Learner Guide Cambridge O Level Chemistry 5070 EXAMINATION ADVICE AND PAPER SOLVING SKILLS GENERAL ADVICE . • • • • • • • • • The most basic advice for being successful in examinations is to: read each question carefully and then answer it write to the point, being specific not vague. Read the question so that you understand exactly what is required before you start to answer. You must do what the question asks. You must focus the terms to make it clear what is required e.g. explain, describe, suggest. The meaning of these words can be found in the appendices section at the end of this document and also in the syllabus. Take careful note of how many marks there are for a question. Generally the number of marks indicates the number of points you need to cover in your answer. Provide what the question asks for. If a question asks for two examples, give two – extra wrong answers may lose you marks. ‘Describe what you observe’ requires you to write down what you see, hear or feel (e.g. the test-tube gets hot). Write to the point. There is a lot of time wasted in exams by writing the same answer more than once and by including irrelevant material. In calculations always show your working – even if your answer is wrong, you may get marks for your method. Give units when needed. Your answer should be given in the manner requested or to the same number of significant figures as the data provided in the question. Make sure that you understand the difference between significant figures and decimal places. For example the mass 11.445 g is to 5 significant figures and 3 decimal places. If it was corrected to 4 significant figures it would become 11.45 g and corrected to 3 significant PAPER1 • • • • • Attempt every question. There are forty questions and forty marks available. Do not make any assumptions about the order of responses – just because the last two answers have been D, it does not mean that the next one cannot be D as well. Read each question carefully. If possible work out the answer before you look at the four alternatives provided – you can usually do this with calculations. If this is not possible, eliminate options which are clearly incorrect and then choose between those left. While you must select an answer, you should never be making a guess from four. Do not try to work out the answers in your head. Write on the question paper to help you understand and solve the problems. Practice is important in making you better at multiple choice. You may find it useful to get your teacher or someone else to mark your answers to a paper and then look over your mistakes without being told the correct answers. This will allow you to spot careless errors, which are the result of poor technique, e.g. poor reading of the question and also to identify material which is not known or well understood. Examples 13. What are the correct anode (positive electrode) and cathode (negative electrode) products when aqueous copper(II) sulfate is electrolyzed using copper electrodes? anode product cathode product A aqueous copper(II) ions copper metal B aqueous copper(II) ions hydrogen gas C oxygen gas copper metal D oxygen gas hydrogen gas • 18. Which row correctly describes the oxides? 24 Aluminium reacts with chromium(III) oxide as shown. aluminium + chromium(III) oxide → chromium + aluminium oxide Which statements are correct? 1 Aluminium is more reactive than chromium. 2 A similar reaction would also take place between aluminium and iron(III) oxide. 3 Iron(III) oxide is reduced by another metal in the blast furnace. A 1, 2 and 3 B 1 and 2 only C 1 and 3 only D 2 and 3 only 5070 Chemistry Principal Examiner Report for Teachers paper1 Question 13 • The syllabus requires a good knowledge of the electrolysis of aqueous copper(II) sulfate with copper electrodes as a means of purifying copper. This knowledge leads to the choice of alternative A as the correct answer. Question 16 • Curve 2 showed that in the second experiment more oxygen was produced. The only method of obtaining more product from a reaction which is not reversible is to use more of the starting reagents. Question 18 • Most candidates appreciated that aluminium oxide is amphoteric, though many did not realise that magnesium oxide is a basic oxide. Question 24 • In the blast furnace, iron(III) oxide is reduced by either carbon or carbon monoxide neither of which is a metal, thus statement 3 is not correct. Question 26 • The metals used in the sacrificial protection of iron are always more reactive than iron. Magnesium is higher than iron and copper is lower than iron in the reactivity series. Therefore alternative D was the correct answer. Question 30 • Nitrogen dioxide is an acidic gas and reacts with the oxygen and water in the atmosphere to form nitric acid which is a constituent of acid rain. Buildings made from limestone will as a consequence be damaged when nitrogen dioxide is present in the atmosphere, eliminating alternatives B and C. PAPER 2 ADVICE • • • • • • • You need to read the question so that you know what is required to answer it. The following ideas seek to explain the advice given through examples. You must answer the question set. State one specific use of nickel other than its use in alloys. (1 mark) The word ‘specific’ is important here. You will get no credit for ‘nickel is used as a catalyst’ because you must also identify the industrial reaction catalysed i.e. the hydrogenation of vegetable oils. Suggest two possible consequences of an increase in global warming. (2 marks) The answer requires two effects of global warming, e.g. polar ice melting, sea level rising but does not want an explanation of how or why global warming occurs. Describe how the boiling points of unbranched alkanes vary with the size of their molecules. (1 mark) An answer like ‘the boiling points depend on the size of the molecules’ is not good enough. It indicates the candidate knows size affects boiling point but it does not state how changing size changes boiling point. ‘As the molecules get larger the boiling points increase’ answers the question set. You must be specific not vague in writing an answer. How do fertilisers increase crop yields? (1 mark) Vague