CJC 2026 Prelim P2 Ans

1 [Turn over 9476/02 CJC JC2 Preliminary Examination 2026 CANDIDATE NAME CLASS 2T INDEX NUMBER CHEMISTRY 9476/02 Paper 2 Structured Questions 28 August 2026 2 hours You must answer on the Question Paper. You will need: Data Booklet INSTRUCTIONS • Answer all questions. • Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs. • Write your name and index number in the boxes at the top of the page. • Write your answer to each question in the space provided. • Do not use an erasable pen. Do not use correction fluid or tape. • You may use an approved calculator. INFORMATION • The total mark for this paper is 75. • The number of marks for each question paper or part question is shown in brackets [ ]. This document has 20 printed pages. For Examiner’s Use Paper 1 30 Paper 2 Q1 /16 Q2 / 8 Q3 /10 Q4 /18 Q5 /11 Q6 /12 75 Paper 3 75 Paper 4 50 OVERALL (100%) GRADE Catholic Junior College JC2 Preliminary Examination Higher 2 WORKED SOLUTIONS

2 9476/02/ CJC JC2 Preliminary Examination 2026 [Turn over Answer all questions in the spaces provided. 1 (a) Calcium fluoride, CaF2, is an ionic compound that occurs naturally in the mineral fluorspar. It is the major source of fluorine for the chemical industry including that of making fire retardants. Calcium fluoride can dissolve in water resulting in a decrease in temperature. (i) Name the force of attraction between particles in CaF2(s) and water molecules. …………………………………………………………………………………………..…[1] (ii) Fig. 1.1 shows the ions present in CaF2. Complete Fig. 1.1 to show: • the arrangement of two water molecules around one Ca2+(aq) ion • the arrangement of two water molecules around one F‒(aq) ion. Show clearly all the possible interactions between the ions and water molecules. Fig. 1.1 [1] (iii) Hence, suggest why the temperature decreases when CaF 2(s) dissolves in water. State and use the forces of attraction between the particles in CaF2(s) and between water molecules in your answer. …..……………………………………………………………………………...…………… …..……………………………………………………………………………...…………… ………………………………………………………………………………….…………… …………………………………………………………………………………………….… ……………………………………………………………………………………………[2] The Ca2+ and F– form ion-dipole attractions with the water molecules. The energy released from the formation of ion-dipole attractions is less than the energy required to break the strong ionic bonds between Ca2+ and F– and the intermolecular hydrogen bonding in water, hence heat energy is absorbed from the surrounding. [1, identify ionic bonds and hydrogen bonding] [1, compare energy released is less than energy required] The Ca2+ and F– form ion-dipole attractions with the water molecules [1] [1 correct partial charges; two oxygen attracted to cation and 4 hydrogens attracted to anion]

3 9476/02/ CJC JC2 Preliminary Examination 2026 [Turn over (b) The net effect of the individual processes that occur when CaF 2(s) dissolves in water produces an overall negative entropy change. (i) Explain what is meant by entropy. …………………………………………………………………………………………….… ……………………………………………………………………………………………[1] (ii) The processes that give positive entropy change include ions becom ing more mobile in the solution and an increase in number of particles. Describe one process, related to the arrangement of particles, that has a negative entropy change when CaF2(s) dissolves in water. …………………………………………………………………………………………….… …………………………………………………………………………………………….… ……………………………………………………………………………………………[1] (iii) The standard enthalpy change of solution of CaF2 is +46.0 kJ mol‒1 and the standard entropy change of solution is ‒44.9 J mol‒1 K‒1. Calculate the standard Gibbs free energy change of solution, G ꝋ , at 25 o C and state whether dissolving CaF2(s) in water is spontaneous. ……………………………………………………………………………………………[2] ∆Go = ∆Ho ‒ T∆So = +46.0 ‒ [298 (‒44.9 x 10‒3)] = +59.4 kJ mol‒1 [1 for numerical value and units] Since G ꝋ > 0, reaction is not spontaneous. [1, with positive sign] Entropy of a system measures the degree of disorder. It gives a measure of the extent to which particles and energy are distributed within the chemical system. [1, measure is a keyword as entropy has a numerical value] The water molecules becomes more ordered as these are orientated in a fixed arrangement due to ion-dipole attraction between the ions and water, resulting in a decrease in disorder. [1]

4 9476/02/ CJC JC2 Preliminary Examination 2026 [Turn over (c) Enthalpy change of hydration of ions is not easily measured directly. By using Hess’ Law and the energy cycle, it can be obtained from experimental data. (i) Explain what is meant by enthalpy change of hydration. …………………………………………………………………………………………….… …………………………………………………………………………………………….… …………………………………………………………………………………………….… ……………………………………………………………………………………………[1] (ii) Table 1.1 gives data relevant to this question process Ho / kJ mol‒1 standard lattice energy of CaF2(s) ‒2653 standard enthalpy change of solution of CaF2(s) +46.0 standard enthalpy change of hydration of calcium ions ‒1577 Table 1.1 Use the data from Table 1.1 to calculate the standard enthalpy change of hydration of fluoride ions. [2] (iii) Suggest why the standard enthalpy change of hydration of chloride ions has a lower magnitude than that of fluoride ions. …………………………………………………………………………………………….… …………………………………………………………………………………………….… ……………………………………………………………………………………………[1] The enthalpy change of hydration, ΔHhydo, of ion s is defined as the enthalpy change when one mole of the gaseous ions is dissolved in such a large amount of water that addition of more water produces no further heat change. [1] By Hess’ Law, ∆Hsol = − LE + ∑∆Hhyd [1 for method] 46 = − (−2653) + (−1577) + 2(Hhyd ꝋ (fluoride)) 46 = 1076 + 2(Hhyd ꝋ (fluoride)) Hhyd ꝋ (fluoride) = (46 − 1076) ÷ 2 = − 515 kJ mol−1 [1, CAO] ΔHhyd ∝ | 𝑞− 𝑟− | . The ionic charge is constant (– 1), but the ionic radius of Cl– is larger than F–, hence Cl– has a lower charge density and weaker polarising power. Thus Cl– attract H2O molecules less strongly and the ΔHhyd has a lower magnitude. [1]

5 9476/02/ CJC JC2 Preliminary Examination 2026 [Turn over (d) Table 1. 2 shows the decomposition temperatures of two metal carbonates which are commonly used as flame retardants. Table 1.2 metal carbonate decomposition temperature / oC MgCO3 400 SrCO3 1340 (i) Write an equation for the thermal decomposition of MgCO3. …………………………………………………………………………………………..[1] (ii) Explain why the decomposition temperature of MgCO3 is lower than that of SrCO3. …..……………………………………………………………………………...…………… …..……………………………………………………………………………...…………… …..……………………………………………………………………………...…………… ………………………………………………………………………………….…………… …………………………………………………………………………………………….… ……………………………………………………………………………………………[3] [Total: 16] Mg2+ has a smaller ionic radius than Sr2+ while their ionic charges are the same. Charge density of Mg2+ is higher [1] and hence has higher polarising power than Sr2+. Mg2+ is able to polarise / distort the electron cloud of CO32‒ to larger extent[1], and weaken C‒O bond to larger extent [1] and easier to break. MgCO3 is less thermally stable / easier to decompose and decompose at a lower temperature. MgCO3