CJC 2024 Prelim H1 Chem Paper 2 Solutions

CANDIDATE NAME CLASS 2T CHEMISTRY 8873/02 Paper 2 Structured Questions 26 August 2024 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. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected where appropriate. You are reminded of the need for good English and clear presentation in your answers. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part of the question. This document consists of 28 printed pages Section A Q1 10 Q2 9 Q3 10 Q4 21 Q5 10 Section B Q6 20 Q7 20 Paper 2 (67%) 80 Paper 1 (33%) 30 Percentage Grade Catholic Junior College JC2 Preliminary Examinations Higher 1

2 8873/02/CJC JC2 Preliminary Examinations 2024 1 Dioxygen, O2 and ozone, O3 are two allotropes of oxygen found in the atmosphere. The thermochemical equation for the reversible conversion between ozone and dioxygen is shown below. 2O3(g) ⇌ 3O2(g) H = ‒286.0 kJ mol−1 (a) Draw the structure and suggest the shape of ozone, O3. Structure of O3 Shape of ozone: Bent [2] (b) Using Le Chatelier’s Principle, explain why an increase in temperature causes the ozone concentration to increase. ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ... [2] (c) Sketch, on the same axis below, two graphs that show how the rate of forward and reverse reactions vary over time. Label your graphs clearly and indicate t, the time when equilibrium was established. [1] time rate By Le Chatelier’s Principle, an increase in temperature favours the endothermic reaction which is the backward reaction. in this instance. Hence, position of equilibrium shifts to the left resulting in an increase in the concentration of ozone. t Rate of forward reaction, Rf Rate of reverse reaction, Rb

3 8873/02 CJC JC2 Preliminary Examinations 2024 [Turn over (d) The diagram below shows that the concentration of ozone in the atmosphere varies with altitude above sea level. Diagram taken from https://ozonewatch.gsfc.nasa.gov/facts/SH.html Between about 20 to 50 km above sea level, at the stratosphere, the ozone layer consists of a mixture of dioxygen and ozone molecules. Different types of Ultraviolet (UV) radiation from the Sun is absorbed by the ozone layer during the dioxygen-ozone reversible reaction. The table below shows information on the different types of ultraviolet radiation. Type of ultraviolet rays Relative activation energy Absorption by ozone layer UV-A Low No UV-B Moderate Yes UV-C High Yes Table 1.1 (i) Give a reason why the reversible conversion between dioxygen and ozone in the atmosphere is unlikely to reach a state of dynamic equilibrium. ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ... [1] Ozone concentration is regulated by the UV-rays from the Sun, hence the ozone concentration is constantly changing at differing times of the day. As a result the position of equilibrium will be shifting regularly to offset the constant decrease in the concentration of ozone in the atmosphere OR it is not a closed system OR Intensity of ultraviolet radiation is not constant, rate of forward and backward reaction is unequal

4 8873/02/CJC JC2 Preliminary Examinations 2024 (d) (ii) Complete the energy profile diagram of the dioxygen -ozone reversible reaction using the axes below. The sketch should include labels for • The formula, stoichiometric coefficient, and state symbols of both reactants and products; • The enthalpy change of reaction, H; • Based on Table 1.1, i ndicate the type of ultraviolet rays, UV −A, UV−B or UV−C, absorbed to the relative activation energies of both the forward and backward reactions respectively. Each type of UV rays should be used only once or not at all. [3] Reaction progress energy 3O2(g) UV-B H = ‒286.0 UV-C 2O3(g)

5 8873/02 CJC JC2 Preliminary Examinations 2024 [Turn over (e) The Northern Lights or Aurora Borealis occurs near the North Pole about 100 to 300 km above sea level. Absorption of UV rays by an oxygen molecule causes the O=O bond to break and two oxygen atoms are formed as shown in the equation below. O2(g) → 2O(g) When an oxygen atom in its ground state collides with a high energy electron from the Sun, one of its valence electrons (bolded in the energy level diagram below) is excited to a higher energy arrangement as seen in excited state 2. As energy is released, green light is emitted with an electron arrangement in excited state 1 that violates Hund’s rule. When the electron transits to a lower energy level, coloured light is emitted as shown in the energy level diagram below. In the diagram above, draw the electron arrangement in the 2p subshell of an oxygen atom in excited state 1 by changing the arrangement of the bold electron only. [1] [Total: 10]

6 8873/02/CJC JC2 Preliminary Examinations 2024 2 25.0 cm3 of H2O2 solution reacted with 23.40 cm3 of 0.0250 mol dm‒3 KMnO4 in a titration in the presence of excess dilute sulfuric acid according to the following equation: 2MnO4‒(aq) + 5H2O2 (aq) + 6H+(aq) → 2Mn2+ (aq) + 8H2O(l) + 5O2(g) (a) Explain in terms of change in oxidation number why the above reaction is considered a redox reaction. Identify all the relevant species in your explanation. ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ... [2] (b) Calculate the concentration, in mol dm−3, of the H2O2 solution. [2] (c) With reference to structure and bonding, explain why the melting point of H2O2 is higher than O2. ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ................................ ................................ ................................ ................................ ....... ..