ACJC 2015 H1 CHEM P2 Answers Prelims

2 © ACJC 2015 8872/02/Prelim/15 [Turn over Section A Answer all questions in this section in the spaces provided. 1 Oxides of nitrogen are by -products of combustion of hydrocarbon fuels in internal combustion engines. One of them is nitrogen monoxide. At 700 °C, nitrogen monoxide and hydrogen react as follows: 2NO(g) + 2H2(g)  N2(g) + 2H2O (g) Keeping the concentration of nitrogen monox ide to be constant at 0.012 mol dm-3, the following graph was obtained. For Examiner’s use (a) (i) Calculate its initial rate of this reaction. [1] Initial rate = 6.67 x 10-6 mol dm-3 s-1 (ii) The results of some investigation of the rate of reaction are shown below. Experiment Number Initial [NO]/ mol dm-3 Initial [H2]/ mol dm-3 Initial Rate/ x 10-6 mol dm-3 s-1 1 0.0120 0.002 13.0 Using the given information and your answer in (i), determine the order of reaction with respect to hydrogen. [1] Comparing experiment 1 and keeping initial [NO] constant, the initial rate doubles when the initial [H 2] doubles, hence order of reaction wrt H2 is 1. (iii) Using the information below, d educe the order of reaction with respect to NO. Show your working clearly. Experiment Number Initial [NO]/ mol dm-3 Initial [H2]/ mol dm-3 Initial Rate/ x 10-6 mol dm-3 s-1 2 0.0020 0.012 2.2 3 0.0040 0.012 8.7 [2] [H2]/ mol dm-3 Time/ s

3 x x .0040)k(0.012)(0 .0020)k(0.012)(0 3 Rate 2 Rate  x      2 1 6- -6 8.7x10 2.2x10 x = 2 (iv) Determine a value of k, stating the units. [2] Using experiment 1, 13 x 10-6 = k(0.012)2(0.0020) k = 45.1 mol-2 dm6 s-1 (b) Write equations inclu ding state symbols for the reaction of the following oxides with water. In each case, state the pH of the resulting solution. (i) Na2O [1] Na2O(s) + H2O(l)  2NaOH(aq) pH=13 (ii) P4O6 [1] P4O6(s) + 6H2O(l)  4H3PO3(aq) pH 2 or 3 [Total: 8 marks] 2 During the OPEC 1973 oil crisis, methanol from coal is proposed as a proven liquid fuel with well -established manufacturing technology and sufficient resources to replace gasoline. T he enthalpy of combustion of methanol in a typical car engine is -715 kJ mol–1. In a methanol plant, its production is carried in 2 steps. The first step is to convert the feedstock natural gas into a synthesis gas stream consisting of carbon monoxide, carbon dioxide, steam and hydrogen. Methanol can then be produced by using the following reversible reaction between carbon monoxide and hydrogen. 2H2(g) + CO(g) CH3OH(g) For Examiner’s use (a) (i) It is recommended that the temperature of a stationery or slow moving car should reach the optimum temperature of 34.0 °C before acceleration to 80 km per hour should take place to achieve optimum fuel efficiency. The efficiency of energy transfer in the engine from the combustion of fuel is 95% and the mass of engine oil found in the engine is 2.5 kg. Calculate the mass of methanol that should be burnt in order to for the car to reach its optimum temperature from an initial temperature of 20.0 °C. Given that the specific heat capacity of engine oil is 1.8 kJ K–1 kg–1. Heat required to raise to optimum temperature [3]

4 = 2.5 x 1.8 x (34.0 – 20.0)= 63 kJ No. of moles of methanol = 63 / 715 (0.95) = 0.09275 mol Mass of methanol = 0.09275 x 32.0 = 2.97 g (ii) Given the following standard enthalpy change of combustion data and energy cycle, apply Hess’s Law and calculate the enthalpy change for the production of methanol using carbon m onoxide and hydrogen. ΔHc (H2) =  286 kJ mol1 ΔHc (CO) =  283 kJ mol1 2H2(g) + CO(g) CH3OH(g) +O2(g) 2H2O(l) + CO2(g) +1/2O2(g) +3/2O2(g) Hreaction = 2H1 + H2  H3 = 2(286)  283 (715) = 140 kJ mol1 [2] (iii) Compound A can be used as low toxic ity solvent in paints, glues and nail polish remover. It can be synthesized using methanol and ethanoic acid in the presence of concentrated sulfuric acid. Draw the displayed formula of compound A and state the type of reaction. Condensation/esterification [2] (b) Methanol can be used to determine the concentration of iron in an impure solid. 7.00 g of the impure solid was first dissolved in an excess of sulfuric acid and completely oxidised to form Fe3+ (aq) solution. The resulting solution containing Fe3+ was made up to 250 cm3. 10.0 cm 3 of this solution was titrated against a solution of methanol of concentration 0.01 mol dm 3. During the titration, effervescence was observed. The gas evolved formed white precipitate in Ca(OH)2 (aq).

5 It was found that 24.00 cm3 methanol was required and Fe3+ is reduced to Fe2+ upon complete reaction. The half equation of the reduction of Fe3+ is: Fe3+ + e → Fe2+ (i) Write a balanced oxidation half equation of methanol. CH3OH + H2O → CO2 + 6H+ + 6e [1] (ii) Write a balanced redox equation with state symbol for the reaction between Fe3+ and CH3OH. 6 Fe3+(aq) + CH3OH (aq) + H2O(l) → 6Fe2+(aq) + CO2 (g) + 6H+(aq) [1] (iii) Calculate the percentage purity of iron in the impure solid. No. of moles of CH3OH used = 24.00 x 103 x 0.01 = 2.4 x 104 mol No. of moles of Fe3+ in 10.0 cm3 = 6 x 2.4 x 104 = 1.44 x 103 mol No. of moles of Fe 3+ in 250 cm 3 =1.44 x 103 x 250/10 = 0.03 60 mol Mass of Fe in sample = 0.0360 x 55.8 = 2.009 g % of Fe in sample = 2.009/7.00 x 100 = 28.7% [3] [Total: 12 marks]

6 3 (a) Compound Z is a component found in aviation fuel. Some chemical transformations of Compound Z are given below. For Examiner’s use (i) Draw the structural formula of Compound Z. [1] (ii) State the reagents and conditions used in reaction (ii). [1] Ethanolic NaOH, heat with reflux (iii) State the type of reaction for reaction (i) and (ii). [2] Reaction (i) : Substitution Reaction (ii) : Elimination (b) Compound E(C7H8) is an essential starting material for organic chemistry synthesis. When E reacts with bromine in presence of iron, it forms compound F. Compound G can be obtained from E under suitable condition with manganate(VII) solution. Compound G and H produce effervescence when reacted with sodium. G dissolves in aqueous sodium hydroxide but H cannot. G can be obtained from H from treatment with hot acidified potassium dichromate. Suggest the

7 structures for E, F, G and H. [4] [Total: 8 marks] 4 Data about HF, HCl, HBr and HI are given below. HF HCl HBr HI Boiling Point / oC 20 -85 -67 -35 Bond Energy / kJ mol-1 562 431 366 299 For Examiner’s use (a) (i) Draw the dot-and-cross diagram to illustrate the bonding in HF. [1] (ii) Hence, explain why the bond energy of the hydrogen halides decreases down the group.  As the size of the halogen atom increases, effectiveness of the overlap of the bonding orbitals decrease [1] (iii) Whilst the bond energy of the hydrogen halides shows a neat trend, the bond energy of the halogens do not. Based on the data available from your data booklet, identify the halogen that does not fit in with the trend and suggest an explanation f