answers simply state that fertilisers contain food for plants or similar whereas a specific answer would note that the fertiliser contains nitrogen and link this to protein and growth. Explain why carbon monoxide must not be allowed to escape from the furnace. (1 mark) ‘Carbon monoxide is toxic or poisonous’ is the answer whereas ‘pollutant’, ‘harmful’ and ‘causes breathing difficulties’ are vague. The number of marks for a question can give you an indication of the number of points you should make in your answer. Explain why zinc chloride conducts electricity when molten, not when solid. (2 marks) ‘In molten zinc chloride the ions can move’ is one point and scores( 1 mark). Because the ions cannot move’ is the other scoring point. Cont… • • • • • It is common to find that candidates score the first mark but not the second. Explain how the processes of photosynthesis and respiration help regulate the amount of carbon dioxide in the atmosphere. (3 marks) There are three points to make – one about photosynthesis, another about respiration and a final one about ‘regulate’, i.e. maintaining the balance. Consequently the following answer scores all the marks – ‘Plants take up carbon dioxide from the atmosphere in photosynthesis. In respiration carbon dioxide is released to the atmosphere. These processes maintain a balance because the amount of carbon dioxide taken up is equal to the amount being released.’ It is important that you know what is meant by the scientific terms or phrases used in the questions. State two differences in the physical properties of the metals potassium and iron. (2 marks) State one difference in the chemical properties of the metals potassium and iron. (1 mark) The fi rst question is about properties such as melting point, density, hardness so ‘potassium has a low melting point and iron has a high melting point’ scores 1 of the 2 marks. The second question is about the chemical reaction of the metals and an answer such as ‘potassium is very reactive and iron is not very reactive’ or ‘potassium reacts with water and iron does not’ will gain the mark. -Write an equation for the complete combustion of methane. (1 mark) ‘Write an equation’ in the exam means a symbol equation i.e. a balanced formula equation – it must contain the correct formulae for the reactants and products and be balanced. While formula writing and balancing equations are skills acquired through practice, marks are often lost because candidates forget the nonmetals made up of diatomic molecules, e.g. O2, N2, H2, Cl2. Paper 2 A2. Both respiration and combustion add carbon dioxide to the atmosphere. (a) Give one reason why scientists are concerned about the increasing use of fossil fuels. • .......................................................................................................................................... • ..................................................................................................................................... [1] (b) Respiration is a process that occurs in living organisms where glucose, C6H12O6, reacts with oxygen. Write the overall equation that represents respiration. • ..................................................................................................................................... [1] (c) Respiration is an exothermic reaction. (i) Explain, in terms of the energy changes that occur during bond breaking and bond making, why respiration is an exothermic reaction. • .................................................................................................................................. • .................................................................................................................................. .................................................................................................................................. • ............................................................................................................................. [2] (ii) Complete the energy profile diagram for respiration. On your diagram label the • products, • enthalpy change for the reaction, ΔH, • activation energy, Ea. [3] B9. Ethanol is manufactured by the hydration of ethene. C2H4(g) + H2O(g) C2H5OH(g) ΔH = – 45 kJ / mol This reaction is exothermic. The reaction is normally carried out at a pressure of 70 atmospheres and a temperature of 300 °C. (a) The reaction is carried out at 70 atmospheres pressure and at 600 °C rather than 300 °C. Predict and explain the effect of raising the temperature on (i) the rate of reaction, • .................................................................................................................................. • .................................................................................................................................. • ............................................................................................................................. [2] (ii) the position of equilibrium. • .................................................................................................................................. • .................................................................................................................................. • ............................................................................................................................. [2] (b) The reaction is carried out at 20 atmospheres rather than 70 atmospheres, and at 300 °C. Predict and explain the effect of decreasing the pressure on (i) the rate of reaction, • .................................................................................................................................. • .................................................................................................................................. • ............................................................................................................................. [2] (ii) the position of