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, henc