ACJC_Chemistry_H1 2008 Prelims P2

Examiner’s Report 2008 1 Examiner’s report H1 Prelims 2008 General Comments for Section A The examiners felt that there was visible attempt form the students to answer the questions as thoroughly as they can. However, what was most glaring was the insufficient amount of key-words and jargons provided by the students’ answers, which actually highlights that there was insufficient detailed studying of the topics. Of the 4 questions in Section A, Q2 and Q4 were generally poorly answered. 1a No. of mol of Cu2O = 0.286 ÷ (63.5+63.5+16.0) = 0.00200 I mol of C6H12O6 ≡ 1 mol of Cu2O No. of mol of C6H12O6 = 0.00200 Mr of C6H12O6 = 12.0(6) + 1.0(12) + 16.0(6) = 180.0 Mass of C6H12O6 = 0.00200 x 180.0 = 0.360g Percentage of glucose in the food sample = 0.360 / 5.00 x 100% = 7.20% Note: Answers are to show 3 significant figures to gain full marks. b 1s2 2s2 2p63s2 3p63d9 c i Isotopes are atoms of the same element with the same number of protons and different number of neutrons. ii Relative atomic mass of copper = (69.2/100 x 63) + (30.8/100 x 65) = 63.62 = 63.6 (1 dec place) d Number of protons= 29 Number of neutrons= 34 Number of electrons= 29 e i ii As angle of deflection α charge / mass, the angle of deflection for a sample of 32S2- is predicted to be twice that of 63Cu2+ as its mass is two times smaller than 63Cu2+. 63Cu2+ 32º 32S2- 16º ACJC 2008 8872/02/Aug/08 Preliminary Examination

Examiner’s Report 2008 2 2a Dynamic equilibrium is reached when the rate of the forward reaction and the rate of the backward reaction is the same. Comments: • Generally well answered. b i CH3CH2COOH (l) + CH3CH2OH (l) CH3CH2COOCH2CH3 (l)+H2O(l) No marks awarded if no state symbols given ii Ethyl propanoate c CH3CH2COOH + CH3CH2OH CH3CH2COOCH2CH3+H2O Initial no. of mole 0.6 0.5 0 0 Δ in no. of mole 0.4 – 0.6 = -0.2 -0.2 +0.2 +0.2 No. of mole at eqm 0.4 0.5 – 0.2 = 0.3 0.2 0.2 Let the volume of the vessel be V dm3. Kc = [CH3CH2COOCH2CH3][H2O] [CH3CH2COOH][CH3CH2OH] = (0.2/V) (0.2/V) (0.4/V) (0.3/V) = 0.333 ACJC 2008 8872/02/Aug/08 Preliminary Examination

Examiner’s Report 2008 3 d i Position of equilibrium will shift to the right hand side. By Le Chatelier’s principle, when ethanol is added, the system will favour the forward reaction so as to decrease the concentration of ethanol. Hence, the position of equilibrium will shift to the right hand side. ii The position of equilibrium shifts to the left-hand side. By Le Chetalier’s principle, increasing temperature of the system wil