equilibrium. • .................................................................................................................................. • .................................................................................................................................. • ............................................................................................................................. [2] Principal Examiner Report for Teachers PAPER2 Question A2 • • • • • Most candidates knew the overall equation for photosynthesis but many added energy or heat as a product of the equation. Part (c)(i) about bond breaking and bond making in an exothermic reaction was reasonably well done. A significant number of candidates confused the relationship between energy changes and bond breaking and formation. Many could draw an energy profile diagram for an exothermic reaction. (a) Most candidates referred to the formation of carbon dioxide or the finite nature of fossil fuels. Some candidates gave non-specific answers that referred to air pollution. A significant minority referred incorrectly to ozone depletion. (b) Many candidates could reproduce the overall equation for respiration. The most common error was to include heat or energy in the equation. Other common errors included: the inclusion of alcohol as a product; the omission of oxygen as a ‘reactant’; incorrect balancing (often with 3O2 or 3CO2 and 3H2O) (c) (i) In comparison with previous series, more candidates gave good answers. There was far less reference to bond making needing energy than in previous series. Good answers stated that bond breaking is endothermic and bond making exothermic and then explained that more energy is released than absorbed. Other candidates did not gain marks because they contradicted themselves when writing about energy absorbed and released. (ii) Many candidates could draw a correct energy profile diagram. The main errors arose from the enthalpy change. Many candidates drew double-headed arrows or a line without arrows for the enthalpy change. The activation energy was drawn better than the enthalpy change. Some candidates drew the arrows too short or too long to be credited with marks, especially the arrow for the activation energy. Question B9 This question focused on the hydration of ethene. (a) (i) Candidates often confused rate of reaction and position of equilibrium and as a result gave answers that referred to a lower rate of reaction because the reaction was exothermic. Other candidates just referred to the number of collisions or collision frequency rather than focusing on the speed of particles and the number of successful collisions. The best answers appreciated that the rate of reaction increased because there were more successful collisions. (ii) Candidates expressed the idea that the position of equilibrium moved to the left in a variety of ways and often described the backward reaction being favoured. The best answers referred to the reaction being exothermic so the position of equilibrium moves to the left. (b) (i) Candidates often did not appreciate that the particles would be less crowded and that as a result the collision frequency decreases. Candidates often did not include the idea of collision frequency and just referred to the number of collisions. Other candidates linked the rate of reaction with le Chatelier’s principle. (ii) As in (a)(ii) candidates expressed the idea that the position of equilibrium moved to the left in a variety of ways. Only the best answers linked this to the reactants having the greater number of moles. (c) Some candidates were able to calculate the energy released as 450 kJ. (d) Often candidates not only referred to the lowering of the activation energy but described the idea of an alternative pathway. PAPER4 ADVICE • • • • • • • • This is a paper which tests your knowledge of practical work so in revision make sure that you study all the experiments you have seen demonstrated. You do not get the Qualitative Analysis Notes in the exam and need to learn all those tests for anions, cations and gases. You also need to learn tests for oxidising and reducing agents and know the reactions of acids, metals and organic compounds from the syllabus. You will be expected to be familiar with the practical techniques and equipment identified in the Practical assessment section of the syllabus content for e.g Question 1 is a quantitative problem involving measuring, recording data and using the data. You should record readings as instructed – burette readings to the nearest cm3, e.g. 23 cm3, thermometer readings usually to the nearest 0.5oC and timings to the nearest second. In titration-based problems you record burette readings in the table provided. From your titrations you must obtain two results which are within 0.2 cm3, tick these results and then average them. Some candidates forget to identify the results they average and others average all their results rather than just the ones ticked. In experiments where times or temperatures are recorded, the data is often used to produce a graph. Plot the results carefully and use the points to draw a graph line (or lines) as instructed. Usually you are told whether the line is straight or curved but do recognize that ‘a line of best fit’ can be straight or curved. When using your data in calculations, make sure that you act upon the advice in the questions. You should give answers to three significant figures unless instructed otherwise. • The Qualitative Analysis Notes will be on the last page of the exam paper and you must make sure that you have carried out and understand all the tests described. • You also need to know tests for oxidising and reducing agents and be familiar with other test-tube reactions such as those of acids and metals. • You must record observations carefully using the correct words. The way the test results are recorded in the Qualitative Analysis Notes is ideal. Substances should always be described – solids have a colour, e.g. a white precipitate, solutions have a colour or are colorless (clear does not mean colorless) and gases are generally colorless but when they are formed in liquids, bubbles are seen. • You must describe a positive test for a gas before it can be identified. Recording ‘the gas is hydrogen’ will score no marks but ‘the gas pops with a lighted splint (1 mark) and is hydrogen (1 mark) scores 2. ATP EXAMPLES 8 Ammonium sulfate is a nitrogenous fertiliser. Nitrogen in the fertiliser promotes the growth of plants. A student is given an aqueous solution of ammonium sulfate, R, and is asked to determine its concentration. Using a pipette, 25.0 cm3 of R is placed in a beaker and 50.0 cm3 of 2.00 mol / dm3 sodium hydroxide, an excess, is added. The following reaction takes place. (NH4)2SO4 + 2NaOH Na2SO4 + 2H2O + 2NH3 The mixture is heated until all the ammonia has evolved. (a) How can the student confirm that all the ammonia has evolved? ................................................................................................................................................... ...............................................................................................................................................