ACJC H2 Chem 2013 Prelim P3 Soln

2 © ACJC 2013 9647/03/Prelim/13 [Turn over ACJC Chemistry Prelim Paper 3 Suggested Solution 1 (a) (i) CH 2=CHCH2OCOCH3 + Br2  CH2BrCHBrCH2OCOCH3 (ii) Compare Expt No. 1 and 2, [Br 2] constant, when [propenyl ethanoate] increases by 1.5 times, rate increases by 1.5 times. Order of reaction with respect to propenyl ethanoate is 1 Compare Expt No. 1 and 3, [propenyl ethanoate] constant, when [Br 2] doubles, rate increases by 4 times. Order of reaction with respect to bromine is 2 Rate = k[Br2]2[propenyl ethanoate] (iii) 1% of ester = 1/100 x 4.0 x 10 -2 = 4.0 x 10-4 mol dm-3 Time taken = (4.0 x 10-4) / (5.16 x 10-3) = 0.0775 s (assume constant rate) (iv) Chlorine is electron-withdrawing; hence will lower the electron density on the carbon-carbon double bond, hence less susceptible to electrophiles. Rate is expected to be slower. OR Chlorine is electron-withdrawing, hence makes the carbocation intermediate less stable as positive charge on carbocation is more intensified. Thus rate is expected to be slower. (v) CH2BrCHOHCH2OCOCH3 / CH2OHCHBrCH2OCOCH3 (vi) Rate of reaction would be faster. Water is more polar than ethanoic acid. It can form ion-dipole interaction with carbocation, hence stabilize it. Activation energy would be lowered. OR Rate of reaction would be slower. Water dilutes the concentration of Br 2. Since rate depends on concentration of Br2 and concentration of Br2 is decreased, rate is expected to be slower. (b) (i) From the graph, 20 cm 3 of NaOH reacted with MgCl2. No. of moles of NaOH in 20 cm 3 = 1.00 x 20/1000 = 0.02 mol MgCl 2 + 2NaOH  2NaCl + Mg(OH)2 No. of moles of MgCl2 that reacted with NaOH = 0.02/2 = 0.01 mol Initial concentration of MgCl2 = 0.01/(50/1000) = 0.200 mol dm-3 (ii) pH = 9 [H+] = 10-9 mol dm-3 [OH-] = 10-5 mol dm-3 No. of moles of Mg 2+ = 0.01/2 = 0.005 mol [Mg2+] = 0.00500/(60/1000) = 0.0833 mol dm-3 297

3 © ACJC 2013 9647/03/Prelim/13 [Turn over Ksp of Mg(OH)2 = [Mg2+] [OH-]2 = (0.0833) (10-5)2 = 8.33 x 10-12 mol3 dm-9 (iii) Solubility decreases with increasing temperature Less OH- needed to reach Ksp. pH would be less than 9. (iv) Mg 2+ has high charge density and can polarize H2O molecules slightly, weakening the O-H bond in water. Mg(H2O)6 2+ (aq) Mg(H2O)5 (OH)+ (aq) + H+ (aq) 2 (a) (i) Two mono-brominated products: (ignore stereoisomers) (ii) Ratio of the above products = 6:4 = 3:2 (b) (i) Step I is a first order reaction. Write a balanced equation for the reaction, stating suitable reagents and conditions required. Hence, state the rate equation of the reaction. Br + NaOH  OH + NaBr aq NaOH & heat under reflux or heat rate = k[C 5H9Br] (ii) Step III: HCN in the presence of traces of NaCN or NaOH & 10C - 20C Mechanism: Nucleophilic