2024 TMJC H2 Chem Prelim P2 (QP)

CANDIDATE NAME CIVICS GROUP H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 12 September 2024 2 hours Candidates answer on the Question Paper. Additional materials: Data Booklet This document consists of 22 printed pages. READ THESE INSTRUCTIONS FIRST Write your name and civics group in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams and graphs. Do not use 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. The number of marks is given in brackets [ ] at the end of each question or part question. TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION For Examiner’s Use Percentage Paper 1 / 30 / 15 Paper 2 1 / 13 2 / 13 3 / 17 4 / 16 5 / 16 Paper 2 Total / 75 / 30 Paper 3 / 80 / 35 Paper 4 / 55 / 20 Grand Total / 100

2 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry 1 Some main group elements are similar to transition elements as they are capable of showing different oxidation states. (a) Fig. 1.1 shows a sketch of the nine successive ionisation energies of element A, a main group element. Fig. 1.1 (i) Write the equation for the third ionisation energy of element A. [1] (ii) Explain the general trend in successive ionisation energy shown in the graph of Fig. 1.1. [2] (iii) Suggest, with reasoning, the group number of element A and state its identity, given that it belongs to Period 4. [2] 1 2 3 4 5 6 7 8 9 log (IE) number of electrons removed

3 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry [Turn Over Transition elements form complexes that are responsible for the colour of many gemstones such sapphires and rubies. (b) Define the term transition element. [1] (c) In an isolated gas phase transition metal ion, the d-orbitals have the same energy. However, in the octahedral complex ion, [Cr(H2O)6]3+, the d subshell is spilt into two energy levels. (i) Complete the electronic configuration of Cr and Cr3+. Cr : 1s2 Cr3+ : 1s2 [1] (ii) Explain why the splitting of d-orbitals occurs in the presence of ligands. [1] (iii) Draw and name the shape of one of the d orbitals in the lower energy level and one of the d orbitals in the upper energy level for the octahedral complex ion. Indicate the axes labels clearly. Name of lower energy d orbital: __________________ Name of upper energy d orbital: __________________ [2]

4 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (iv) Cr3+ ions forms a complex that gives rise to the red colour of rubies. Explain how the colour arises. [2] (d) Chromium(III) picolinate, Cr(C 5H4N(COO))3, another bright red complex , is used to treat Type 2 diabetes. Picolinate is bidentate and forms an octahedral complex ion with chromium(III). The nitrogen atom in the benzene ring of picolinate has a lone pair of electrons available for coordination. picolinate ligand Using the information provided, draw the octahedral structure of chromium( III) picolinate in the space below showing the coordinate bonds clearly. [1] [Total: 13]

5 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry [Turn Over 2(a) Magnesium oxide reacts reversibly with chlorine according to the following equation. equation 2.1 2MgO(s) + 2Cl2(g) 2MgCl2(s) + O2(g) Under certain conditions, a dynamic equilibrium is established. (i) Explain what is meant by the term dynamic equilibrium. [1] (ii) At 1.00  105 Pa and 500 K, 70% of the initial amount of Cl2(g) has reacted with MgO. Calculate the equilibrium constant, Kp for the reaction. [2]

6 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (iii) Predict and explain the effect of decreasing the volume of the system in equation 2.1 on the • composition of reactants and products [2] • equilibrium constant, Kp. [1] (b) Hard water is water that contains high concentrations of calcium and magnesium ions. Temporary hard water consists of dissolved hydrogencarbonate (HCO 3 – ) ions. Heating HCO3 – causes it to decompose according to equation 2 .2. The carbonate then forms an insoluble metal carbonate with one of the metal ions according to equation 2.3, which gives the characteristic white ‘scales’ found in kettles used to boil such water. equation 2.2 2HCO3 – (aq) CO3 2– (aq) + CO2(g) + H2O(l) equation 2.3 M 2+ (aq) + CO3 2– (aq) MCO3(s) where M = Ca or Mg (i) With reference to the equations given above, explain why the hardness of temporary hard water can be reduced by boiling. [1]

7 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry [Turn Over The numerical value of the solubility products of some of the compounds involved in the processes in part (b) are shown in Table 2.1. Table 2.1 Compound Solubility product, Ksp, at 25 C CaCO3 2.8  10–9 MgCO3 3.5  10–8 Ca(OH)2 5.5  10–6 Mg(OH)2 1.8  10–11 (ii) Deduce whether calcium or magnesium ion is likely to be precipitated out first in equation 2.3. [1] Hard water can be softened by raising the pH of the water to precipitate out some metal ions. A sample of hard water contained equal concentrations of calcium and magnesium ions at 8.00  10–3 mol dm–3 each. Its pH was raised to 10.0 at 25 C. (iii) By considering the relevant ionic product, deduce which metal ion will be precipitated out. [2]

8 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (c) Phosphates from fertilizers used in agriculture could enter the water supply and combine with calcium ions present to form the sparingly soluble salt calcium phosphate, Ca3(PO4)2. The value of Ksp for calcium phosphate is 1.0  10–26. (i) Write the Ksp expression and calculate the solubility, in mol dm –3, of calcium phosphate. [2] (ii) Suggest why calcium phosphate is expected to be less soluble in phosphoric acid than it is in water. [1] [Total: 13]

9 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry [Turn Over 3(a) In the presence of a base catalyst, aldehyde can react with alcohol via nucleophilic addition to give hemiacetal. An example involving propanal and methanol is shown below. This reaction occurs via a three-step mechanism. Step 1 The base removes a proton to form a methoxide anion, CH3O−. Step 2 The methoxide anion undergoes nucleophilic addition with the carbonyl group generating an anionic intermediate, in a rate determining step. Step 3 The anionic intermediate abstracts a proton from a CH3OH molecule. Draw the mechanism for steps 2 and 3 of the reaction between propanal and methanol. In your answer, you should show all relevant charges, dipoles, and lone pairs; and show the movement of electrons using curly arrows. [3]

10 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (b) Butanone undergoes the same nucleophilic addition reaction with methanol. O butanone Observations were made about the reactions as shown in Table 3.1. Table 3.1 Reaction between propanal and methanol Reaction between butanone and methanol Relative rate of reaction faster slower Optical activity of product does not rotate plane polarised light does not rotate plane polarise