2025 SAJC H2 Chem Prelim P3 Questions

1 [TURN OVER ST. ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 4 S CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 15 September 2025 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. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. 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 35 printed pages (including this cover page). For Examiner’s Use Q1 22 Q2 18 Q3 20 Q4 / Q5 20 Total 80

2 [TURN OVER Section A Answer all the questions in this section. 1 (a) Calcium carbide, CaC₂, is an ionic compound primarily used in industry to generate acetylene gas, C2H2, which has numerous applications in welding, cutting and chemical synthesis. reaction 1 CaC2(s) + 2H2O(l) → Ca(OH)2(s) + C2H2(g) (i) The carbon atoms in CaC2 and C2H2 have sp hybridisation and hence they have similar bonding between the two carbon atoms. Explain what is meant by sp hybridisation with reference to the carbon atoms in C2H2. [2] (ii) Suggest how the electrons are arranged in the second shell of sp carbon atom so that the bonding in the two carbon atoms in C2H2 can occur. [2] (iii) Explain, in terms of structure and bonding, why CaC 2 is a solid at room temperature and pressure while C2H2 is a gas. [2] ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… …………………………………………………………………………………

3 [TURN OVER ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… (b) Calcium carbide reacts vigorously and explosively with water. Using the data in Table 1.1, calculate the standard enthalpy change of reaction for reaction 1. Table 1.1 compound Hf⦵ / kJ mol–1 [2] CaC2(s) − 59.0 H2O(l) −285.8 Ca(OH)2(s) − 985.2 C2H2(g) +226.6 …………………………………………………………………………...………...… …………………………………………………………………………...………...… …………………………………………………………………………...………...… …………………………………………………………………………...………...… …………………………………………………………………………...………...… …………………………………………………………………………...………...… …………………………………………………………………………...………...… …………………………………………………………………………...………...…

4 [TURN OVER (c) Caution is required when handling calcium carbide to minimise its exposure to water. Flammable C₂H₂ gas is produced upon reaction of calcium carbide with water and poses explosion risks. (i) Define the term standard enthalpy change of combustion, Hc⦵, of C2H2. [1] (ii) Use bond energy values from the Data Booklet to calculate the standard enthalpy change of combustion of C2H2. [2] (iii) The actual standard enthalpy change of combustion of C2H2 is −1300 kJ mol−1. Other than the use of average bond energies, suggest one other reason for the difference between the actual value and the value calculated in (c)(ii). [1] (iv) Suggest an appropriate storage method for calcium carbide to minimise explosion risks. [1] ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… …………………………………………………………………………………

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6 [TURN OVER (d) C₂H₂ is a weak Brønsted-Lowry acid and reacts with sodium amide, NaNH2, to form an equilibrium mixture containing two acid-base pairs. equilibrium 2 C2H2 + NaNH2 ⇌ Na(C2H) + NH3 Kc = 1 x 1013 (i) With reference to equilibrium 2, explain the terms Brønsted-Lowry acid and conjugate acid-base pair. [2] (ii) Identify another acid in the mixture in equilibrium 2 and suggest whether the acid is a stronger or weaker acid than C2H2. Explain your answer. [2] ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… …………………………………………………………………………………

7 [TURN OVER (e) C₂H₂ can undergo polymerisation in the presence of suitable catalysts to form polyacetylene, an organic polymer with alternating single and double bonds . It is used in organic semiconductors and its reaction with iodine produces highly electrical conducting material. n HC≡CH → –[CH=CH]n– –[CH=CH]n– is known as a repeating unit in polyacetylene. (i) Two isomers can be obtained when C2H2 undergoes polymerisation. Draw the structures of the two isomers, showin