SAJC Prelim P3 Answers

Section A Answer all the questions in this section. 1 Electrosynthesis in organic chemistry is the synthesis of compounds in an electrolytic cell using inert electrodes. One important process is the Kolbe reaction. The Kolbe reaction is the decarboxylative dimerization of two carboxylate ions. Alkanes can be formed at the anode by the following reaction. (a) Explain why alkanes are generally unreactive. [1] Alkanes have C-C and C-H bonds which are strong and non-polar. (b) (i) Calculate the volume of carbon dioxide produced when a steady current of 5A is passed through the electrolyte for 32 minutes at standard temperature and pressure. [2] Q = It = 32 x 60 x 5 = 9600 C Amt of CO2 = Amt of e = Q/F = 9600/(96500) = 0.09948 mol Vol of CO2 collected = 0.09948 x 22.7 = 2.26 dm3 (ii) Methanoate ions undergo the Kolbe reaction to form only 2 gases. Using the axes given below, sketch the variation of pV/RT against p for one mole of each gas at the same temperature. Briefly explain your answer. [2] pV/RT p 0 1.0 ideal gas

2 CO 2 deviates more from ideal behaviour because it experiences stronger instantaneous dipole-induced dipole interactions / intermolecular forces between the molecules compared to H2 which has a smaller electron cloud. (c) The cathodic reaction for the Kolbe reaction can be represented by the following half equation: 2H2O + 2e  H2 + 2OH-. Write an overall equation for the Kolbe reaction involving ethanoate ions. Hence, calculate ΔGƟ for the Kolbe reaction given that the E Ɵcell value for the reaction is +2.28 V. [2] 2CH3COO-  CH3-CH3 + 2CO2 + 2e 2H2O + 2e  H2 + 2OH- Overall: 2CH3COO-+ 2H2O  CH3-CH3 + 2CO2 + H2 + 2OH- ∆GƟ = − nFE = − (2)(96500)(2.28) = − 440 kJ mol-1 (d) The reaction mechanism of the Kolbe reaction involves a 4-step reaction mechanism as described below: Step 1: Dissociation of carboxylic acid to form a carboxylate ion. Step 2: Oxidation of carboxylate ion to form a carboxyl radical, RCOO•. Step 3: Decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R•. Step 4: Formation of a covalent bond between two alkyl radicals. (i) Using ethanoic acid as a starting reagent, suggest suitable equations for Steps 1, 2 and 4 on how ethane can be produced using the Kolbe reaction. [3] pV/RT p 1.0 H2 ideal gas CO2

3 [Please Turn Over Step 1: CH3COOH → CH3COO- + H+ Also accept “CH3COOH + H2O  CH3COO- + H3O+”. Step 2: CH3COO-→ CH3COO• + e Step 4: CH3• + CH3• → CH3CH3 (accept C2H6 also) (ii) The decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R •, in Step 3 can be represented by the following equation: RCOO•  CO2 + R• Outline the mechanism of this step by copying the diagram below and include relevant curly half arrows. [1] (e) In another separate experiment, a new alkane A, C6H12, was produced. When reacted with bromine under ultraviolet light, A produced only one isomeric monobromo compound, B, which does not have a chiral centre. Draw the skeletal formulae of A and B. [2] (f) (i) Using monohalogenoethane, C 2H5X, as examples, describe and explain the relative reactivities of chloro- and bromo-compounds in hydrolysis reactions. [2] The hydrolysis of C 2H5X will increases in reactivity (C 2H5Cl < C2H5Br) . This is because the bond length / bond energy of C-C l <C-Br, hence, it is most difficult to overcome C-C l bond > C-Br bond.

