CJC Prelim H2 CHEM P1 Solutions

1 9647/01/CJC JC2 Preliminary Exam 2016 [Turn over CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/01 Paper 1 Multiple Choice Wednesday 31 August 2016 1 hour Additional Materials: Multiple Choice Answer Sheet Data Booklet READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. Write your name and HT group on the Answer Sheet in the spaces provided. There are forty questions on this paper. Answer all questions. For each question there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer Sheet. Read the instructions on the Answer Sheet very carefully. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. The use of an approved scientific calculator is expected, where appropriate. This document consists of 30 printed pages. WORKED SOLUTIONS

2 9647/01/CJC JC2 Preliminary Exam 2016 Section A For each question there are four possible answers, A, B, C and D. Choose the one you consider to be correct and record your choice in soft pencil on the separate Answer Sheet provided. 1 Use of the Data Booklet is relevant to this question. Arsenic(III) sulfide, As2S3, is a bright yellow solid which has been used as a pigment in paintings. When arsenic(III) sulfide is heated in air, it reacts with oxygen to give arsenic(III) oxide and sulfur(IV) oxide. 2As2S3 + 9O2  2As2O3 + 6SO2 What would be the mass of arsenic(III) oxide produced if 550 dm3 oxygen, measured at room temperature and pressure, reacted with arsenic(III) sulfide? A 1010 g B 1070 g C 2010 g D 2160 g Answer: A Number of moles of O2 = 550 24 = 22.9 mol Since 9O2 ≡ 2As2O3 Number of moles of As2O3 = 22.9 9 × 2 = 5.09 mol Mr of As2O3 = 74.9 × 2 + 16.0 × 3 = 197.8 Mass of As2O3 = 5.09 × 197.8 = 1007 g 2 Incomplete combustion of 20 cm 3 of a gaseous hydrocarbon gave carbon dioxide and carbon monoxide in a 6:1 ratio, as well as water vapour. It was found that the carbon dioxide took up a volume of 51.4 cm 3 whereas the water vapour took up a volume of 80 cm3. All gas volumes are measured at the same temperature and pressure. What is the molecular formula of the hydrocarbon? A C2H4 B C2H6 C C3H6 D C3H8 Answer: D 7CxHy + ? O2  6xCO2 + xCO + 7𝑦 2 H2O Reduction of 80 cm3 after passing through dehydrating agent  volume of water vapour was 80 cm3 Reduction of 51.4 cm3 after passing through KOH (aq)  volume of CO2 was 51.4 cm3 Mole ratio of CxHy : CO2 = 7 : 6x = 20 : 51.4

3 9647/01/CJC JC2 Preliminary Exam 2016 [Turn over 6𝑥 7 = 51.4 20 x = 2.99  3 Mole ratio of CxHy : H2O = 7 : 7𝑦 2 = 20 : 80 7𝑦 2 7 = 4 y = 8 Therefore, the molecular formula of the hydrocarbon is C3H8. 3 Polythionates are a series of sulfur -oxo anions with the general formula S nO62‒, where n > 2. A simple example is tetrathionate ion, S4O62‒. Which of the following graphs best illustrates the variation in the average oxidation number of sulfur in SnO62‒ with n? Answer: B Let the average oxidation number of S be y For SnO62‒, ‒2 = 6(‒2) + ny ny = +10 y = + 10 𝑛 Hence the graph is a hyperbolic graph starting from the point (3, +10/3), which corresponds to graph B. 4 Which of the following species has the smallest bond angle around the central atom? A NCl3 B PCl3 C SO3 D BF3 average oxidation number of S 3 number of S atoms, n 0 A B C D

