NJC_H2_CHEM_P3 Prelim

NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Tues 13 Sep 2011 2 hours READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. 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. At the end of the examination, fasten all your work securely behind the cover page. This paper consists of 13 printed pages and 1 cover page.

NJC/H2 Chem/03/2011 [Turn over 2 Answer any four questions. 1 Barium carbonate, BaCO 3, also known as witherite, is often used as a rat poison. (a) Draw the dot-and-cross diagram of barium carbonate. [1] (b) Given the following data, construct an energy level diagram to calculate the standard enthalpy change of solution of barium carbonate. H θ lattice energy (BaCO3) / kJmol1 2554 Hθ hydration (Ba2+) / kJmol1 1305 Hθ hydration (CO3 2– ) / kJmol1 1245 [3] (c) (i) Given that the solubility of barium carbonate in water is 0.024 g dm 3, calculate the value of the solubility product of barium carbonate, stating its units. (ii) G θ solution can be calculated in J mol1 using the formula Gθ solution =  2.303 RT lg Ksp. Calculate a value for Gθ solution of BaCO3 in kJ mol–1. [3]

NJC/H2 Chem/03/2011 [Turn over 3 1 (d) (i) Write an equation, with state symbols, for the thermal decomposition of barium carbonate. (ii) When the same amount of MgCO 3 and BaCO 3 was heated as shown in the above setup, the following data was obtained. Time taken for white precipitate to be observed in X MgCO3 3 minutes BaCO3 9 minutes Explain the above observations. [5] 1 (e) When water is added to the white residue from (di), an aqueous solution of barium hydroxide is formed. Barium hydroxide could be used in organic synthesis as a strong base under suitable conditions. One example is the aldol reaction where barium hydroxide can act as a base to generate a nucleophile from ethanal. CH 3CHO + OH CH2CHO + H2O (nucleophile) (i) Explain why a nucleophile can be generated from an aldehyde molecule but not from an alkane molecule as shown below: CH3CH3 + OH CH2CH3 + H2O (ii) In the aldol reaction between 2 molecules of ethanal, the following product is obtained. OH CH3CHO + CH3CHO 