2025 SAJC H2 Chem Prelim P2_Answers

1 [TURN OVER ST. ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 4 S CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 2 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. 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 21 Q2 7 Q3 16 Q4 12 Q5 19 Total 75

2 [TURN OVER 1 (a) Titanium dioxide, TiO2, is a white solid, which is an amphoteric oxide. In the structure of titanium dioxide, the titanium ion is bonded to six oxide anions. (i) Complete the electronic configuration of a titanium atom. 1s2 …………………………………… [1] 1s2 2s2 2p6 3s2 3p6 3d2 4s2 (ii) Suggest the shape around the titanium ion in titanium dioxide. [1] Octahedral (b) (i) Aluminium oxide is another example of an amphoteric oxide. Write two equations to illustrate the reaction of Al2O3 with an acid and a base of your choice respectively. [2] Al2O3 + 6HCl → 2AlCl3 + 3H2O Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4] (ii) The ionic radius of Al3+ is 0.050 nm and Ti2+ is 0.086 nm. Explain the difference in ionic radii between Al3+ and Ti2+. [2] Ti2+ has a higher nuclear charge than Al3+. Ti2+ also has more filled inner shells / electron shells / (principal) quantum shells and the valence electrons are further away from the nucleus and shielding effect increases. The valence electrons are less strongly attracted to the nucleus. Ti2+ has a larger ionic size than Al3+. (c) Titanium(II) chloride is prepared by the thermal decomposition of TiCl3 at 500°C. The reaction is driven by the loss of volatile TiCl4. 2TiCl3 (s) → TiCl2 (s) + TiCl4 (g) (i) State and explain the sign for ∆So. [1] ∆So is positive or >0 as there is an increase in disorderliness / decrease in orderliness / more ways of arranging when gaseous TiCl4 / more gaseous molecule is formed.

3 [TURN OVER (ii) Deduce the sign of the enthalpy change , ∆H, of the thermal decomposition of TiC l3, given that the decomposition is spontaneous only at high temperature. Explain your answer. [1] ∆G = ∆H − T∆S The reaction is spontaneous only at high temperature, hence ∆G is negative at high temperature. Since ∆S is positive, ∆H must be positive. (iii) Explain wh