4 9647/01/CJC JC2 Preliminary Exam 2016 Answer: B 3 bp, 1 lp 107° 3 bp, 1 lp And P is less electronegative than N, so electron pairs less attracted to P, hence smallest bond angle, <107° 3 bp, 0 lp 120° 3 bp, 0 lp 120° 5 Which of the following shows the correct bonds present in solid CHI3? A Covalent bonds only B Ionic bonds only C Covalent bonds & temporary dipole-induced dipole forces of attraction D Covalent bonds & permanent dipole-permanent dipole forces of attraction Answer: D CHI3 (triiodomethane) is a simple covalent molecule which is polar, hence it contai ns covalent bonds between atoms and permanent dipole-permanent dipole forces of attraction between molecules. 6 Polymerisation is a process in which large number of small molecules (monomers) are joined together to form long–chain molecules. Considering that polymerisation is a spontaneous reaction , what are the correct signs of S and H for the process? S H A – – B – + C + – D + + Answer: A G = negative because it is a spontaneous reaction S = negative because the degree of disorderness decreases as polymerisation form orderly long-chain molecules. Since G = H - TS, with G = negative and (-TS) = positive, H has to be negative

5 9647/01/CJC JC2 Preliminary Exam 2016 [Turn over 7 S4N4 is a thermoch romic solid which changes colour with temperature. S 4N4 has a cage structure as shown in the diagram. Given the following data, what is the average bond energy of dissociation of S-N, in kJ mol-1? ΔHf° (S4N4) = +460 kJ mol-1 ΔHat° (sulfur) = +279 kJ mol-1 ΔHat° (nitrogen) = +497 kJ mol-1 (S-S) bond energy of dissociation in S4N4 = +204 kJ mol-1 A +150 B +280 C +395 D +559 Answer: B 4S(s) + 2N2(g) S4N4(s) 4S(g) + 4N(g) H = -[4(+279) +4(497)] +460 = -2644 kJ mol-1 Total Bond Energy = 2644 kJ mol-1 = 8 BE (S-N) + 2 BE (S-S) 8 BE (S -N) = 2644 – (2)(+204) BE (S-N) = +279.5 = 280 kJ mol-1 8 At 25 C, a dented ping pong ball has an internal volume of 31.0 cm 3 and internal pressure of 110 kPa . It is then placed in a water bath maintained at 60 C, which returned the ball to its original spherical shape with internal volume of 33.5 cm3. Assuming ideal gas behaviour, what is the pressure of air inside the ball at 60 C in its original spherical shape? A 101 kPa B 114 kPa C 122 kPa D 244 kPa H +460 4(+279) 4(+497)

6 9647/01/CJC JC2 Preliminary Exam 2016 Answer: B P1V1 T1 = P2V2 T2 (110)(31.0) (298) = P2(33.5) (333) p2 = 113.7 kPa 9 Gas X decomposes to two other gases, Y and Z, according to the following equation: 2X(g) ⇌ 3Y(g) + Z(g) The graph below shows the decomposition of 1.0 mol of pure gas X in the presence of a catalyst at various temperatures. Which one of the following statements about the above system is correct? A The decomposition of X is endothermic. B The Kp of the system decreases with increasing temperature. C The percentage decomposition of X is 40 % at 300 oC. D The system becomes less disordered when it reaches equilibrium state. Answer: B A – As T increased, amount of X remaining at equilibrium increased. This shows that position of equilibrium shifts left, hence backward reaction is endothermic. Forward reaction is thus exothermic. ⇒ incorrect B – Position of equilibrium shifts left with increasing T to favour endotherm ic reaction (since forward reaction is exothermic). Thus Kp will decrease with increasing T. ⇒ correct C – At 300 oC, amount of X remaining is 0.4 mol. Hence, percentage decomposition of X is 60 % instead of 40%. ⇒ incorrect D – There is an increase in the number of moles of gaseous particles resulting in an increase in degree of randomness. ⇒ incorrect 1.0 0.8 moles of X remaining time 300 °C 0.6 0.4 0.2 0.0 250 °C

7 9647/01/CJC JC2 Preliminary Exam 2016 [Turn over 10 Malonic acid, HO2CCH2CO2H, is a weak dibasic acid. Malonic acid is titrated against 20 .0 cm 3 of sodium hydroxide of the same concentration, the following pH-volume curve is obtained. At which point on the titration curve is the mixture most able to resist pH change upon addition of