Introductory Chemistry — Final Exam Equations and Data pV = nRT where p = pressure, V = volume, n= no. of moles, R = ideal gas constant T = temperature R = 8.3144 J K–1mol–1 or R = 0.082057 L atm K–1mol–1 ∆Hrxn = ∑ ∆Hf products – ∑∆Hf reactants Avagadro number = 6.022 x 1023 mol–1 Solubilities of various substances in H2O: calcium carbonate: 14 mg dm–3 calcium acetate: 29.7 g dm–3 sodium carbonate: 450 g dm–3 calcium chloride: 75 g dm–3 sodium acetate: 46.4 g dm–3 Enthalpies of formation: ∆Hf CH4(g) = –74.81 kJ mol–1 ∆Hf COCl2(g) = –223.0 kJ mol–1 ∆Hf H2O(g) = –241.8 kJ mol–1 ∆Hf CH3CH2OH(l) = –277.7 kJ mol–1 ∆Hf CO2(g) = –393.5 kJ mol–1 1 Introductory Chemistry — Final Exam 2 Introductory Chemistry — Final Exam 3 Question 1 A) [Part A omitted from this sample] B) ‘Hard’ water is saturated with calcium carbonate (CaCO3). i) Kettles in CaCO3
layer
areas with hard tap water often build up solid layers of CaCO3 on their surfaces over a period of several months. Why? water
Some of the boiling water will evaporate, which will increase the concentration of remaining solutes. Since the tap water is already saturated with CaCO3, CaCO3 will precipitate out of solution as the water is boiled off. [2] ii) Devices called ‘water softeners’ exchange the Ca2+ ions in hard water with Na+ ions, forming sodium carbonate, which does not form layers on the surfaces of kettles. Complete the equation below and make any necessary corrections to the formulae: Ca2+ CO32– + 2Na+ —> Na+2 CO32– + Ca2+ [1] iii) Use information from the data sheet to explain why sodium carbonate does not form layers on the surfaces of kettles. Na2CO3 has a solubility of 450 g dm–3. This is a very high solubility and normal tap water will not be close to saturation with Na2CO3. [2] Introductory Chemistry — Final Exam 4 Question 2 A) An engineer claims that refrigerators reduce the entropy of their contents by lowering their temperature. Does this statement contradict the second law of thermodynamics? Why? No, it does not conflict with the second law of thermodynamics since refrigerators are open systems. (Refrigerators increase the entropy of their surroundings) [2] B) Some cars now use methane gas (CH4(g)) instead of petrol (gasoline). They use gas cylinders that can carry about 10 kg of methane when full. An engineer plans to use hydrogen gas (H2(g)) instead of methane and wants to use the same volume gas tank at the same pressure. Use information from the data sheet to calculate the total energy released from burning i) a full tank of methane and ii) a full tank of hydrogen. Include balanced chemical equations for the oxidation of CH4 and H2, assume both fuels behave as ideal gasses. CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g) Mr CH4 = (12.01 + 4(1.008)) kJ mol–1 = 16.04 g mol–1 10 000 g CH4 / 16.04 g mol–1 = 623.4 mol CH4 ∆Hrxn = (–393.5 kJ mol–1 + 2(–241.8 kJ mol–1)) – (–74.81 kJ mol–1) = –802.3 kJ mol–1 => Total energy released from 10 kg CH4 = –802.3 kJ mol–1 x 623.4 mol = 5.0 x 105 kJ (2 s.f.) (note, the question asks for how much energy is released so the minus sign is not used (or to be specific, the answer is multiplied by –1)) H2(g) + ½ O2(g) —> H2O(g) ∆Hrxn = –241.8 kJ mol–1 (Assume gasses are ideal so…) Amount of H2(g) = 623.4 mol Total energy release from 623.4 mol H2 = –241.8 kJ mol–1 x 623.4 mol = 1.5 x 105 kJ (2 s.f.) [6] iii) Which fuel releases the most energy per kilogram? Show your working. CH4: 5.002 x 105 kJ / 10 kg = 5.0 x 104 kJ kg–1 Mr H2: 2.016 g mol–1 => 1 kg H2 releases ((–241.8 kJ mol–1) / 2.016 g) x 1 000 = 1.2 x 105 kJ kg–1 H2(g) releases the most energy per kilogram. [2] Introductory Chemistry — Final Exam 5 Question 3 A) Use