cm3 of a 0.100 mol dm−3 solution of X2O5 reacts with a reducing agent, X is reduced to a lower oxidation state. To oxidise X back to its original oxidation state required 50.0 cm3 of 0.0200 mol dm−3 acidified potassium manganate(VII) solution. The half-equation for acidified manganate(VII) is Calculate the oxidation state of X after it has been reduced. (3) (Total for question = 13 marks) (Q18 WCH15/01, Oct 2023) Q17. This question is about electrochemical cells. *(a) Describe how you would carry out an experiment to determine the standard electrode potential of the electrode system shown. Assume that you have access to the equipment and chemicals that you need, describing their use in your answer. You may include a labelled diagram. (6) (b) Acidified potassium dichromate(VI) is an oxidising agent. The acid commonly used is sulfuric acid. Explain, using reference to the electrode potentials shown, why concentrated hydrochloric acid should not be used to acidify potassium dichromate(VI). (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (c) The cell diagram for an electrochemical cell is shown. Write the reduction half-equation for this electrochemical cell. State symbols are not required. (1) (d) The Direct Methanol Fuel Cell (DMFC) uses methanol and oxygen to generate electricity. When the cell operates, the methanol reacts with water at one electrode to produce carbon dioxide and hydrogen ions. At the other electrode the oxygen reacts with hydrogen ions to form water. (i) Write the oxidation half-equation for this fuel cell. State symbols are not required. (1) (ii) Write the overall equation for this fuel cell. State symbols are not required. (1) (iii) The reduction half-cell in the DMFC has a standard electrode potential . When the solution concentration changes, the electrode potential changes so that E = +1.20 V. The relationship between these values is given by the equation where X is the concentration of the solution in mol dm−3 . Calculate the value of X in the non-standard half-cell. (1) (Total for question = 13 marks) (Q17 WCH15/01, Oct 2024) Q18. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross The half-equation for the reaction taking place at the positive electrode in an alkaline hydrogen-oxygen fuel cell is . O2 + 2H2O + 4e− → 4OH− What is the half-equation for the reaction taking place at the negative electrode? A B C D H2 + 2OH− → 2H2O + 2e− H2 + OH− → H2O + H+ + 2e− H2 → 2H+ + 2e− H2 + H2O → 3H+ + OH− + 2e− (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q09 WCH15/01, Oct 2023) Q19. *Fuel cells are an alternative to internal combustion engines and batteries for powering vehicles. A schematic diagram of a fuel cell is shown. Explain how a fuel cell works, discussing their advantages and disadvantages. Use the acidic hydrogen-oxygen fuel cell as an example. Include, in your answer, half-equations for the electrode reactions and an example of another fuel suitable for use in fuel cells. values and values are not required. (6) (Total for question = 6 marks) (Q18 WCH15/01, June 2024) Q20. (a) 2.72 g of a type of brass is dissolved in excess concentrated nitric acid, forming a solution containing both Cu2+ and Zn2+ ions. A solution containing hydrogensulfate(IV) ions, , is then added. The addition of ammonium thiocyanate, NH4SCN, gives a precipitate of copper(I) thiocyanate, CuSCN. Cu+(aq) + SCN−(aq) → CuSCN(s) The precipitate of copper(I) thiocyanate is collected, dried and found to have a mass of 4.69 g. (i) Determine whether or not this type of brass is a gilding metal, by calculating its percentage by mass of copper. (4) (ii) Explain, by considering both thermodynamic and kinetic factors, why but does not then reduce Cu+ to Cu. Use the data in the table. reduces Cu2+ to Cu+ (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) After the copper(I) thiocyanate is precipitated, Zn2+ ions remain in solution. A student suggested that these Zn2+ ions can be precipitated by adding a large excess of aqueous sodium hydroxide. Comment on this suggestion by describing the reactions that take place as a large excess of aqueous sodium hydroxide is gradually added. (4) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (c) Suggest why gilding metals are less malleable than pure copper, by considering their structure. