ACJC_Chemistry_Paper 3_Answer Prelims

Mark Scheme 2008 H2 Chemistry Preliminary Examination Paper 3 Mark Scheme 1 (a)(i) Order of rxn wrt to Br2 Comparing expt 1 & 3, when [Br 2] triples from 0.01 to 0.03 moldm -3, the rate of rxn also triples. Hence, [Br 2] is directly proportional to the rate of rxn. Order of rxn wrt Br 2 = 1 Order of rxn wrt to HCOOH Comparing expt 1 & 2, 8.0 x 10 -6 (0.02)1(0.04)x 2.0 x 10-6 (0.01)1(0.02)x x = 1 Order of rxn wrt HCOOH = 1 * can used any 2 sets of expts to compare too. Rate = k[Br 2][HCOOH] (a)(ii) 6.0 x 10-6 = k (0.03) (0.02) k = 0.01 mol-1 dm3 s-1 (a)(iii) The reaction involves 1 mole of Br 2 and 1 mole of HCOOH in the rate determining step. Overall, this is a one-step reaction. (a)(iv) After mixing, [Br 2] = 0.005 mol dm-3 [HCOOH] = 0.01 mol dm-3 Rate = (0.01)(0.005)(0.01) = 5.0 x 10 -7 mol dm-3 s-1 1 (b)(i) Methanal, methanol and methanoic acid are all polar molecules and exist as simple molecular structure. Methanal has permanent dipole-permanent dipole interaction between molecules. Methanol and methanoic acid have hydrogen bonds between molecules. Since permanent dipole-permanent dipole is weaker than hydrogen bonds, less energy is needed to over the attractions between methanal. Hence, methanal has the lowest boiling point. Methanoic acid forms more extensive hydrogen bonds than methanol. = © ACJC 2008 9746/03/Prelim/08 1

Mark Scheme Thus, more energy is needed to overcome the attractions between methanoic acid. Hence, boiling point of methanoic acid is higher than that of methanol. [Generally, compare: *difference between methanal and ‘methanol and methanoic acid’ *difference between methanol and methanoic acid] (b)(ii) In benzene solution, methanoic acid dimerises through hydrogen bonds. CH3 C O OH CH3C O HO hydrogen bond 1 (c)(i) (c)(i) con’t (1) 2,4-DNPH, at room temp - methanal ⇒ orange ppt - methanoic acid ⇒ no orange ppt HCHO + NO2 NHNH2 NO2 NO2 NHN NO2 CH2 +H 2O (2) Na2CO3(aq), room temp - methanal ⇒ no CO2(g) - methanoic acid ⇒ C O 2(g) evolved which forms white ppt with Ca(OH)2 (aq) 2 HCOOH + Na2CO3 (aq) 2 HCOONa + CO2 + H2O (3) Na(s), room temp - methanal ⇒ no H2(g) - methanoic acid ⇒ H 2(g) which gives a ‘pop’ sound with lighted splinter HCOOH + Na (s) HCOO-Na+ + ½ H2 (g) (c)(ii) (1) KMnO4/H+, heat - methanoic acid ⇒ decolourise purple KMnO 4 and give CO 2(g) which forms white ppt with limewater. - ethanoic acid ⇒ no decolourisation of purple KMnO4 HCOOH + [O] CO2 + H2O (2) aq. silver nitrate in excess ammonia solution, warm © ACJC 2008 9746/03/Prelim/0