2024 SCGS Chemistry Prelim P2 answers

CANDIDATE NAME CLASS 4 REGISTER NUMBER CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Theory 6092/02 20 August 2024 1 hour 45 minutes READ THESE INSTRUCTIONS FIRST Write your class, index number and name on all the work you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid/tape. Section A Answer all questions. Write your answers in the spaces provided. Section B Answer one question. Answer all questions in the spaces provided. The number of marks is given in brackets [ ] at the end of each question or part question. A copy of the Periodic Table is printed on page 20. The use of an approved scientific calculator is expected, where appropriate. This question paper consists of 20 printed pages. SINGAPORE CHINESE GIRLS’ SCHOOL Preliminary Examination Secondary Four For Examiner’s Use Section A 70 Section B 10 Total 80

2 [Turn over] Section A Answer all questions 1 These diagrams show the electron arrangement in the outer shells of five elements, A to E. All elements are from Period 3 of the Periodic Table. Use the letters A to E to answer the following questions. You may use each letter once, more than once or not at all. (a) Which elements are most likely to be non-metals? ………………………………………………………………………………………………………. [1] (b) Which elements can act as reducing agents? ………………………………………………………………………………………………………. [1] (c) Which element has an atomic number of 16? ………………………………………………………………………………………………………. [1] (d) Which element will form three covalent bonds when it forms compounds? ………………………………………………………………………………………………………. [1] (e) Which two elements will form a compound with the formula of the type YZ 2? ………………………………………………………………………………………………………. [1] (f) Which element will form an oxide that reacts with both acids and bases? ………………………………………………………………………………………………………. [1] [Total: 6] A, B, E C, D A E D and B / A and B C

3 [Turn over] 2 Ethylamine, CH3CH2NH2, behaves similarly to ammonia in terms of its chemical properties. The equation below shows what happens when ethylamine is dissolved in water. CH3CH2NH2 + H2O ⇌ CH3CH2NH3+ + OH– (a) According to the Br Ønsted-Lowry theory, an acid is defined as a species that can donate protons (H+), while a base is a species that can accept protons. Based on the Br Ønsted-Lowry theory and the given equation, explain whether ethylamine acts as an acid or a base. ………………………………………………………………………………………………………….. ……………………………………………………………………………………………………….[1] (b) The pH scale is a method to measure the acidity or alkalinity of a substance. Predict the pH of an aqueous solution of ethylamine. …………………………………………………………………………………………………….…[1] (c) Explain, in terms of structure and bonding, why ethylamine has a low boiling point. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ……………………………………………………………………………………. …………………[2] (d) A student conducted an experiment to measure the electrical conductivity of an aqueous solution of sodium hydroxide and an aqueous solution of ethylamine. Predict and explain which solution will be a better electrical conductor. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ……………………………………………………………………………………………………….[2] (e) Ethylamine can react with acids to form a salt. For example, ethylamine reacts with hydrochloric acid according to the equation below: CH 3CH2NH2 + HCl → CH3CH2NH3Cl Deduce the formula of the salt formed when ethylamine reacts with sulfuric acid. ……………………………………………………………………………………………………….[1] [Total: 7] Ethylamine acts as a base as it accepts a proton from water. 8–11 Only a small amount of energy is required to overcome the weak intermolecular forces of attraction [1] in ethylamine’s simple molecular structure. [1] NaOH [1] will be a better electrical conductor as it is a strong base that is fully ionised to produce more mobile ions [1]. (CH3CH2NH3)2SO4

4 [Turn over] 3 Nitrogen oxides in the upper atmosphere cause damage to the ozone layer. Aircraft engines are one source of nitrogen oxides. (a) (i) Explain how nitrogen oxides are formed in the engine of an aircraft. …………………………………………………………………………………………………... …………………………………………………………………………………………………[1] (ii) Give one natural source of nitrogen oxides in the atmosphere. …………………………………………………………………………………………………[1] (b) Nitrogen monoxide, NO, damages the ozone layer by reacting with ozone in a two- step reaction. NO + O 3 → NO2 + O2 step 1 NO2 + O3 → NO + 2O2 step 2 (i) Use oxidation states to identify which element is oxidised in step 1. element ……………………………………. change in oxidation state ……………………………………………….. [2] (ii) Write the equation for the overall reaction between nitrogen monoxide and ozone. ………………………………………………………………………………………………..[1] (iii) Hence, state the role of NO in the reaction. ……………………………………………………………………………………………… [1] (c) Nitrogen oxides are removed from car exhaust emissions with the aid of the platinum catalyst within catalytic converters. In a converter, nitrogen monoxide reacts with carbon monoxide. (i) Briefly explain why carbon monoxide is harmful to humans. ...………………………………………………………………………………………………… ...………………………………………………………………………………………………… …………………………………………………………………………………………………[1] (ii) Explain, in terms of colliding particles, how the presence of the platinum catalyst speeds up the reaction between nitrogen monoxide and carbon monoxide. …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …..…………………………………………………………………………………………… [2] Catalyst [1] At the high operating temperature of an aircraft engine, nitrogen and oxygen from air react to form nitrogen oxides. Lightning activity N [1] 2O3 → 3O2 [1] +2 to +4 [1] CO binds irreversibly / binds more readily to haemoglobin in blood and reduces the ability of blood to transport oxygen in the body. [1] The catalysts on a catalytic converter lower the activation energy of the reaction by providing an alternative reaction pathway [1]. More particles possess energy greater than or equal to activation energy, resulting in a higher frequency of effective collisions. [1]

5 [Turn over] (iii) Cars fitted with catalytic converters still give out environmentally harmful gases. Name one environmentally harmful gas that is emitted in large amounts and describe the problem it causes. …………………………………………………………………………………………………... …………………………………………………………………………………………………[1] [Total: 10] Carbon dioxide is emitted in large quantities, which leads to global warming and causes the melting of polar ice caps and flooding of low-lying areas.

6 [Turn over] 4 Cobalt can be extracted from one of its ores, linnaeite (a cobalt sulfide compound containing traces of other metal compounds), through a 3-stage process. (a) (i) Describe a method to confirm the identity of the main gas in waste gas A. (a) (i) Why is copper metal left after treating with sulfuric acid in Stage 1? …………………………………………………………………………………………………….. …………………………………………………………………………………………………. [1] (ii) Draw a labelled diagram in the box below to show the structure of copper metal. [1] (iii) A sample of cobalt sulfide contain 58% of cobalt and 42% by mass of sulfur. Determine the empirical formula of the cobalt sulfide. element Co S Percentage by mass / % 58 42 Number of moles in 100 g / mol 0.9831 1.313 Mole ratio 3 4 empirical formula …………………… [2] Stage