2024 Prelims SAJC H2 Chem P3 (QP)

1 [TURN OVER ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 3 S CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 9 September 2024 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work that 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, 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. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 32 printed pages (including this cover page). For Examiner’s Use Q1 8 Q2 21 Q3 20 Q4 11 Q5 or Q6 20 Total 80

2 [TURN OVER Section A Answer all the questions in this section. 1 (a) (i) Define the term relative atomic mass. [1] (ii) Diamond, which is made up of both 12C and 13C atoms, has a relative atomic mass of 12.011. With reference to Table 1.1, calculate the percentage abundance of 13C in diamond. Table 1.1 Relative isotopic mass 12C 12.000 13C 13.003 [1] ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… (b) (i) Define the term enthalpy change of atomisation of C(graphite). [1] ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….………………………

3 [TURN OVER (b) (ii) Some enthalpy change values are shown in Table 1.2. Table 1.2 value / kJ mol–1 enthalpy change of atomisation of C(diamond) +717 enthalpy change of combustion of C(diamond) –395 enthalpy change of combustion of C(graphite) –393 Using the data in Table 1.2, construct a suitable energy cycle to show that the enthalpy change of atomisation of C(graphite) is +719 kJ mol–1. [3] (iii) Even though graphite and diamond are allotropes of carbon, they have different enthalpy change of atomisation. With reference to hybridisation of the carbon atoms, explain this difference. [2] ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… ………………………………………………………………….……………………… [Total: 8]

4 [TURN OVER 2 This question is on the chemistry of main group elements and their compounds. (a) (i) The melting point of beryllium and magnesium are shown in Table 2.1. Table 2.1 melting point / oC beryllium 1287 magnesium 650 Explain, in terms of structure and bonding, the difference in melting point between beryllium and magnesium. [2] (ii) Explain why beryllium is a weaker reducing agent than magnesium in terms of shielding and nuclear charge. [2] (iii) Explain why the radius of beryllium ion is smaller than the radius of beryllium atom. [2] (iv) Unlike other Group 2 carbonates, beryllium carbonate is unstable as it will quickly decompose to form beryllium oxide and carbon dioxide. On Fig. 2.1, draw the mechanism for the decomposition of beryllium carbonate. Show the relevant lone pair and movement of electron pairs by using curly arrows on the carbonate ion. Be2+ O C-O O- Be2+ O2- + CO2 Fig 2.1 [1]

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6 [TURN OVER (b) Beryllium can be extracted from ores. An ore containing beryllium is first heated to a very high temperature to form beryllium fluoride, BeF2, before it is dissolved in water. KOH pellets are then added to 1 mol dm -3 of aqueous BeF2 to precipitate Be(OH)2. The pH of the resultant solution is 12.4. Be(OH)2 is sparingly soluble in water. The Ksp of Be(OH)2 is 2.55 x 10–4 at 25 ⁰C. (i) Write an expression for the Ksp of Be(OH)2, stating its units. [1] (ii) Calculate the percentage of Be2+ that has been precipitated. [2] (iii) HCl(aq) and NaOH(aq) are added to separate saturated solutions containing Be(OH)2, which is a white solid. The observations are recorded in Table 2.2. Table 2.2 procedures observations HCl(aq) is added to Be(OH)2. White solid dissolve s to form colourless solution. NaOH(aq) is added to Be(OH)2. White solid dissolve s to form colourless solution. Explain how the solubility of Be(OH) 2 is affected by the addition of HC l and NaOH. [2] (iv) In the gaseous state, BeF2 has the same number of σ and π bonds as CO2. Draw the structure of BeF2 in the gaseous state. [1]

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8 [TURN OVER (c) 25.0 cm3 of 0.100 mol dm–3 of benzoic acid, C6H5COOH, was titrated with 0.100 mol dm –3 of KOH. The change in pH was monitored and shown in the graph below. (i) Show that Ka of benzoic acid is 6.61 x 10–5 mol dm–3. [1] (ii) Calculate the pH of the solution when 12.5 cm3 of KOH was added. [1] (iii) Calculate the pH at equivalence point. [3] ………………………………………………………………….……………………… ………………………………………………………………….……………………… …………