[1] (b) After cooling, the remaining mixture is transferred to a volumetric flask and made up to 250 cm3 with distilled water. This is solution T. 25.0 cm3 of T is transferred to a conical flask and a few drops of phenolphthalein indicator are added. A burette is filled with 0.100 mol / dm3 hydrochloric acid. 0.100 mol / dm3 hydrochloric acid is added to T until the end-point is reached. Phenolphthalein is colourless in acid solution and pink in alkaline solution. What is the colour of the solution in the conical flask • before hydrochloric acid is added, .......................................... • at the end-point? .......................................... [1] Summary Tick (✓) the best titration results. Using these results, the average volume of hydrochloric acid is ................................................... cm3. [4] (d) Calculate the number of moles of hydrochloric acid in the average volume of 0.100 mol / dm3 hydrochloric acid in (c). .................................................moles [1] HCl + NaOH NaCl + H2O deduce the number of moles of sodium hydroxide in 25.0 cm3 of T. .................................................moles [1] (ii) Calculate the number of moles of sodium hydroxide in 250 cm3 of T. .................................................moles [1] (f) Calculate the number of moles of sodium hydroxide in 50 cm3 of 2.00 mol / dm3 sodium hydroxide. .................................................moles [1] (g) By subtracting your answer to (e)(ii) from your answer to (f) calculate the number of moles of sodium hydroxide which reacts with 25.0 cm3 of R. .................................................moles [1] (h) When 1 mole of sodium hydroxide reacts with ammonium sulfate, 17 g of ammonia are produced. Calculate (i) the mass of ammonia produced from the original 25.0 cm3 sample of R, .........................................................g [1] (ii) the mass of ammonia produced from 1 dm3 of R. .........................................................g [1] (i) The formula of ammonium sulfate is (NH4)2SO4. Using your answer to (h)(ii) calculate the concentration of ammonium sulfate in R. [Ar: N, 14; H, 1] .............................................mol / dm3 [2] [Total: 15] • (d) Use your graphs to answer the following questions. What is the mass of each compound that dissolves in 10 g of water at 0 °C? (i) ammonium chloride ........................................................ g [1] (ii) potassium nitrate ........................................................ g [1] (e) The solubility of a compound is defined as the maximum mass of compound that will dissolve in 100 g of water at a given temperature. (i) At what temperature is the solubility of each compound the same? .......................................................°C [1] (ii) Calculate the solubility of both ammonium chloride and potassium nitrate at the temperature you have given in (e)(i). .................................g / 100 g of water [1] (f) The student is given two boiling tubes, one containing 5.0 g of ammonium chloride in 10 g of water, the other containing 5.0 g of potassium nitrate in 10 g of water. Both are stirred and heated to a temperature of 45 °C. Describe the appearance of the contents of each boiling tube at 45 °C. ammonium chloride ................................................................................................................................................... potassium nitrate ................................................................................................................................................... [2] Principal Examiner Report for Teachers PAPER 4/ATP Q 8. (a) Gas no longer turns litmus blue (1) [1] (b) Pink to colourless (1) [1] (c) 29.5 28.8 39.9 1 mark for each correct row or column • 8.9 7.9 19.5 to the benefit of the candidate (3) • 20.6 20.9 20.4 Mean value 20.5 cm3 (1) [4] (d) 0.00205 moles (1) [1] (e) (i) 0.00205 (1) [1] (ii) 0.0205 (1) [1] (f) 0.1 (1) [1] (g) 0.0795 (1) [1] (h) (i) 1.352 g (1) [1] (ii) 54.06 g (1) [1] (i) One mole of (NH4)2SO4 produces 34 g / 2 moles of ammonia (1) Concentration = 54.06 / 34 = 1.59 mol dm3 (1) [2] [Total: 15] Question 10 (a) The thermometers were generally read correctly. (b) Many candidates realized that the problem concerned the low temperature but most suggested that this was due to the type of thermometer used, rather than the difficulty of reaching the temperature which is well below room temperature. This could be achieved by, for example, placing the tube in a beaker containing ice. (c) The points were generally plotted accurately and joined with smooth curves extended to the y-axis. (d) The candidates needed to read from their graph the masses of each compound where the curves crossed the y-axis. (e) Most candidates correctly read the temperature at which the solubility of each compound was the same but many gave the mass reading at that temperature as the solubility, failing to realize that this mass needed to be multiplied by 10 to give the solubility in 100g of water. (f) From the curves it can be deduced that the ammonium chloride would not completely dissolve at this temperature resulting in a solution containing undisclosed solid whereas the potassium nitrate would completely dissolve resulting in a (colorless) solution.