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (d) Zinc and copper are also used in electrochemical cells. (i) Draw a labelled diagram of the apparatus used to measure the emf of a cell with copper and zinc electrodes under standard conditions. (3) (ii) The Nernst equation describes the relationship between the concentration of metal ions in a half-cell and its electrode potential. E = electrode potential under non-standard concentrations z = the number of positive charges on the metal ion A cell is set up with Cu2+ ions of concentration 1.00 mol dm−3 and Zn2+ ions of unknown concentration. The emf of the cell is +1.09 V. Calculate the concentration of the zinc ions. Use the data on page 10 of the Data Booklet. (3) (Total for question = 19 marks) (Q17 WCH15/01, June 2022) Q21. Answer the questions with a cross in the boxes you think are correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . The half-equations for a hydrogen-oxygen fuel cell in alkaline solution are shown. 2H2O(l) + 2e– → H2(g) + 2OH–(aq) = –0.83 V ⁄2O2(g) + H2O(l) + 2e– → 2OH–(aq) 1 = +0.40 V (a) The equation for the overall cell reaction is (1) A B C D – 3H2O(l) + ⁄2O2(g) → H2(g) + 4OH (aq) H2(g) + 4OH–(aq) → 3H2O(l) + 1⁄2O2(g) H2(g) + 1⁄2O2(g) → H2O(l) H2O(l) → H2(g) + 1⁄2O2(g) 1 (b) Calculate for the reaction occurring in the hydrogen-oxygen fuel cell, under alkaline conditions. (1) A B –1.23 V –0.43 V C D +0.43 V +1.23 V (Total for question = 2 marks) Use this space for any rough working. Anything you write in this space will gain no credit. (Q02 WCH15/01, Jan 2024) Q22. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . The cell diagram for an electrochemical cell is shown. What are the half-equations for the reactions that occur in the half-cells? (Total for question = 1 mark) (Q08 WCH15/01, Jan 2023) Q23. For each question, select one answer from A to D and put a cross in the box . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . A hydrogen-oxygen fuel cell is used to provide electrical energy for an electric motor in a car. (a) The electrolyte in the fuel cell is acidic. What is the half-equation at the anode? (1) A B C D ⁄2O2(g) + 2H+(aq) + 2e− → H2O(l) H2O(l) → 1⁄2O2(g) + 2H+(aq) + 2e− H2(g) → 2H+(aq) + 2e− 2H+(aq) + 2e− → H2(g) 1 (b) Hydrogen-oxygen fuel cells have advantages over methanol-oxygen fuel cells in vehicles. Which of these is an advantage of the hydrogen-oxygen fuel cell? (1) A B C D more energy is released per mole of fuel used emissions do not contribute to climate change hydrogen is easier to store than methanol only hydrogen can be obtained from renewable resources (Total for question = 2 marks) (Q02 WCH15/01, Jan 2022) Q24. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Which sequence shows the ions in order of increasing strength as a reducing agent? Refer to your Data Booklet. (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q04 WCH15/01, Jan 2024) Q25. Answer the question with a cross in the box you think is correct ( ). If you change your mind about an answer, put a line through the box ( ) and then mark your new answer with a cross ( ) An equation for the reaction of FeSO4 with KMnO4 is shown. 2KMnO4 + 10FeSO4 + 8H2SO4 → K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + 8H2O (a) Which shows the changes in the oxidation states for iron and manganese? (1) (b) A 25.00 cm3 sample of FeSO4 dissolved in acid required 22.50 cm3 of 0.100 mol dm−3 KMnO4 for complete reaction. What is the concentration of the FeSO4 solution? (1) A B C D −3 0.011 mol dm 0.100 mol dm−3 0.450 mol dm−3 0.900 mol dm−3 (Total for question = 2 marks) (Q03 WCH15/01, June 2024) Q26. A Latimer diagram for a chemical element is a summary of the standard electrode potential data for that element. In a Latimer diagram, the form of the element with the highest oxidation state is on the left, with successively lower oxidation states to the right. A Latimer diagram for manganese at pH = 0 is shown. The diagram shows that the standard electrode potential for the reduction of in acidic conditions, is +1.69 V. to MnO2, (a) (i) Justify the assignment of the oxidation state of +5 to manganese in H3MnO4 using oxidation numbers. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Write an equation for the reaction of H3MnO4 in acidic solution to give ions containing manganese(VI) and manganese(IV). Use the Latimer diagram to obtain the formulae of the ions produced. State symbols are not required. (2) (iii) Deduce whether or not this disproportionation reaction is thermodynamically feasible by calculating for the reaction. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) Before use in titration experiments, potassium manganate(VII) solutions must be standardised. One method uses ethanedioate ions to find the exact concentration of the manganate(VII) ions. 250.0 cm3 of a standard solution contained 1.915 g of sodium ethanedioate, Na2C2O4. A potassium manganate(VII) solution of approximately 0.02 mol dm–3 was standardised using this solution. Excess sulfuric acid was added to 25.0 cm3 portions of the potassium manganate(VII) solution which were titrated with the sodium ethanedioate solution. The mean titre was 22.95 cm3. The relevant ionic half–equations are shown. (i) State the colour change at the end-point of the titration. (1) ............................................................................................................................................. ............................................................................................................................................. (ii) Calculate the accurate concentration of the potassium manganate(VII), in mol dm–3, giving your answer to an appropriate number of significant figures. (4) (iii) A second titration carried out without the addition of sulfuric acid resulted in the formation of a brown suspension. Explain how the value of the mean titre would be affected, if at all, by the reaction that forms this suspension. Use the Data Booklet as a source of information. There is no need to calculate Ecell values. (3) (Total for question = 13 marks) (Q16 WCH15/01, Jan 2024) Q27. (a) Some of the properties of manganese and its compounds depend on electronic configurations. (i) Complete the electronic configurations for a manganese atom and a manganese(II) ion, using the electron-in-boxes notation. (1) (ii) The standard electrode potentials, for manganese and iron are shown. , for the half-cells involving the oxidation states +3 and +2 Explain, in terms of electronic configurations, why the significantly higher than that for the Fe2+ / Fe3+ half-cell. value for the Mn2+ / Mn3+ half-cell is (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) Manganese(IV) oxide, MnO2, is an oxidising agent and a catalyst. (i) Chlorine can be prepared in a laboratory by reacting manganese(IV) oxide with concentrated hydrochloric acid. MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O Explain, in terms of oxidation numbers, why this is a redox reaction. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide. 2H2O2(aq) → 2H2O(aq) + O2(g) 100 cm3 of a solution of hydrogen peroxide decomposed to produce 86.0 cm3 of oxygen gas, measured at room temperature and pressure (r.t.p.). Calculate the concentration of the hydrogen peroxide solution in mol dm–3 . [Molar volume of gas at r.t.p. = 24.0 dm3 mol–1] (2) (c) Potassium manganate(VI), K2MnO4, is a green solid. In acidic solution, the manganate(VI) ions disproportionate to form manganate(VII) ions and manganese(IV) oxide. Write the ionic equation for this disproportionation. State symbols are not required. (1) (d) Manganate(VII) ions react with ethanedioate ions in acidic solution. Two experiments were carried out: Experiment 1 Aqueous potassium manganate(VII) was added to a mixture of excess ethanedioic acid and sulfuric acid. The concentration of the manganate(VII) ions was determined as the reaction progressed. Experiment 2 The same volumes and concentrations of solutions as in Experiment 1 were used but some solid manganese(II) sulfate was dissolved in the ethanedioic acid and sulfuric acid mixture before adding the potassium manganate(VII) solution. Graphs of the concentration of manganate(VII) ions against time are shown. Explain, in terms of catalysis, why the graphs have different shapes. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (e) Methylbenzene is oxidised to benzoic acid by heating under reflux with an alkaline solution of potassium manganate(VII). In an experiment, 1.73 g of methylbenzene was mixed with 7.00 g of potassium manganate(VII) and excess potassium hydroxide solution. The overall equation is (i) Show that the potassium manganate(VII) is in excess. [Molar masses / g mol–1: C6H5CH3 = 92 KMnO4 = 158] (3) (ii) State how the benzoate ions, C6H5COO–, can be converted into benzoic acid. (1) ............................................................................................................................................. (f) Alkaline batteries consist of an anode made of a zinc alloy, a cathode made of manganese(IV) oxide and an electrolyte of aqueous potassium hydroxide. The overall equation for the reaction in the cell is (i) Write the half-equations for the reactions at the anode and the cathode. (2) anode half-equation: cathode half-equation: (ii) The standard electrode potential for the Zn / ZnO half-cell is –1.28 V. Calculate the standard electrode potential for the MnO2 / Mn2O3 half-cell. (1) (Total for question = 19 marks) (Q20 WCH15/01, Jan 2023) Q28. Answer the questions with a cross in the boxes you think are correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . An electrochemical cell is set up using the electrode systems shown. (a) What materials will be used for the electrodes in this cell? (1) What is the standard electrode potential for the TiO2+, Ti3+ electrode system? (1) A B −2.47 V −0.19 V C D +0.19 V +2.47 V (Total for question = 2 marks) Use this space for any rough working. Anything you write in this space will gain no credit. (Q03 WCH15/01, Oct 2022) Q29. This question is about mercury, Hg, and its compounds. Mercury is a liquid element in the same group of the Periodic Table as zinc. The electronic configuration of mercury is [Xe]4f 145d106s2. (a) Mercury forms compounds in either the +1 or +2 oxidation states. Explain why mercury is classified as a d-block element but is not a transition element. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) Mercury reacts with nitric acid to form an aqueous solution of Hg(NO3)2 and nitrogen monoxide gas. The unbalanced equation is shown. Hg(l) + HNO3(aq) → Hg(NO3)2(aq) + NO(g) + H2O(l) (i) Explain, using oxidation numbers, why this is a redox reaction. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Deduce the ionic half-equations for this reaction. State symbols are not required. (2) (iii) Complete the equation for this reaction by adding the stoichiometric coefficients. (1) ................Hg(l) + ................HNO3(aq) → ................Hg(NO3)2(aq) + ................NO(g) + ................H2O(l) (c) Mercury(II) fulminate, Hg(CNO)2, is an explosive. (i) It is produced in the reaction between Hg(NO3)2 and ethanol. The other products of the reaction are ethanal and water. Write the equation for the reaction of one mole of Hg(NO3)2 with ethanol to form mercury(II) fulminate. State symbols are not required. (2) (ii) Hg(CNO)2 decomposes as shown. 3Hg(CNO)2(s) → Hg(CN)2(s) + 2Hg(s) + 2CO2(g) + 2CO(g) + 2N2(g) Calculate the total volume, in cm3, of gas produced when 1.00 g of Hg(CNO)2 decomposes at room temperature and pressure. (3) (d) Mercury(I) chloride, Hg2Cl2, is also known as calomel. A saturated calomel electrode may be used as an alternative to the standard hydrogen electrode. The half-equation for the calomel electrode is The standard electrode potential of the calomel electrode is . (i) Suggest why KCl crystals are needed in the outer tube of the electrode. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) A calomel electrode was used to measure the standard electrode potential of the Sn2+(aq) | Sn(s) half-cell. The reading on the voltmeter in this cell was +0.37 V. Deduce the standard electrode potential for the Sn2+(aq) | Sn(s) half−cell. (1) (iii) Write the overall equation for the cell reaction. (1) (iv) Add labels to complete the diagram of a standard hydrogen electrode. Include details of any essential conditions. (3) ............................................................................................................................................. ............................................................................................................................................. (v) Suggest one advantage of using a calomel electrode, in place of a standard hydrogen electrode, when measuring a cell potential. