SAJC Prelim P2 Qns

Turn Over NAME Class   ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 2 Structured Questions 11 September 2017 2 hours Additional Materials: Data Booklet READ THESE INSTRUCTIONS: 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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. For Examiner’s use: Question 1 2 3 4 5 6 Total Marks 17 5 10 16 6 21 75 This document consists of 19 printed pages (including this page).

2   Answer all the questions 1 Sulfur is a common element on Earth t hat forms many im portant chemical compounds. One of these compounds is sodium thiosulfate, Na2S2O3, an ionic compound used to treat several medical conditions, such as cyanide poisoning and fungal growths. (a) (i) Draw a dot-and-cross diagram for sodium thiosulfate, Na 2S2O3. State the shape and bond angle in the thiosulfate ion. [2] (ii) Below are the melting points of sodium thiosulfate and sulfur. Compound Melting point / °C Sodium thiosulfate 49 Sulfur, S8 115 Explain why sulfur has a higher melting point than sodium thiosulfate. …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... ………………………………………………………………………………….. [3]

3   Turn Over   (b) Another important sulfur co mpound is sulfuric acid, H 2SO4. Before the Contact Process was discovered, concentrated sulfuric acid for industrial purposes was produced by the following method. The mineral pyrite, FeS 2, was first heated in air and oxidised to solid Fe2(SO4)3 and sulfur dioxide gas. Fe2(SO4)3 decomposes at 480 °C to form iron( III) oxide and sulfur trioxide gas. The sulfur trioxide gas could be mixed with any volume of water to produce sulfuric acid of the desired concentration. However, this process was expensive and not efficient. (i) Write a balanced equation, with state symbols fo r the reaction between pyrite, FeS2, and oxygen to form Fe2(SO4)3. ………………………………………………………………………………….. [1] (ii) With the aid of an equation, define the term standard enthalpy change of formation of gaseous sulfur trioxide, SO3. …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... ………………………………………………………………………………….. [2]

4   (iii) Given the following information, determine the enthalpy change of formation of gaseous sulfur trioxide SO3. Fe 2(SO4)3(s)  Fe2O3(s) + 3SO3(g) ∆Hrxn = +95 kJ mol–1 Substance ∆Hf / kJ mol–1 Fe 2(SO4)3(s) –2107 Fe 2O3(s) –824 [3] (iv) Use the appropriate bond energies given in the Data Booklet and the data below to calculate another value fo r the standard enthalpy change of formation of gaseous sulfur trioxide SO3. ⅛S8(s) + O2(g)  SO2(g) ∆Hf = –297 kJ mol–1 [3]

5   Turn Over   (v) Suggest a reason for the discrepancy between the values in (b)(iii) and (b)(iv). …………………………………………………………………………………...... ………………………………………………………………………………….. [1] (c) The value of pV is plotted against p for 1 mol of oxygen O 2, where p is the pressure and V is the volume of the gas at 300 K. (i) On the diagram above, draw and label the graph of pV against p for SO3 at 300 K. [1] (ii) Explain the difference between the graph of SO3 and the graph of O2. …………………………………………………………………………………...... ………………………………………………………………………………….. [1] [Total: 17]    pV O2 0 p Ideal gas

6   2 Transition elements show typical properties that distinguish them from s-block elements, such as calcium. These include variable oxidation states in their compounds, and the formation of coloured complex ions. (a) An ion of manganese has 3 electrons in it s 3d subshell. Suggest the oxidation state of this manganese ion. ………...……………………………………………………………………………….. [1] (b) The following table gives data about some physical properties of the elements calcium, chromium, and manganese. Property Ca Cr Mn Atomic radius (metallic) / nm 0.197 0.129 0.132 Ionic radius (2+) / nm 0.099 0.073 0.083 Melting point / K 1112 1907 1246 Density / g cm–3 1.54 7.19 7.43 Electrical conductivity / x 106 S cm–1 0.298 0.0774 0.00695 (i) Explain why the atomic radii of chromium and manganese are similar to each other. …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... ………………………………………………………………………………….. [2]

7   Turn Over   (ii) Explain why the density of manganese is significantly greater than that of calcium, using relevant data from the table and the Data Booklet . (No calculations are required.) …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... ………………………………………………………………………………….. [2] [Total: 5] 3 A student investigated the thermal decomposition of two carbonates, calcium carbonate and barium carbonate in the following way to determine its exact composition. He used the same amount, in moles, of each carbonate. MCO 3 (s)  MO(s) + CO2 (g) M is Ca or Ba He heated the carbonates separately for 5 mi nutes. The solid obtained after heating was then shaken with distilled water to form an alkaline solution. A portion of the solid remained insoluble and was filtered. A sample of the alkaline filtrate was titrated with hydrochloric acid. No effervescence was observed during the titration. (a) (i) Suggest which carbonate is less likely to decompose completely. Explain your answer. …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... ………………………………………………………………………………….. [2]

8   (ii) It was observed that the volume of acid used in the titration was smaller for the carbonate which did not decompose completely. Explain how the incomplete decomposition can result in a lower volume of acid used in the titration. …………………………………………………………………………………...... …………………………………………………………………………………...... …………………………………………………………………………………...... ………………………………………………………………………………….. [2] (b) Explain the difference between the magnit ude of the lattice energies of calcium carbonate and calcium oxide. …………………………………………………………………………………................ …………………………………………………………………………………................ …………………………………………………………………………………................ ………………………………………………………………………………………….. [2] (c) Given that the decomposition of calcium carbonate is an endothermic reaction. ∆H = +178 kJ mol –1 and ∆S = +159 J K–1 mol–1 Calculate the minimum temperature at which this reaction becomes spontaneous. [2]

9   Turn Over   (d) Beryllium oxide can react with sodium hydroxide. BeO + 2NaOH  Na2BeO2 + H2O Explain this behaviour despite Be being a Group 2 element. …………………………………………………………………………………................ …………………………………………………………………………………................ …………………………………………………………………………………................ ………………………………………………………………………………………….. [2] [Total: 10]

10   4 Gaseous phosphorus(V) chloride dissociates according to the following equation. PCl5 (g) PCl3 (g) + Cl2 (g) Different amounts of the three gases were placed in a closed container and allowed to come to equilibrium at 200 °C. The experiment was repeated at 425 °C. The equilibr