ASRJC 2026 Prelim P2 Ans

ASRJC JC2 PRELIM 2026 9476/02/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2026 JC 2 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 26 / _____ CHEMISTRY 9476/02 Paper 2 Structured Questions SUGGESTED SOLUTIONS 1 Chromium, Cr, and its compounds are widely used in many chemical reactions. (a) Cr exists as four stable isotopes. (i) Describe, in terms of the number of particles, how an atom of 54Cr differs from an atom of 52Cr. [1] 54Cr has (two) more neutrons than 52Cr. [1] (ii) Write an equation to represent the first ionisation energy of chromium. [1] Cr(g) Cr+(g) + e [1] correct equation with state symbol (iii) Suggest how the value for the first ionisation energy of 54Cr compares with the first ionisation energy of 52Cr. Explain your answer in terms of the factors that affect ionisation energy. [3] The first ionisation of 54Cr will be the same as that of 52Cr. [1] • Nuclear charge is the same for both atoms (as the number of protons are the same). • Since the number of electrons are the same for both atoms, number of electron shells is the same • and shielding effect is the same for both atoms. • Electrostatic attractions between the valence electrons and the nucleus are of the same strength • And hence energy required to remove the valence electron from both atoms is the same. [2] (b) (i) Complete the following electronic configuration of chromium. [1] 1s2 2s2 2p6 3s2 3p6 3d5 4s1 [1] (ii) Deduce the total number of unpaired electrons in an atom of chromium [1] 6 unpaired electrons [1]

(c) In an acidic solution, a dynamic equilibrium is established between CrO 42−(aq) and Cr2O72−(aq). 2CrO42−(aq) + 2H+(aq) Cr2O72−(aq) + H2O(l) (i) Explain what is meant by the term dynamic equilibrium. [1] A state of dynamic equilibrium is one where rate of forward reaction is equal to rate of backward reaction for a reversible reaction such that there is no net change in the concentration of the reactants and products in a closed system. [1] (ii) CrO42−(aq) ions are yellow and Cr2O72−(aq) ions are orange. State what is observed when water is added to an orange solution containing an equilibrium mixture of acidified CrO42−(aq) ions and Cr2O72−(aq) ions. Explain your answers. [2] observation: yellow solution [1] explanation: Adding water decreases the concentrations of all aqueous ions. Position of equilibrium will shift left to increase the concentration of aqueous ions. [1] (d) Chromium(IV) fluoride, CrF 4, is a covalent molecule that shows similar chemical properties to SiCl4. (i) Write a relevant equation for the hydrolysis of CrF4 in water. [1] CrF4 + 4H2O CrO2.2H2O + 4HF [1] Or CrF4 + 2H2O CrO2 + 4HF (ii) Explain why CrF4 can hydrolyse in water. [1] CrF4 can undergo hydrolysis because Cr has available/vacant and energetically accessible 3d orbitals that allow H2O to form a dative bond with Cr , bringing H 2O to close proximity for reaction. [1] [Total: 12]

ASRJC JC2 PRELIM 2026 9476/02/H2 [Turn over 2 (a) Fig. 2.1 shows how the entropy, S, of pure iron changes with temperature, T. T1 and T2 represent the melting point and boiling point of iron respectively. Fig. 2.1 (i) Explain what is meant by entropy. [1] Entropy is a measure of the degree of disorder of the system or a measure of the extent to which particles and energies are distributed within the system. [1] (ii) Complete Fig. 2.1 above by drawing suitable graph line(s) to show the effect of increasing temperature on the entropy of iron: • at T2, and • from T2 to T3 [1] [1] Both correct points • At T2 – vertical line is longer than that at T1 • From T2 to T3 – line with positive gradient T3

