2019 CJC Prelim H2 Chem P1 ANS

1 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/01 Paper 1 Multiple Choice Tuesday 3 September 2019 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, class and NRIC/FIN number on the Answer Sheet in the spaces provided. There are thirty 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. Catholic Junior College JC 2 Preliminary Examinations Higher 2 WORKED SOLUTIONS

2 9729/01/CJC JC2 Preliminary Examination 2019 1 In a titration experiment, 15 cm3 of 0.10 mol dm−3 phosphorous acid, H3PO3 was found to react with 10 cm3 of 0.30 mol dm−3 sodium hydroxide. Using this information, deduce the formula of the salt formed in this neutralisation process. A NaH2PO3 C Na3PO3 B Na2HPO3 D Na3PO4 Concept: MCS, Acid-base reaction Answer: B Amount of H3PO3 used in experiment = 0.015 x 0.10 = 1.50 x 10-3 mol Amount of NaOH neutralised = 0.010 x 0.30 = 3.00 x 10-3 mol H3PO3 ≡ 2 NaOH 2 mol of NaOH neutralise 2 mol of H+ 1 mol of H+ is left unreacted in 1 mol H3PO3 (since H3PO3 ≡ 3H+) Salt formed contains 1 mol of H+ 2 50 cm3 of a mixture of ethene and propane gas were exploded in excess oxygen. The volume of the residual gases was found to be 200 cm3. On being shaken with potassium hydroxide, the final volume was found to be 90 cm3. What is the composition of the mixture? (All volumes are measured at r.t.p) volume of ethene volume of propane A 10 40 B 20 30 C 30 20 D 40 10 Concept: MCS, calculations involving volume of gases (involving hydrocarbons) Answer: D Let vol of ethene (C2H4) be x. vol of propane (C3H8) = 50 - x Vol of CO2 gas formed in total = 2x + 3(50 - x) = 150 - x Change in vol of residual gases after passing through KOH = vol of CO2 formed = 200 – 90 = 110 cm3 Hence, 150 – x = 110 x = 40 cm3 Vol of ethene = 40 cm3 ; vol of propane = 10 cm3

3 9729/01/CJC JC2 Preliminary Examination 2019 [Turn over 3 Plutonium-239 is a radioactive substance that emits a constant stream of –particles which are similar to the nucleus of 4He. Rubidium-87 is another radioactive substance that emits –particles which can be considered as electrons. A small amount of Plutonium-239 and Rubidium -87 are separately placed in an ionisation chamber to emit a constant stream of radiation and the emitted particles are passed through an electric field. What would be the path of the emitted particles in the electric field? Answer: D Angle of deflection ∝ charge of ion mass Given that alpha particles (–particles) are similar to the nucleus of He (i.e. 4He2+) and beta particles ( –particles) can be considered as electrons ( 0e–), –particles will be deflected towards the negative plate and by a smaller magnitude (due to greater mass no.) than –particles (towards positive plate). Plutonium-239 (–particles) Rubidium-87 (–particles) A 1 2 B 1 3 C 4 1 D 3 1

4 9729/01/CJC JC2 Preliminary Examination 2019 4 Diazene has a molecular formula of N 2H2. Given that the nitrogen and hydrogen show their usual valencies, which of the following correctly describes the shape and bond angle around each central atom? shape bond angle A bent 110⁰ B linear 180⁰ C tetrahedral 109⁰ D trigonal planar 120⁰ Concept: Drawing Lewis structure, identify shape and bond angle Answer: A Around each N atom, there are 2 bonds pairs and 1 lone pair of electrons. Hence, it is bent around each N atom and the bond angle is between 109⁰ and 120⁰. Hence, the most appropriate answer is 110⁰. 5 Which of the following contains delocalised electrons? 1 Graphite 2 Sodium 3 Ethanoate ion 4 Cyclohexene A 1 and 2 only B 2 and 3 only C 1, 2 and 3 only D 3 and 4 only Concept: Structure and bonding, delocalisation of electrons Answer: C 1: Each C atom is bonded to 3 other C atoms and the remaining electron is delocalised along the layers of carbon atoms. 2: Sodium metal has a giant metallic structure, which has strong electrostatic forces of attraction between the metal ion and the sea of delocalised electrons. The valence electron is involved in delocalisation. 3: The negative charge on oxygen can be delocalised across the sp2 hybridised carbon.

5 9729/01/CJC JC2 Preliminary Examination 2019 [Turn over 4: The electrons are localised between the 2 C in the C=C. 6 Which equation corresponds to the enthalpy change stated? A Ba(OH)2(aq) + 2HCl(aq)  BaCl2(aq) + 2H2O(l) Honeutralisation B Na2SO4(s) + aq  2Na+(aq) + SO42-(aq) Hosolution C S8(s) + 8O2(g)  8SO2(g) Hoformation D Br2(l)  2Br(g) Hoatomisation Ans: B From definitions of various enthalpy changes Errors in options given: Honeutralisation – only 1 mol of H2O formed (not 2 mol) Hoformation – only 1 mol of compound formed (not 8 mol) Hoatomisation – breaking of bonds in gaseous phase (not liquid phase) 7 In which of the following processes will S be negative? A Photosynthesis B Electrolysis of water C Boiling of water D Dissociation of solid MgSO4 in water Ans: A A: 6CO2 + 6H2O  C6H12O6 + 6O2 S=-ve B: 2H2O +2e-  H2 +2OH- OR 2H2O  O2 + 4H+ +4e- S=+ve C: phase change from liquid to solid S=+ve D: MgSO4 + aq MgSO4(aq) S=+ve 8 Which of the following diagrams correctly describes the behaviour of a fixed mass of an ideal gas? (P = pressure, ρ = density, V = volume, T = temperature). 1 PV = nRT(K) V = nR P T(K) V  T(K) (  linear line passing through origin) 2 PV = nRT PV T = nR (constant,  horizontal line) constant P T / K PV T P /Pa V

6 9729/01/CJC JC2 Preliminary Examination 2019 3 A 1 only B 1 and 2 only C 2 and 3 only D 1, 2 and 3 PV = nRT(K) P = rM  RT(K) P ρ = r RT(K) M P ρ = o r R(T( C)+273) M P ρ = o rr RT( C) 273R+MM Plotting P/ρ vs T(oC) will give a linear line with a y-intercept of r 273R M 9 A sample of HBr gas was decomposed in a sea led container at temperature T. The decomposition equation is given below. 2HBr(g) H2(g) + Br2(g) H is positive Which of the following statements is false? A Equal amounts of H2 and Br2 are formed at equilibrium. B When a catalyst is added, the rates of the forward and reverse reactions are increased by the same extent. C The Kc will decrease if reaction is carried out at higher temperature. D The equilibrium position is unaffected when the volume of the container is decreased. Concept: Le Chatelier’s Principle, Calculating Kc, mole fraction When the temperature is increased, the position of equilibrium shifts to the right to favour the forward reaction which is heat-absorbing/endothermic. Kc will increase. 2HBr(g) ⇌ H2(g) + Br2(g) P  T / oC

7 9729/01/CJC JC2 Preliminary Examination 2019 [Turn over Since the number of mole of gases on both sides of the equation are the same, there would be no change in equilibrium position when the volume of container is decreased and the pressure is increased. Hence Option D is true. 10 The pH changes when 0.100 mol dm −3 NH3(aq) is added dropwise to 25 .0 cm 3 of 0.100 mol dm−3 HCl as shown below. At which point on the graph does pOH = pKb, where Kb is