SAJC 2022 P2 QP

1 [TURN OVER ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 1 S CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 31 August 2022 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work that you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 24 printed pages (including this cover page). For Examiner’s Use Q1 12 Q2 14 Q3 14 Q4 10 Q5 25 Total 75

2 [TURN OVER 1 Hydrazine, N2H4, is a colourless liquid with an ammonia-like odour. It is an important precursor in the pharmaceuticals industry. (a) Hydrazine exists as a liquid while ammonia exists as a gas at room temperature and pressure. State two reasons to explain this difference in physical state. [2] ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. (b) The Kb values of hydrazine, ethylamine, and phenylamine are shown in Table 1.1. Table 1.1 base Kb / mol dm–3 Hydrazine 1.7 x 10–6 (for Kb1) Ethylamine 4.5 x 10–4 Phenylamine 7.4 x 10–10 (i) Explain what is meant by the term Bronsted-Lowry base. [1] …………………………………………………………………………………………. …………………………………………………………………………………………. (ii) Explain the relative magnitudes of the Kb values in Table 1.1. [2] …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

3 [TURN OVER (iii) The Kb values of diethylamine and triethylamine are shown in Table 1.2. Table 1.2 base Kb / mol dm–3 Diethylamine 6.9 x 10–4 Triethylamine 6.5 x 10–5 Suggest why the Kb value of triethylamine is significantly smaller than the Kb values of ethylamine and diethylamine. [1] …………………………………………………………………………………………. …………………………………………………………………………………………. (c) The Wolff-Kishner reaction is a valuable synthetic method to convert carbonyl compounds into alkanes. This is done by reacting a carbonyl compound with excess hydrazine in the presence of potassium hydroxide. O RR + RR + +N2 H2ON2H4 KOH (aq), heat (i) Suggest a simple chemical test to monitor the completion of the Wolff-Kishner reaction. [2] …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

4 [TURN OVER (ii) Propan-1-ol can be synthesised from propene by the following 3-step route that incorporates the Wolff-Kishner reaction. CH3CH=CH2 CH3CH2CH2OH step 1 step 2 N2H4, KOH (aq) heat Suggest the structures of intermediate products A and B and state the reagents and conditions for each step. Reagents and conditions Step 1: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Step 2: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [4] [Total: 12] A B (C3H5OBr)

5 [TURN OVER 2 Organic matter is known to decay under low oxygen conditions, such as in swamps. The sulfate- reducing bacteria present in the organic matter will reduce the various sulfates into hydrogen sulfide, H 2S. Some of the hydrogen sulfide will react with Fe 2+ present in swamp to produce insoluble FeS, which is responsible for the brown colour of sludge in the swamp. reaction 1 H2S(g) + aq ⇌ 2H+(aq) + S2–(aq) reaction 2 Fe2+(aq) + S2–(aq) ⇌ FeS(s) Hꝋppt (a) (i) In a saturated solution of hydrogen sulfide, [H +]2[S2–] is 1.0 × 10–23 mol3 dm–9. Calculate the maximum concentration of sulfide ions present in the swamp, given that the pH of swamp water is 6.8. [1] (ii) Hence, calculate the minimum concentration of Fe 2+ in the swamp required for the precipitation of FeS. (Ksp of FeS = 4.9 × 10–18 mol2 dm–6) [1]

6 [TURN OVER (iii) Gꝋppt, can be determined by using the following expression , where R is the molar gas constant and T is the temperature measured in K. Gꝋppt = 2.303RT lg Ksp Using the Ksp in (a)(ii), calculate Gꝋppt for the precipitation of FeS. Express your answer in kJ molꟷ1. [2] (iv) Predict how the brown colour intensity of sludge will change when pH decreases. Explain your answer. [2] …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

7 [TURN OVER (b) Using data from Table 2 below, together with relevant data from the Data Booklet, draw an energy cycle and calculate Hꝋppt for reaction 2. Table 2 standard enthalpy change of formation of FeS(s) –102 kJ mol–1 standard enthalpy change of atomisation of Fe(s) +415 kJ mol–1 standard enthalpy change of atomisation of S(s) +279 kJ mol–1 sum of first and second electron affinity of sulfur +337 kJ mol–1 standard enthalpy change of hydration of Fe2+(g) –1970 kJ mol–1 standard enthalpy change of hydration of S2–(g) –1372 kJ mol–1 [4]

8 [TURN OVER (c) (i) Use your answers in (a)(iii) and (b), calculate the Spptꝋ for the formation of FeS precipitate. [1] (ii) Hence, explain the significance of the sign of Spptꝋ in (c)(i). [1] …………………………………………………………………………………………. …………………………………………………………………………………………. (d) Although hydrogen sulfide and water molecules have the same shape, they have slightly different bond angle s. State and explain which species has a larger bond angle. [2] ……………………………………………………………………………………………….…. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. [Total: 14]

9 [TURN OVER 3 Ionisable drugs have acidic, basic or amphoteric properties. An example of an acidic drug is benzylpenicillin G, which is an antibiotic used to treat certain bacterial infection. 1 2 O N N O S O OH H benzylpenicillin G (a) Both nitrogen atoms in benzylpenicillin G are sp2 hybridised. (i) Draw the shape of the hybrid orbitals around N1. [1] (ii) Suggest why sp2 hybridisation at N1 and N2 will make benzylpenicillin G more stable. [1] ………………………………………………………………………………………….. ………………………………………………………………………………………..… (iii) Suggest a reason why CꟷN2 bond is weaker than CꟷN1 bond. [1] ………………………………………………………………………………………….. ………………………………………………………………………………………..…

10 [TURN OVER (b) The solubility of three different ionisable drugs is shown in Fig. 3.1. Fig. 3.1 (i) Explain why the solubility of benzylpenicillin G in aqueous increases from low to high pH. [2] …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. (ii) Classify the following compounds according to the type of ionisable drug (acidic, basic and amphoteric). [1] A) Chlorpromazine: ……………………………………………………………... B) Oxytetracycline: ……………………………………………………………… solubility pH Benzylpenicillin G Chlorpromazine Oxytetracycline