RJC Chemistry H2 P2 with Ans Prelims

RAFFLES JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 HIGHER 2 CANDIDATE NAME C L A S S I N D E X N U M B E R CHEMISTRY 9746/02 Paper 2 Structured 15 September 2008 1 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided on the top of this page. Write in dark blue or black pen in the space provided. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good Eng lish and clear presentation in your answers. 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 /10 /10 /10 /10 /10 /10 /60 This question booklet consists of 16 printed pages. ©RJC 2008 [Turn over

2 Answer all the questions in the spaces provided. 1 (a) Household bleach contains sodium chlorate( I), NaC lO. When bleach solution is acidified with dilute hydr ochloric acid, chlorine is produced by the following reaction: ClO–(aq) + 2H+(aq) + Cl–(aq) Æ Cl2(aq) + H2O(l) Chlorine oxidises iodide to iodine, and the amount of iodine produced can be estimated using a standard sodium thiosulphate solution. A 25.0 cm 3 sample of a household bleach is diluted to 250 cm 3. To a 25.0 cm 3 portion of this diluted solution, dilute hydrochloric acid is added. This is followed by the addition of excess potassium iodide solution. The resultant mixture is then titrated against 0.200 mol dm –3 sodium thiosulphate solution. The volume required is 18.50 cm 3. What is the concentration of sodium chlorate( I) in the household bleach? [ 3 ] ©RJC 2008 [Turn over

3 (b) The rate of reaction between peroxodisulphate ions, S2O8 2–, and iodide ions, I–, can be increased by the addition of Fe3+(aq). (i) With the aid of the Data Booklet, explain how Fe 3+ catalyses the reaction between S2O8 2– and I–. (ii) With the aid of a suitable diagram , explain why the rate of the reaction can also be increased by heating. [ 6 ] ©RJC 2008 [Turn over

4 (c) Certain gas–phase or liquid–phase r eactions which are catalysed by solid catalysts often ex hibit zero order kinetics with respect to the reactants at high concentrations of reactants. Suggest a reason why this might be so. [1] [Total: 10] ©RJC 2008 [Turn over

©RJC 2008 [Turn over 5 2. The first and second ionisation ener gies (IE), standard enthalpy changes of atomisation and hydr ation for some Group II elements and their ions are shown in the table below. Element 1 st IE / kJ mol–1 2nd IE / kJ mol–1 ΔHO atom / kJ mol–1 ΔHO hyd of M2+(g) / kJ mol–1 Mg +737 +1450 +148 –2003 Ca +590 +1146 +178 –1557 (a) The standard enthalpy change of hydration of Mg2+ is more exothermic than that of Ca2+. Explain why. [ 1 ] (b) (i) Define standard enthalpy change of formation, ΔH O f, of Mg2+(aq). (ii) Making use of the data in the t able above, construct an energy cycle and calculate ΔH O f of Mg2+(aq).

(iii) Group II elements with more exothermic ΔHO f of its aqueous ions also have more negative EO values. Explain why. (iv) Given that the standard enthalpy change of solution, ΔH O soln, of MgCl2 is –150 kJ mol–1, explain why MgCl2 is soluble in water at all temperatures. [ 7 ] (c) On heating, Group II peroxides decompose to their respective oxides and oxygen. Explain why strontium peroxide (SrO 2) decomposes at a lower temperature than barium peroxide (BaO2). [ 2 ] [Total: 10] ©RJC 2008 [Turn over

3 (a) The graph below shows the variation of the first to fourth ionisation energies for the 1st row d−block elements scandium to zinc. 716 762 736 908 1240 1310 1370 1590 1510 1560 1640 1750 1960 1730 2390 2720 2870 2990 3250 2960 3230 3390 3350 3238 7110 4170 4600 4700 5190 5400 5100 5400 5690 5980 632 661 745757648 653 0 1000 2000 3000 4000 5000 6000 7000 8000 20 22 24 26 28 30 4th IE Ionisation Energy / kJ mol−1 3rd IE 2nd IE 1st IE Sc Ti V Cr Mn Fe Co Ni Cu Zn (i) Give an equation that represent s the second ionisation energy of chromium. (ii) Why is the second ionisation energy of chromium and copper higher than that of manganese and zinc respectively? ©RJC 2008 [Turn over

(iii) Explain briefly why the fourth ionisation energy of cobalt is lower than that of iron and nickel. [5] (b) The colours and standard reduction pot ential of two complex ions of iron( III) are shown below: Ion Colour Equation and EO [Fe(H2O)6]3+ Violet [Fe(H2O)6]3+ + e− [Fe(H2O)6]2+, EO = +0.77 V [Fe(CN)6]3− Yellow [Fe(CN)6]3− + e− [Fe(CN)6]4−, EO = +0.36 V (i) Why do transition metal ions form complexes readily? (ii) Explain why standard reduction potential of [Fe(CN) 6]3− is lower than that of [Fe(H2O)6]3+. ©RJC 2008 [Turn over

9 (iii) Outline one simple experiment that could be performed to demonstrate that [Fe(CN) 6]3− is a weaker oxid ising agent than [Fe(H2O)6]3+. (iv) Explain briefly why the colours of the two complexes of iron( III) are different. [5] [Total: 10] ©RJC 2008 [Turn over

10 4 (a) To combat pollution and soaring fuel cos t, luxury sports car maker Ferrari is experimenting with ethanol to help produce cars with lower carbon dioxide emissions and improv e fuel economy. Titled “Drunk with Power”, this projec t using ethanol as fuel also claims to bring about an increase in the horsepower of the automobile too! (i) Write an equation for the complete combustion of ethanol. (ii) Draw a dot–and–cross diagram of ethanol and hence suggest a value for the C–O–H bond angle. [3] (b) Ethanol is often used in the sy nthesis of many organic compounds, such as ethanal, ethanoic acid and sodium methanoate. (i) Write an equation for the synthesis of sodium methanoate from ethanol. (ii) Draw a dot–and–cross di agram of sodium methanoate. [2] ©RJC 2008 [Turn over