some or all of the following terms to answer parts i)—iv). exothermic entropy enthalpy energy activation energy endothermic The oxidation of diamond (a carbon crystal) in oxygen follows the equation: C(diamond) + O2(g) —> CO2(g) i) This reaction releases heat so it is exothermic. ii) The total entropy of the products is greater than that of the reactants. iii) The total energy of the products is lower than that of the reactants. iv) Diamonds can last for millions of years without noticeably oxidising so the activation energy must be very high for this process. [4] B) Oxygen reacts with carbon monoxide gas according to the following equation: 2CO(g) + O2(g) —> 2CO2(g) ∆H = –283 kJ mol–1 Can we predict the rate of reaction from this data? Why? No. We cannot predict the rate of a reaction from thermodynamic data. [2] ii) In terms of kinetic theory, how does raising the temperature and/or pressure affect the rate of this reaction. How often molecules collide with enough energy for a reaction and how often they collide in the right orientation for reaction. [2] iii) This reaction occurs slowly at low temperature. How could we make it occur quickly at low temperature and at the same pressure? Give one reason why we would do this. Use a catalyst. Cars use catalysts to convert CO to CO2 at low temperatures before the gas leaves the exhaust. (Other answers may be acceptable) [2] Introductory Chemistry — Final Exam Question 4 [This question is omitted] 6 Introductory Chemistry — Final Exam 7 Question 5 [This question is omitted.] [2] Introductory Chemistry — Final Exam 8 Question 6 The ease of oxidation of the following metals are in the order: <— Easier to oxidise magnesium (Mg —> Mg2+), iron (Fe —> Fe2+), tin (Sn —> Sn2+), copper (Cu —> Cu2+), silver (Ag —> Ag+) harder to oxidise —> A) i) Steel (mostly iron, Fe) is often protected from rusting by applying paint. Explain two ways that paint prevents rusting. Paint prevents water from reacting with Fe2+ ions and Fe2+ ions from travelling to cathodic regions. (Other answers may be acceptable) [2] ii) Underground steel pipes are often protected from rusting by using sacrificial anodes. Choose any metal from the above list that would make a suitable sacrificial anode for steel pipes. Explain your choice. Magnesium is the only metal from the list that can be used as a sacrificial anode as sacrificial anodes must be easier to oxidise than the metals they are protecting. [2] B) i) a) Will any change be observed if Sn wire is immersed in a beaker of copper sulfate (CuSO4)? b) Will any change be observed if Ag wire is immersed in a beaker of magnesium sulfate (MgSO4)? a) Yes. The tin wire will dissolve and copper will precipitate. b) No. [2] ii) Choose a reaction from part i) and write the full equation, the ionic equation and the pair of half equations: [For example] Sn + CuSO4 —> Cu + SnSO4 Sn + Cu2+ —> Sn2+ + Cu Sn – 2e– —> Sn2+ Cu2+ + 2e– —> Cu [4] Introductory Chemistry — Final Exam 9 Question 7 A) Which of the following diagrams are correct? Mark correct diagrams with a tick (√) Correct any mistakes. O
i) NH2 ✓ –
O
ii) –
(other answers may be acceptable) CH3
CH3
O
iii) NH2
H3C
✓ NH2
iv) H
H3C
NH2
H
(other answers may be acceptable) [4] B) What is the chemical formula of the following compounds? (Write CnHn followed by other elements, e.g. C2H4O2) OH
H 2N
i) C3H6 ii) OH C3H9NO2 [2] C) Which of the following diagrams show a pair of isomers (constitutional or configurational)? Write ‘isomers’ or ‘not isomers’. i) and isomers ii) and not isomers [2] O
NH2 D) a) b) c) d) Cl Which of the above functional groups i) is the strongest base ii) has the strongest inductive effects? i) a ii) b [2]