2023 DHS H2 Chem Prelim Paper 3 Suggested Solutions

© DHS 2023 9729/03 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 3 Free Response Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 21 September 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class at the top of this page. 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 questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 25 printed pages and 1 blank page.

2 © DHS 2023 9729/03 Section A Answer all the questions in this section. 1 (a) Group 2 salts, such as carbonates, decompose when heated. Table 1.1 shows information of some Group 2 salts that undergo thermal decomposition. Table 1.1 Group 2 salt thermal decomposition temperature / °C MgCO3 350 CaCO3 825 (i) Explain the difference in thermal decomposition temperature of MgCO3 and CaCO3. [2] The Ca 2+ cation has a lower charge density and hence lower polarising power compared to the Mg2+ cation. The electron cloud of CO 3 2– anion is distorted/polarised by Ca 2+ to a lesser extent compared to Mg 2+. Hence the covalent bonds in CO 3 2– are weakened to a lesser extent by Ca 2+ and higher energy is required for the thermal decomposition of CaCO3. (ii) Hence predict the thermal decomposition temperature for SrCO3. [1] Any temperature higher than 825°C (iii) Write an equation for the thermal decomposition of CaCO 3. Include state symbols in your answer. [1] CaCO3 (s) → CaO (s) + CO2 (g) (iv) State and explain the sign of S for the thermal decomposition of CaCO 3. Hence explain how spontaneity of the reaction changes as temperature increases. [2] S > 0 / is positive since there is an increase in number of gaseous particles from 0 to 1. G = H – TS. Hence G becomes more negative as temperature increases and spontaneity increases. (v) Outline a suitable experiment to verify that the thermal decomposition of a sample of Mg(OH)2 is complete. No details regarding use of specific glassware are required. [2] 1. Transfer the sample to a crucible. 2. Weigh the sample using an electronic mass balance. 3. Heat the sample for a few minutes. 4. Remove the sample from the flame and let it cool. 5. Re-weigh the sample.

3 © DHS 2023 9729/03 [Turn over 6. Repeat until there is negligible / no change in mass of the sample upon further heat-cool-weigh cycles. (b) Group 2 carbonates are sparingly soluble in water. Table 1.2 shows the solubility product, Ksp, of two Group 2 carbonates. Table 1.2 Group 2 carbonate Ksp MgCO3 2.61 10–3 CaCO3 5.83 10–5 (i) Write an expression for Ksp of MgCO3, giving its units. [1] mol2 dm–6 (ii) Describe and explain how the solubility of MgCO 3 is affected by the addition of ethanoic acid. [2] MgCO3(s) ⇌ Mg2+(aq) + CO3 2–(aq) Ethanoic acid undergoes neutralisation / acid -base reaction / acid - carbonate reaction with CO3 2–, hence decreases [CO 3 2–]. Position of equilibrium shifts to the right to increase [CO3 2–], hence solubility increases. Excess solid magnesium nitrate and calcium nitrate were added to a solution containing carbonate ions. The mixture was stirred and left to equilibrate at room temperature. (iii) Calculate the [Mg2+]/[Ca2+] ratio in the resulting mixture. [1] Both carbonate salts are saturated in the resulting mixture. (c) Magnesium is also used in the pinacol coupling reaction. A new C–C bond is formed between two carbonyl compounds, as shown in Fig. 1.1. R'R O 2 (i) Mg (ii) dilute acid OH R R' OH R' R Fig. 1.1

4 © DHS 2023 9729/03 (i) When butanone undergoes the pinacol coupling reaction, a mixture of 3 stereoisomers are formed. Draw the structure of the product that does not rotate plane-polarised light, showing the stereochemistry of the molecule clearly. [1] OH OH (ii) Phenaglycodol is a sedative. It can be synthesised from compounds P and Q using the pinacol coupling reaction. Cl OH OH Phenaglycodol Suggest possible structures for P and Q. [2] Cl O O (d) Compound R, C8H8O2, reacts with Fehling’s reagent and aqueous bromine. It is also soluble in NaOH(aq). When R is heated with acidified potassium manganate( VII), C8H8O3, is first formed. Further heating produces 4-hydroxybenzoic acid. For each reaction, state the type of reaction described and explain what the information tells you about the functional groups present in R. Hence suggest a possible structure for R. [5] Observations Type of reaction Deductions R has the molecular formula C8H8O2 - The high C:H ratio indicates that a benzene ring is likely present R reacts with Fehling’s reagent. Oxidation R contains an aliphatic aldehyde R reacts with aqueous bromine Electrophilic addition or Electrophilic substitution R contains a C=C bond / alkene group or R contains a phenol group R is soluble in NaOH(aq) Neutralisation R contains a –COOH or phenol group.

5 © DHS 2023 9729/03 [Turn over Since R only contains two oxygen atoms and must contain an aldehyde, it must also contain a phenol group. When R is heated with acidic potassium manganate(VII), C8H8O3 is first formed. Oxidation Aldehyde is oxidised to a carboxylic acid. Further heating with acidic potassium manganate(VII) produces 4- hydroxybenzoic acid. Side-chain oxidation There are two substituents on the benzene ring with a 1,4 - substitution pattern OH H O [Total: 20] 2 (a) (i) State the two main assumptions of ideal gases. [1] Negligible intermolecular forces of attraction between gas particles. Gas particles have negligible volume compared to volume of container/gas. (ii) Explain why a real gas will deviate more from ideality at a lower temperature. [1] At lower temperature, the average kinetic energy of the gas particles decreases and they have less energy to overcome the intermolecular forces of attraction. Hence intermolecular forces of attraction between particles become more significant. (b) Pure hydrogen iodide, HI, partially dissociates into hydrogen and iodine as shown in the equation below. 2HI(g) ⇌ H2(g) + I2(g) The reaction was carried out at 500 K. At the start of the reaction, 9.0 x 10 –4 mol of HI were introduced into a 2 dm 3 vessel. When equilibrium was established, it was found that 85% of the hydrogen iodide was left. (i) Write an expression for the equilibrium constant, Kc, for the dissociation of HI. [1] Kc = (ii) Determine the equilibrium constant, Kc, for this reaction at 500 K. [2]

6 © DHS 2023 9729/03 2HI(g) ⇌ H2(g) + I2(g) Initial mole/ mol 9.0 x 10–4 0 0 Change in mole / mol –1.35 x 10–4 +6.75 x 10–5 +6.75 x 10–5 Equilibrium mole / mol 7.65 x 10–4 +6.75 x 10–5 +6.75 x 10–5 Equilibrium conc / mol dm–3 3.825 x 10–4 3.375 x 10–