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (Total for question = 20 marks) (Q21 WCH15/01, June 2023) Q30. Answer the question with a cross in the box you think is correct ( ). If you change your mind about an answer, put a line through the box ( ) and then mark your new answer with a cross ( ). Naphthalene is an aromatic molecule with two aromatic rings joined together. It can be nitrated at position 1 as shown. (a) In an experiment, a 2.00 g sample of naphthalene was nitrated and produced 2.57 g of 1-nitronaphthalene. What is the percentage yield of 1-nitronaphthalene? (1) A B C D 95.1% 77.8% 74.0% 57.6% (b) A student carrying out this experiment calculated the percentage yield of the reaction to be greater than 100%. What is the most likely explanation for this result? (1) A B C D there was unreacted naphthalene present the naphthalene was impure the sample of 1-nitronaphthalene was damp other isomers of 1-nitronaphthalene were produced (Total for question = 2 marks) (Q06 WCH15/01, Oct 2024) Q31. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross When copper is added to concentrated nitric acid, a brown gas is given off and the final solution is blue. In terms of oxidation number and electron transfer, how does the nitrogen change in this reaction? . (Total for question = 1 mark) (Q01 WCH15/01, Oct 2022) Q32. Vanadium forms compounds with oxidation states +2, +3, +4 and +5. (a) State the oxidation number of vanadium in Na3VO4. (1) ............................................................................................................................................. (b) In aqueous solution, vanadium(II) exists as [V(H2O)6]2+ but vanadium(V) exists as [VO2(H2O)4]+. Explain why vanadium(V) does not exist as [V(H2O)6]5+. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (c) Hydrated potassium vanadium(III) sulfate contains 7.9 % potassium, 10.2 % vanadium, 12.9 % sulfur, 4.8 % hydrogen and 64.2 % oxygen. Calculate the empirical formula of this compound and hence write the overall formula showing the potassium (K+), vanadium (V3+) and sulfate ions and the water of crystallisation. (3) (d) An acidified aqueous solution of potassium manganate(VII) oxidises vanadium(III) ions. In an experiment, 10.0 cm3 portions of a solution containing 0.132 mol dm–3 vanadium(III) ions are titrated against 0.0200 mol dm–3 potassium manganate(VII) in acidic conditions. The mean titre is 26.40 cm3 . (i) Calculate the ratio of the number of moles of manganate(VII) ions, of vanadium(III) ions, V3+, in this oxidation. You must show all your working. , to the number of moles (3) (ii) Write the equation for the reaction between manganate(VII) ions and vanadium(III) ions in acidic solution. State symbols are not required. (2) * (e) Vanadium(V) ions, , in acidic solution may be reduced step by step through its oxidation states. Compare and contrast the use of iron and tin in this reduction process. Include equations and values for any reactions that occur. State symbols are not required. (6) Use only data from these tables. Vanadium systems Reducing agents ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (Total for question = 18 marks) (Q18 WCH15/01, Jan 2023) Q33. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . In which species does the transition metal have the lowest oxidation number? A B C D [Cu(NH3)2]+ [Fe(CN)6]3– MnSO4 Ni(CO)4 (Total for question = 1 mark) (Q01 WCH15/01, Jan 2023) Q34. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross Persulfate ions react slowly with iodide ions at room temperature unless a catalyst of Fe2+ or Fe3+ is added to the reaction mixture. Which of these is the most likely reason why this reaction is slow without a catalyst? . (Total for question = 1 mark) (Q07 WCH15/01, Jan 2023) Q35. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Excess aqueous sodium thiosulfate is added to an aqueous solution of ammonium vanadate(V). What colour is the mixture when the reaction is complete? Refer to page 10 of the Data Booklet. (1) A B C D yellow blue green violet Use this space for rough working. Anything you write in this space will gain no credit. (Total for question = 1 mark) (Q03 WCH15/01, June 2022) Q36. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross Which reaction occurs at the negative electrode in a hydrogen-oxygen fuel cell? A B H2 + 2OH− → 2H2O + 2e− 2H+ + 2e− → H2 . C D O2 + 4H+ + 4e− → 2H2O 4OH− → O2 + 2H2O + 4e− (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q05 WCH15/01, June 2023) Q37. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross The emf, . , of a cell is +0.57 V. The numerical values of the standard electrode potentials of the two half-cells joined in this cell are 0.17 V and 0.40 V. What are the signs of the standard electrode potentials of the right-hand half-cell and the left-hand half-cell? (Total for question = 1 mark) (Q10 WCH15/01, Oct 2023) Q38. This question is about silver and silver compounds. Glass decorations are made reflective by coating their inner surface with silver. This is achieved by using the reaction between silver nitrate solution, ammonia and glucose, under alkaline conditions. Initially the colourless complex ion diamminesilver(I), [Ag(NH3)2]+ , forms. (a) (i) Explain the shape of [Ag(NH3)2]+ . (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Explain why [Ag(NH3)2]+ is colourless. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) The diamminesilver(I) complex then reacts with glucose forming silver and an organic compound, Y. Two other products also form. (i) Complete the equation for the reaction. (2) C6H12O7 + .......Ag +........................................................ + .................................................... (ii) Draw the structure of Y. (1) (c) The overall reaction in a silver cell used in watch batteries is Ag2O(s) + H2O(l) + Zn(s) → 2Ag(s) + Zn(OH)2(s) The half-equation for the reaction at the positive electrode is Ag2O(s) + H2O(l) + 2e− → 2Ag(s) + 2OH−(aq) Deduce the half-equation for the reaction at the negative electrode. State symbols are not required. (1) (Total for question = 9 marks) (Q10 WCH15/01, Jan 2022) Q39. Answer the questions with a cross in the boxes you think are correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Sodium thiosulfate can be used to determine the concentration of an iodine solution by titration using starch indicator. (a) 5.00 g of sodium thiosulfate was dissolved in deionised water and the solution made up to 250.0 cm3 in a volumetric flask. The volumetric flask has an uncertainty of ± 0.25 cm3. What is the minimum uncertainty of the balance required to match the uncertainty of the volumetric flask? Assume two weighings are needed. (1) A B C D ± 0.0025 g ± 0.005 g ± 0.01 g ± 0.05 g (b) The titration was carried out with sodium thiosulfate in the burette and starch was added just before the end-point. What would be the colour of the solution in the conical flask at the end-point? (1) A B C D blue-black brown colourless yellow (Total for question = 2 marks) (Q12 WCH15/01, Jan 2024) Q40. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Standard electrode potentials can also be given an alternative name. The electrochemical series lists standard electrode potentials in order. Which of these is correct? (Total for question = 1 mark) (Q08 WCH15/01, Oct 2023) Q41. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross What is the pressure of hydrogen gas used in the standard hydrogen electrode? A B C 1 Pa 100 Pa 1 000 Pa . D 100 000 Pa (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q02 WCH15/01, Oct 2022) Q42. Answer the question with a cross in the box you think is correct ( ). If you change your mind about an answer, put a line through the box ( ) and then mark your new answer with a cross ( ). The emf, conditions. , of the cell made up of the copper and magnesium half-cells is measured under standard The voltmeter used has a measurement uncertainty of 0.50%. What is the highest emf that could be measured in this experiment? Use your Data Booklet. A B C D 2.70 V 2.72 V 2.74 V 2.85 V (Total for question = 1 mark) (Q04 WCH15/01, June 2024) Q43. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross The possibility of a reaction between potassium dichromate(VI) and hydrochloric acid may be assessed using standard electrode potentials but also depends on the activation energy, Ea, of the reaction. . When potassium dichromate(VI) and hydrochloric acid are mixed, very little chlorine is formed under standard conditions but a significant amount of chlorine is produced when concentrated hydrochloric acid is used. What is the effect on Ecell and on Ea of using concentrated hydrochloric acid? (Total for question = 1 mark) (Q04 WCH15/01, Oct 2022) Q44. This question is about the chemistry of vanadium. The standard electrode potentials of some vanadium species are shown. (a) Explain the highest stable oxidation state formed by vanadium, by referring to its electronic configuration. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (b) A student suggests that the ion VO2+ may be converted into V3+ using sodium thiosulfate, Na2S2O3, with no other vanadium species being formed by reduction. (i) Justify the use of sodium thiosulfate for this reaction by writing the relevant equations and calculating their values. Use the standard electrode potentials given in the table and values from your Data Booklet. State symbols are not required in the equations. (4) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Explain why nickel, Ni, is not a suitable reagent to convert VO2+ into V3+, with no other vanadium species being formed. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (c) Most vanadium produced is used to make a steel alloy called ferrovanadium. The vanadium content of ferrovanadium may be determined by a titration method. Procedure The sample of ferrovanadium is dissolved in chloric(V) acid. The vanadium species formed is . 3 The resulting solution is transferred to a 250.0 cm volumetric flask, washings added and the solution made up to the mark with distilled water and mixed. Using a pipette, 25.0 cm3 of the solution is transferred to a conical flask and 25.0 cm3 of a 0.250 mol dm−3 solution of iron(II) sulfate, FeSO4, is added. The iron(II) ions react with the ions: + 4H+ + Fe2+ ↔ VO2+ + 2H2O + Fe3+ The resulting solution is titrated against potassium manganate(VII) to determine the amount of iron(II) ions remaining. + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O In an experiment, the mass of ferrovanadium used was 4.87 g, the concentration of potassium manganate(VII) was 0.0195 mol dm−3 and a mean titre of 22.50 cm3 was obtained. (i) Give the colour of the solution at the end-point of the titration. (1) ............................................................................................................................................. (ii) Suggest why the VO2+ ions formed do not affect the titration. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (iii) Calculate the percentage by mass of vanadium in the ferrovanadium. (7) (d) In the manufacture of sulfuric acid, vanadium(V) oxide, V2O5, is the catalyst used in the conversion of sulfur dioxide to sulfur trioxide: 2SO2 + O2 ↔ 2SO3 Write two equations to show a possible mechanism for this reaction. State symbols are not required. (2) (Total for question = 21 marks) (Q19 WCH15/01, Oct 2022) Q45. Answer the questions with a cross in the boxes you think are correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . The apparatus can be used to measure 2Cr3+(aq) + for the reaction shown. Zn(s) → 2Cr2+(aq) + Zn2+(aq) (a) Which electrodes are used for this cell? (1) (b) A student wishes to measure the standard cell potential, Cr3+ and Cr2+ ions. , of this cell. The right-hand cell requires What mass of Cr2(SO4)3⋅18H2O must be dissolved in 1.00 dm3 of deionised water to give the concentration of Cr3+ ions required to measure this ? (1) (c) What can be deduced from the fact that, for this reaction, is positive? (1) A B C D ΔStotal and ln K are positive ΔStotal and ln K are negative ΔStotal is positive and ln K is negative ΔStotal is negative and ln K is positive (Total for question = 3 marks) Use this space for any rough working. Anything you write in this space will gain no credit. (Q01 WCH15/01, Jan 2024) Q46. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . is directly proportional to A B C D ΔrH and ln K ΔrH and ln RT ΔStotal and ln K ΔStotal and ln RT (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q07 WCH15/01, Oct 2023) Q47. Answer the question with a cross in the box you think is correct ( ). If you change your mind about an answer, put a line through the box ( ) and then mark your new answer with a cross ( ). Which graph shows the relationship between electrochemical system? plotted on the x-axis and ΔStotal on the y-axis for any (Total for question = 1 mark) (Q06 WCH15/01, June 2024) Q48. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . In which of these pairs does the transition metal have the same oxidation number? (Total for question = 1 mark) (Q02 WCH15/01, Oct 2023) Q49. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross Some standard electrode potentials are shown. Which reaction is thermodynamically feasible? A B C D 2Ce + 3Mg2+ → 2Ce3+ + 3Mg 4Ce3+ → Ce + 3Ce4+ 3Mn2+ → Mn + 2Mn3+ Mg + 2Ce4+ → Mg2+ + 2Ce3+ . (Total for question = 1 mark) (Q02 WCH15/01, June 2023) Q50. