NJC_2008 H2 CM Prelim P3

NATIONAL JUNIOR COLLEGE 2008 PRELIMINARY EXAMINATIONS C H E M I S T R Y 9 7 4 6 / 0 3 Higher 2 2 1 A u g u s t 2 0 0 8 Paper 3 Free Response 2 hours READ THESE INSTRUCTIONS FIRST Write your Name, Registration Number and Chemistry group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use correction fluid. Answer each question on a fresh sheet of paper. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. This question paper consists of 11 printed pages (including this page)

Answer any four questions. 1 (a) (i) What do you understand by the term enthalpy change of atomisation of magnesium? (ii) Use the following data together with relevant data from the data booklet, construct an energy level diagram to calculate the enthalpy change of atomisation of magnesium. ΔH / kJ mol −1 Lattice energy of magnesium chloride −2500 1st electron affinity for chlorine −349 Standard enthalpy change of formation of magnesium chloride −652 (iii) Suggest, with reason, how the magnitude of the enthalpy change of atomization of magnesium might compare with that of chromium. [7] (b) (i) Explain why a solution of chromium(III) chloride is green while aqueous magnesium chloride is colourless. [4] (ii) When aqueous sodium carbonate is added to aqueous chromium(III) chloride, effervescence was observed. Explain the observation with equations where necessary. [2] (c) Grignard reagent, RMgCl, is useful in organic synthesis as it enables the combination of two organic molecules into a bigger one by forming a carbon- carbon bond. It is prepared by the reaction of magnesium with a halogenoalkane. One of the most important uses of the Grignard reagent is its reaction with carbonyl compounds to form alcohols as shown below. RM g C l +C O R' H C R' R H OH + Mg(OH)ClH2O (i) Given the carbon-magnesium bond in Grignard reagent is highly polar, state the type of reaction between R –MgCl and the aldehyde. [1]

(ii) R-MgCl reacts with propanal to give compound A. Compound A gives a yellow precipitate with I2 in aqueous NaOH. When compound A reacts with excess concentrated H 2SO4 at 170 oC, compounds B and C are formed. Compound B upon reaction with hot acidified potassium manganate(VII), produces compound D a