2025 SAJC H2 Chem Prelim P3 Answers

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 XX 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] The mixing/combining of one (2)s and one (2)p orbital of C to form (two) sp hybrid orbitals, which are arranged in a linear manner. (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] sp orbitals p orbital There are two sp hybrid orbitals and two p orbitals with 1 electron each . C-C σ bond is formed from (head on) sp-sp overlap, when the sp hybrid orbital overlaps head-on with the sp hybrid orbital of C that also contains 1 electron. There is (unhybridised) p orbital with 1 electron. C-C π bond is formed when the p orbital of C overlaps side-on with the p orbtial of C that also contains 1 electron. (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] CaC2 has a giant ionic lattice structure with strong electrostatic attractions/ionic bond between oppositely charged ions. C2H2 has a simple molecular structure with weak intermolecular instantaneous dipole - induced dipole attractions are easily broken. More energy required to break the stronger ionic bonds, hence CaC2 has higher melting point than C2H2 and is a solid.

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 CaC2(s) + 2H2O(l) → Ca(OH)2(s) + C2H2(g) Hr⦵ = Hf⦵ (products) – Hf⦵ (reactants) = +226.6 – 985.2 – [–59.0 – (2  285.8)] = –128 kJ mol-1 (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] The standard enthalpy change of combustion, Hco, is the enthalpy change/energy change/released when one mole of gaseous C2H2 is completely burnt in excess oxygen at standard conditions of 298 K and 1 bar. (ii) Use bond energy values from the Data Booklet to calculate the standard enthalpy change of combustion of C2H2. [2] H–CC–H (g) + 5 2 O=O (g) → 2 O=C=O (g) + H–O–H (l) Bonds broken Bonds formed 2 BE(C–H) 4 BE(C=O) BE (CC) 2 BE(O–H) 5 2 BE(O=O) Hc⦵ = [2 BE(C–H) + BE (CC) + 5 2 BE(O=O)] − [4 BE(C=O) + 2 BE(O–H)]

4 [TURN OVER Hc⦵ = [2  (410) + (840) + 5 2  (496)] − [4  (805) + 2  (460)] = −1240 kJ mol−1 (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] Bond energy values are for gaseous compounds, whereas water is a liquid , hence the difference between the standard enthalpy changes of combustion. OR The value calculated in (c)(ii) did not take into account the standard enthalpy change of vapourisation/condensation of water. (iv) Suggest an appropriate storage method for calcium carbide to minimise explosion risks. [1] *Condition must be dry, no water, no flame, no oxidiser Use airtight, waterproof containers or (Prevents contact with atmospheric humidity or accidental water exposure) Store in a cool, dry, well -ventilated area away from water sources (e.g., sprinklers, pipes) or No open flames/ignition sources in storage areas (acetylene is highly explosive) or Store away from oxidisers (nitrates, chlorates, peroxides etc) and water or For bulk storage, use nitrogen (N₂) or argon purging to displace oxygen and prevent acetylene accumulation / store in inert gas Or Store in anhydrous condition (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] C2H2 is a Brønsted-Lowry acid as it donates proton to NH2−.

5 [TURN OVER In the acid-base reaction, a H+ is transferred from C2H2 to a NH2−. C2H2 and C2H– is a conjugate acid-base pair, which differs by one H+. Or NH2− and NH3 (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] NH3 It is a weaker acid that C2H2 as the large K (>1) value implies that the position of equilibrium lies more on the right, favouring the donation of H+ from C2H2. (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, showing at least two repeating units, and state their isomeric relationship. [2] n trans n cis Cis-trans isomerism Polyacetylene (–[CH=CH]ₙ–) becomes highly conductive when doped with iodine. The reaction removes an electron from a double bond, creating a radical cation called polaron. –[CH=CH]n– + 3 2I2 → –[CH=CH]n•+ + I3− polaron (ii) State the role of iodine in the reaction. Explain your answer. [2]

6 [TURN OVER I2 serves as the oxidising agent as it gains electron from polyacetylene. Or the oxidation number of iodine changes from 0 to -1 or -1/3, implying it is reduced. (iii) Suggest why the doping process increases the electrical conductivity of polyacetylene. [1] Polarons formed can act as mo