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . A titre has an uncertainty of 0.32 %. The uncertainty of each burette reading is ±0.05 cm3 . What is the most likely value of the titre in cm3 ? A B C D 6.40 15.60 31.25 32.00 (Total for question = 1 mark) (Q06 WCH15/01, June 2022) Q51. This question is about transition metal compounds and their quantitative analysis. (a) Potassium dichromate(VI), K2Cr2O7, is present in very small amounts in cement, to help increase the time for the cement to set. A 50.0 g sample of cement was washed with portions of deionised water to dissolve the potassium dichromate(VI). Any insoluble residues were removed by filtration and the filtrate was transferred to a volumetric flask. The volume was made up to 100.0 cm3 , using 2 mol dm−3 sulfuric acid. 50.0 cm3 of this solution was transferred to a conical flask and titrated with a solution of ammonium iron(II) sulfate, (NH4)2Fe(SO4)2(aq), of concentration 3.24 × 10−4 mol dm−3 . The indicator N-phenylanthranilic acid was used, which gave an intense red-violet colour at the end-point. The mean titre of ammonium iron(II) sulfate was 10.90 cm3 . The ionic equation for the redox reaction in the titration is shown. 6Fe2+(aq) + (aq) + 14H+(aq) → 2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq) (i) State the colour of each chromium species in the reaction. (1) .............................................................................................................................. Cr3+ ............................................................................................................................... (ii) Suggest a reason why an indicator is needed in this titration. (1) ............................................................................................................................................. ............................................................................................................................................. (iii) Calculate the percentage by mass of potassium dichromate(VI) in the cement sample. (5) (b) N-phenylanthranilic acid has the structure shown. The solution used as an indicator was prepared by mixing 100 mg of this acid in 5 cm3 of sodium hydroxide solution, NaOH(aq). The mixture was then diluted to 100 cm3 with deionised water. Explain why the N-phenylanthranilic acid is added to the sodium hydroxide solution before it is mixed with water in the preparation of this solution. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (c) The concentration of chromium(VI) in aqueous solution may also be determined using a colorimeter. On adding 1,5-diphenylcarbazide, DPC, to a solution of chromium(VI) ions, an intensely coloured octahedral complex forms. The formula of the complex is (i) The structure of DPC is shown. . Describe how DPC is able to act as a bidentate ligand, using your diagram to show the atoms involved. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) The intense colour of this complex is due to the transfer of electrons from the ligand to the chromium(VI) ion. Suggest a possible reason why the colour is not due to the transfer of electrons between split d-orbitals in the ion. Refer to the electronic configuration of the chromium(VI) ion. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (d) The concentration of nickel(II) ions, Ni2+(aq), can be determined by forming a complex with the ligand dimethylglyoxime, C4H8N2O2. [Ni(H2O)6]2+ + 2C4H8N2O2 + 2OH− → [Ni(C4H7N2O2)2(H2O)2] + 6H2O Explain why the formation of the dimethylglyoxime complex is favoured, in terms of entropy. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (Total for question = 15 marks) (Q15 WCH15/01, June 2022) Q52. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . A sample of air contains 405 parts per million (ppm) of carbon dioxide. What is the approximate volume, in cm3 , of carbon dioxide in 250 cm3 of this air? A B C D 0.001 0.01 0.1 1 (Total for question = 1 mark) (Q10 WCH15/01, Jan 2023) Q53. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . The formula of hydrated copper(II) bromide is CuBr2⋅4H2O. What is the percentage, by mass, of water of crystallisation in this compound? A B C D 6.10 % 8.06 % 24.4 % 32.2 % (Total for question = 1 mark) Use this space for any rough working. Anything you write in this space will gain no credit. (Q05 WCH15/01, Jan 2023)
