2023 CJC H2 Chem Prelims P3 Examiners comments

9729/03/CJC JC2 Preliminary Examination 2023 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/03 Paper 3 Free Response 12 September 2023 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use an 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. 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 the questions. Section B Answer one question. The use of an approved scientific calculat or is expected, where appropriate. A Data Booklet is provided. At the end of 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 28 printed pages For Examiner’s Use Section A Q1 /23 Q2 /17 Q3 /20 Section B Q4 /20 OR Q5 /20 TOTAL 80 Catholic Junior College JC2 Preliminary Examinations Higher 2 Mark Scheme

2 9729/03/CJC JC2 Preliminary Examination 2023 Section A Answer all the questions in this section. 1 The element lithium and its compounds have been pivotal in the area of development for clean automotive power, based on the use of lithium-ion batteries in electric vehicles and the potential use of lithium hydride and lithium aluminium hydride in hydrogen fuel cell vehicles. (a) Lithium hydride can be prepared by passing hydrogen gas over heated lithium. The following data will be useful in this question. enthalpy change of formation of LiH(s) = ‒90.5 kJ mol‒1 enthalpy change of atomisation of Li(s) = +159.5 kJ mol‒1 electron affinity of hydrogen atoms = ‒73.0 kJ mol‒1 enthalpy change of formation of AlCl3(s) = ‒704 kJ mol‒1 enthalpy change of formation of LiCl(s) = ‒408.5 kJ mol‒1 (i) Calculate the lattice energy of LiH(s) using relevant data from the above list, together with relevant data from the Data Booklet. [2] Li(s) + ½ H2(g) ‒ 90.5 LiH(s) +159.5 ½ (+436) Li(g) H(g) LE = ? +519 ‒73 Li+(g) + H−(g) Using Hess’ Law, ‒ 90.5 = +159.5 + ½ (+436) + 519 + (‒73.0) + LE LE = ‒914 kJ mol−1 The relevant data from the Data Booklet include the 1st IE of Li(g) (+519 kJ mol ‒1) and the BE of H2(g) (+436 kJ mol‒1). This question was not well attempted despite it being a fairly straightforward Born Haber cycle for ionic compounds. Common errors include: - Not starting with ½ H2 (g) for hydrogen element. - Forgetting to include the enthalpy change of atomisation of H2(g) or not dividing the BE of H2(g) by 2 to obtain this enthalpy value. - Attempting to use an energy level diagram but getting confused or unsure of the sign of the arrows and hence the sign of the enthalpy values in the calculations.

3 9729/03/CJC JC2 Preliminary Examination 2023 [Turn over (ii) When heated with anhydrous aluminium chloride, lithium hydride forms lithium aluminium hydride, LiAlH4. Lithium chloride is also produced. The enthalpy change for this reaction is ‒276kJ per mole of LiAlH4 formed. Write an equation , with state symbols, for this reaction and use it together with the relevant data from the above list to calculate a value for the enthalpy change of formation of LiAlH4. [3] (b) LiAlH4 and sodium borohydride, NaBH4, are useful reducing agents in organic chemistry. An organic compound W, C3H4O3, undergoes reduction to form X, C3H8O2 and Y, C3H6O3 via LiAlH4 and NaBH4 respectively. All three compounds W, X and Y form a yellow precipitate when reacted with alkaline aqueous iodine, and all three compounds react with sodium metal. Compounds W and Y also react with Na2CO3(aq), but compound X does not. (i) Identify the structures for W, X and Y. [3] Positive iodoform test suggests the group, CH3—C—R or CH3—CH—R ║ │ O OH present in W, X and Y Reacts with Na suggests the alcohols or carboxylic acids functional groups present in W, X and Y. Reacts with Na2CO3(aq) suggests the presence of –CO2H in W and Y only. (W) (Y) (X) 4LiH(s) + AlCl3(s) LiAlH4(s) + 3LiCl(s) 4(‒90.5) ‒704 ‒276 ΔHf = ? 3(‒408.5) ΔHr = ∑ ∆𝑯𝒇(𝒑𝒓𝒐𝒅𝒖𝒄𝒕𝒔) ‒ ∑ ∆𝑯𝒇(𝒓𝒆𝒂𝒄𝒕𝒂𝒏𝒕𝒔) ‒276 = [ΔHf + 3(‒408.5)] ‒ [4(‒90.5) + (‒704)] ΔHf = ‒116.5 kJ mol‒1 This question was also not well attempted. Common errors include: - Forgetting or giving the wrong state symbols for the chemical equation. - Getting the formula for ΔHr = ∑ ∆𝐻𝑓(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) ‒ ∑ ∆𝐻𝑓(𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠) wrong. - Forgetting to include factor 3 for LiCl(s) (or 4 for LiH(s)) in calculation. This question was fairly well attempted. Common errors include: - Wrongly deducing W contains an aldehyde. This would not be possible in this case as W undergoes a positive iodoform reaction. As a result of the wrong structure of W, the structures for X and Y were subsequently incorrect.

4 9729/03/CJC JC2 Preliminary Examination 2023 (ii) Predict the structures of the products P and Q of the following reactions. [2] (c) Lithium-ion batteries work by the transfer of lithium ions and electrons from the anode to the cathode. At the anode, elemental lithium inserted between graphite layers is oxidised to Li+. The Li+ ions migrate to the cathode via the electrolyte LiBF 4, where they react with cobalt oxide, CoO2, to form lithium cobalt oxide, LiCoO2. This is illustrated in the following diagram in which C-C-C-C-C-C is a simplified representation of a layer of carbon atoms in graphite. (i) State the oxidation states of cobalt in CoO2 and LiCoO2. [1] (ii) Construct an equation for the cathode reaction. [1] Li+ + CoO2 + e‒ → LiCoO2 Oxidation state of Co is +4 in CoO2 and decreases to +3 in LiCoO2 This question was well attempted. - Some candidates were unfamiliar with the reduction of esters to form alcohols. - Some were also unfamiliar with the specificity of the effects of different reducing agents NaBH4 and LiAlH4 on different organic compound functional groups. Most candidates were able to score this mark. Common error involves using ⇌ instead of single arrow → in the equation. Note that only reduction occurs at the cathode. Some candidates missed out accounting for the electron gained in the equation (either not writing e- at all or writing e- on the right side, representing electron loss instead!)

5 9729/03/CJC JC2 Preliminary Examination 2023 [Turn over (iii) Hence, construct the equation for the overall reaction. [1] (iv) The lithium-ion battery is capable of producing an e.m.f. of 3.60V. By using suitable data from the Data Booklet, suggest a value for the Eo for the cathode reaction. [1] (d) Aside from its use in lithium-ion batteries, cobalt is also widely used in alloys in gas turbines and aircraft jet engines as these a