ACJC Prelim P3_Answers

1 © ACJC 2018 9729/03/Prelim/2018 Section A Answer all questions in this section. 1 (a) (i) Due to reactivity of A lCl3 and PCl5 in water, the following reactions are carried out under anhydrous conditions. Write equations showing the reactions of aluminium chloride and phosphorus pentachloride in water. State the pH of resulting solutions. [3] A lCl3 + 6H2O [A l(H2O)6]3+ + 3Cl– [Al(H2O)6]3+ [Al(H2O)5(OH)]2+ + H+ pH 3 PCl5 + H2O POC l3 + 2HCl POCl3 + 3H2O H 3PO4 + 3HCl OR PCl5 + 4H2O H 3PO4 + 5HCl pH 2 (ii) Chlorobenzene and 2-chloro-2-methylbutane are separately boiled with aqueous sodium hydroxide and acidified with nitr ic acid before aqueous silver nitrate is added. State and explain the difference in observations. Write equations for the reactions that occur. [3] 2-chloro-2-methylbutane forms a white ppt while chlorobenzene does not. The C–C l bond in chlorobenzene is strengthened by partial double bond character due to the overlap of p-orbital on C l atom and π-orbital of benzene. Hence chlorobenzene is resistant to hydrolysis. CH 3C(CH3)ClCH2CH3 + OH– CH 3C(CH3)OHCH2CH3 + Cl – Ag+ + Cl – AgC l (b) (i) To study the kinetics of the reaction between 2-chloro-2-methylbutane and sodium hydroxide in alcoholic medium, two experiments with different initial concentrations of 2-chloro-2-methylbutane were carried out at constant temperature. A [NaOH]–time graph was plotted using the results obtained from the experiments.

2 © ACJC 2018 9729/03/Prelim/2018 (i) Deduce the order of reaction with respect to 2-chloro-2-methylbutane and NaOH. [2] Using Reaction 1 (or 2), when [NaOH] decreases, gradient remains constant, hence rate of reaction is constant. Rate of reaction is independent of [NaOH], hence the reaction is zero order wrt [NaOH]. Comparing reactions 1 and 2, when [2-chloro-2-methylbutane] doubles from 0.25 to 0.50 mol dm -3, the gradient doubles, showing that the rate of reaction doubles from 1.60 × 10 -4 to 3.15 × 10 -4 mol dm -3 min-1. Rate of reaction ∝ [2- chloro-2-methylbutane], hence the reaction is first order wrt [2-chloro-2- methylbutane]. (ii) Hence, state the rate equation of this reaction. [1] Rate = k[2-chloro-2-methylbutane] Reaction 1: 0.25 mol dm-3 of 2-chloro-2-methylbutane Reaction 2: 0.50 mol dm-3 of 2-chloro-2-methylbutane 0 0.002 0.004 0.006 0 5 10 15 20 25 0.008 0.010