2024 DHS Y6 H1 Prelim Paper 2 Suggested Solutions

© DHS 2024 8873/02 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H1 CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 8873/02 11 September 2024 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. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 10 2 13 3 17 4 20 Section B 5 / 6 20 Total 80 This document consists of 20 printed pages.

2 © DHS 2024 8873/02 Section A Answer all the questions in the spaces provided. 1 (a) Petrol is a mixture of hydrocarbons that includes octane, C8H18. The density of petrol is typically 748.9 kg per litre and comprises 95% octane by mass. (i) Write an equation for the complete combustion of octane. [1] C8H18(l) + 25 2 O2(g) → 8CO2(g) + 9H2O(l) (ii) Calculate the volume of carbon dioxide gas that will be emitted at r.t.p., if a full tank carrying 50 litres of petrol is completely combusted. You should assume that only octane undergoes combustion. [3] Mass of octane combusted = 50 × 0.95 × 748.9 = 35572.75 kg No. of mol of octane combusted = 35572.75 ×1000 114 = 3.12 × 105 mol Since C8H18  8CO2, no of mol of CO2 emitted = 3.12 × 105 × 8 = 2.50 × 106 mol Volume of CO2 emitted at r.t.p. = 2.50 × 106 × 24 = 5.99 × 107 dm3 (iii) Ethanol is sometimes added to the petrol for internal combustion engine. The standard enthalpy change of combustion of ethanol, Hc, can be determined by applying Hess’ Law to the energy cycle in Fig. 1.1. Fig. 1.1 Express the standard enthalpy change of combustion of ethanol, Hc, in terms of x, y and z. [1] Hc = –x + y + z (iv) State the type of enthalpy change that x represents. [1] standard enthalpy change of formation of ethanol (b) Exhaust gases from internal combustion engine are harmful to the environment. These include oxides from carbon and nitrogen. (i) Write an equation to show the removal of carbon monoxide and nitrogen monoxide from the exhaust gases. [1] 2NO(g) + 2CO(g) → 2CO2(g) + N2(g) 7 2ൗ Hc

3 © DHS 2024 8873/02 [Turn over (ii) State one reason why carbon monoxide is harmful. [1] Carbon monoxide is harmful because it can cause suffocation/ death if inhaled in high concentration. (iii) Butane is a fuel that is commonly used in gas canisters that light up stoves. Suggest, with reference to structure and bonding, why butane is used as a fuel in gas canisters but not octane. [2] Butane has a smaller and less polarisable electron cloud resulting in weaker instantaneous dipole –induced dipole interactions between molecules compared to octane. Hence, less energy is required to overcome these weaker intermolecular forces of attraction and butane has a lower boiling point/more volatile than octane. Therefore, butane exists as a gas that can be stored in a canister but not octane. [Total: 10] 2 (a) Describe and explain the variation in ionic radius across Period 3 of the Periodic Table for the elements Na to Cl. [4] Across the period from Na+ to Si4+ and from P3− to Cl−, number of protons and hence nuclear charge increases. However, screening effect is the same as the isoelectronic ions have the same number of electrons. Therefore effective nuclear charge increases. This results in a progressively stronger electrostatic forces of attraction between nucleus and valence electrons/ valence electrons are progressively pulled closer to the nucleus. Thus, the ionic radius decreases as the atomic number increases. Anions have an additional electron shell as compared to the cations. Despite the larger nuclear charge of anions, valence electrons of anions are more shielded and further away from the nucleus than the valence electrons of cations. Thus, ionic radii of anions are larger than cations in the same period. (b) (i) State what is meant by the term electronegativity. [1] Electronegativity refers to the ability of an atom to attract a shared pair of electrons in a covalent bond.

4 © DHS 2024 8873/02 (ii) State how the bonding in the oxides of the elements sodium to sulfur changes across Period 3. Explain this change in terms of the electronegativities of these elements. [2] Bonding in the oxides of the elements changes from ionic to covalent across Period 3. Across the period from Na to S, electronegativity of the elements increases so the electronegativity difference between the element and oxygen in the oxides decreases. (c) Ammonia, NH3, forms ammonium cation, NH4+, when dissolved in water. (i) Draw a ‘dot-and-cross’ diagram to show the bonding in NH4+. [1] (ii) State the shape and bond angle of NH3 and NH4+. Explain your answers using the Valence Shell Electron Pair Repulsion (VSEPR) theory. shape of NH3: trigonal pyramidal bond angle of NH3: 107○ shape of NH4+: tetrahedral bond angle of NH4+: 109.5○ There are different number of bond pairs and lone pairs of electrons around the central N atom in both NH 4+ and NH3, with four bond pair and zero lone pairs of electrons in NH4+, and three bond pairs and one lone pair of electrons in NH3. By VSEPR theory, these electron pairs are arranged in a manner to minimise repulsion with lone pair -bond pair repulsion > bond pair -bond pair repulsion . Hence, the shapes and bond angles are different. [5] [Total: 13] H ● x H ● ● ● N H ● x x H +

5 © DHS 2024 8873/02 [Turn over 3 (a) The structure of compound M, C8H15COOH, is shown below. CH2CH CCH2 C C CH2CH3 CH3 OH OH H M (i) Draw the skeletal formula of M. [1] OH O In the liquid state, M forms a dimer, (C8H15COOH)2. (ii) By representing the structure of M as RCOOH, draw a suitable diagram to illustrate the formation of the dimer. Include lone pairs, dipoles and label the interactions formed. [2] R C O O H RC O O H + + + + hydrogen bonding (b) Dinitrogen tetroxide, N2O4, and nitrogen monoxide, NO, can exist in equilibrium with dinitrogen trioxide, N2O3. N2O4(g) + 2NO(g) ⇌ 2N2O3(g) H = −26 kJ mol−1 (i) Write an expression for the equilibrium constant, Kc, including its units. [2] Kc = 2 23 2 24 [N O ] [N O ][NO] mol−1 dm3

6 © DHS 2024 8873/02 (ii) A mixture containing a 1 :2 ratio of N 2O4 and NO was allowed to reach equilibrium in a 5 dm3 sealed vessel. At equilibrium, the amount of each compound was determined. compound amount / mol N2O4 0.153 NO 0.307 N2O3 0.0511 Calculate the value for Kc. [1] Kc = 2 2 0.0511 5 0.153 0.307 55              Kc = (0.01022)2 (0.0306)(0.0614)2 Kc = 0.905 mol−1 dm3 (iii) State and explain how the position of equi