(iii) With reference to the entropy change observed at T1, explain your answer in (a)(ii) for the entropy change you have expected at T2. [1] Compared to melting at T1, the entropy increase/change at boiling is (much) larger (∆Svap > ∆Sfus). This is because upon boiling at T2, gaseous Fe particles move in an even larger volume of space than in liquid Fe. There are even more ways to distribute the gas particles and their energies, resulting in a greater disorder in the system. [1] Hence, a greater increase in entropy. (b) Iron is extracted from iron(III) oxide (Fe2O3) primarily through smelting in a blast furnace. Reaction I shows the reduction of Fe2O3 by carbon monoxide in the furnace at 450 °C. I Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) ∆Gr = –36.2 kJ mol–1 Table 2.1 shows enthalpy change values of some reactions. Table 2.1 Enthalpy change of formation of Fe2O3(s) ∆Hf = –824 kJ mol–1 Enthalpy change of formation of CO(g) ∆Hf = –110 kJ mol–1 Enthalpy change of combustion of C(s) ∆Hc = –394 kJ mol–1 (i) Using the data given, determine the enthalpy change, ∆Hr, for reaction I. [1] C(s) + O2(g) CO2(g) ∆Hc(C) = ∆Hf(CO2) = –394 kJ mol–1 ∆Hr =  ∆Hf (products) –  ∆Hf (reactants) = [3(–394)] – [(–824) + 3(–110)] = –28.0 kJ mol–1 [1] with correct sign (ii) Hence, calculate the entropy change, ∆Sr, for reaction I. [1] ∆Gr = ∆Hr – T∆Sr –36.2 = (–28.0) – (450 + 273) ∆Sr ∆Sr = +0.0113 kJ mol–1 K-1 = +11.3 J mol–1 K-1 [1] with correct sign and correct units

ASRJC JC2 PRELIM 2026 9476/02/H2 [Turn over (c) Iron can also be obtained from the reduction of iron(II) oxide, FeO, using carbon monoxide. FeO(s) + CO(g) Fe(s) + CO2(g) Given ∆Hro = –11.1 kJ mol–1, and ∆Sro = –15.2 J mol–1 K–1 for this reaction, predict the effect of increasing temperature on the spontaneity for the formation of iron. Explain your answer. [2] ∆Gro = ∆Hro – T∆Sro As temperature increases, –T∆Sro becomes more positive. [1] ∆Gro becomes less negative / more positive and the reaction is less spontaneous. [1] (d) Fe2+ acts as a homogeneous catalyst in the exothermic reaction between I–(aq) and S2O82–(aq). (i) Write equations for the reactions that occur when Fe 2+(aq) is added to a mixture of I–(aq) and S2O82–(aq). [2] Eqn 1: 2Fe2+(aq) + S2O82–(aq) → 2SO42–(aq) + 2Fe3+(aq) [1] Eqn 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2 (aq) [1] (ii) Fig. 2.2 shows the energy profile diagram for the uncatalysed reaction between I–(aq) and S2O82–(aq). On the same axes, complete the energy profile diagram for the catalysed reaction between I–(aq) and S2O82–(aq) using Fe2+ catalyst. Include labels to show all relevant species, the enthalpy change, H, and the activation energy, Ea, for the catalysed reaction. [2] Fig. 2.2

• Correct shape and same H [1] • Correct Ea, and species [1] [Total:11]

ASRJC JC2 PRELIM 2026 9476/02/H2 [Turn over 3 (a) 2-aminobutane, CH3CH(NH2)CH2CH3, is a liquid with Kb = 3.63 × 10⁻4 mol dm–3. (i) Explain why an aqueous solution of 2-aminobutane is alkaline. Include a relevant equation in your answer. [2] CH3CH(NH2)CH2CH3 + H2O ⇌ CH3CH(NH3+)CH2CH3 + OH– [1] 2-aminobutane is a weak base and undergoes hydrolysis (dissociation / ionisation ) in water to produce OH–. Since [OH–] > [H+], pH is greater than 7. [1] Or 2-aminobutane is a weak base and accepts a proton in water to produce OH–. Since [OH–] > [H+], pH is greater than 7. [1] (ii) Calculate the pH of 0.10 mol dm–3 of 2-aminobutane. [1] Kb = [CH3 CH(NH3+)CH2CH3 ][OH-] [CH3CH(NH2)CH2