4 (ii) Halogenoalkanes such as chlorofluoroalkanes, CFCs, were once used as refrigerant fluids and aerosol propellants. In many applications, they have now been replaced by alkanes. This is because CFCs contribute to the destruction of the ozone layer. Explain how CFCs destroy the ozone layer and suggest one potential hazard of using alkanes instead of CFCs. [2] CFCs produce chlorine radicals, which in turn initiate the chain reaction breaking down ozone molecules, O3 to O2. Alkanes are flammable. (iii) Halogenoalkanes can react with lithium to give organolithium compounds. R-Br + 2Li  R-Li + LiBr These organolithium compounds can react with carbonyl compounds to form alcohols. (R is alkyl, R’ and R’’ are either alkyl or H) Using the above reaction sequence, deduce the structure of a suitable bromoalkane, R-Br, and a suitable carbonyl compound to synthesise hexan-2-ol. [2] Ethanal, 1-bromobutane OR Bromomethane, Pentanal [Total: 19]

5 [Please Turn Over 2 Wastewater has to be treated carefully to remove all harmful contaminants before it can be released into water bodies. (a) Industrial wastewater contains high levels of heavy metal ions. Electrocoagulation is used to remove heavy metal ions from wastewater. In this method, heavy metal ions are oxidised to form hydroxides, so that they can coagulate easily and be removed easily in subsequent steps. A sample of wastewater containing Fe 2+ is treated by electrocoagulation. 4Fe2+ (aq) + 10H2O (l) + O2 (g) 4Fe(OH) 3 (s) + 8H+ (aq) (i) By considering the interaction it forms with water, explain why Fe 2+ cannot be easily removed from wastewater. [1] It forms ion-dipole interaction with water. Hence, it is soluble in water and cannot be easily removed. OR It forms dative bond with water to form a soluble complex of [Fe(H2O)6]2+. Hence it cannot be easily removed. (ii) State a suitable physical method to remove Fe(OH)3 from the wastewater. [1] filtration (iii) Using Le Chatelier’s Principle, suggest why the wastewater has to be kept alkaline for a more effective removal of Fe2+. [1] As [OH−] is high / [H+] is low, the position of the equilibrium shifts right to increase [H+]. Hence, more Fe2+ would be oxidised/removed as Fe(OH)3. (iv) With an appropriate sketch of the Boltzmann distribution, explain how an increase in temperature would affect the rate of removal of Fe2+. [3] [1] for diagram When T increases, the KE of the reacting particles increases. Hence, there are more particles with E≥Ea and the frequency of effective collision increases. The rate of removal of Fe2+ would increase. E No of particles with Energy, E 0 Ea T2 T1 T1 < T2

6 (b) Ammonia is a toxic substance present in wastewater from the mining industry. Nitrification is a common biological treatment method to convert ammonia into less toxic nitrate. NH3 (aq) + 2O2 (g) NO3− (aq) + H+ (aq) + H2O (l) (i) Write the expression for the equilibrium constant, Kc, for this reaction given that Kc has units of mol−1 dm3. [1] Kc = [NO3−][H+] / [NH3] [O2]2 (ii) 12 mol of oxygen gas was pumped into 1000 dm 3 of wastewater containing 5 mol of NH3. After nitrification, the resulting treated wastewater had a pH of 4.2. Calculate the value of Kc. Leave your answer to 3 significant figures. [3] NH 3(aq) + 2O 2 NO3−(aq) H +(aq) H 2O Initial conc / mol dm−3 0.005 0.012 0 0 Eqm conc / mol dm−3 0.005 − 10−4.2 = 0.0049369 0.012 – 2 x 10−4.2 = 0.011873 10 −4.2 10 −4.2 Kc = (10−4.2)2 = 5.72 X 10−3 mol−1 dm3 (3sf) (4.9369 X 10−3)(1.1873 X 10−2)2 (iii) State how the Kc in (ii) would change when the amount of NH3 in wastewater is increased to 10 mol. [1] Kc would remain the same. (iv) The nitrification process was carried out at room temperature. It was observed that the outer surface of the reaction vessel was cold after some time. By considering the effect of temperature on the equilibrium, suggest and explain how you can change the reaction temperature to increase the conversion of ammonia. [2] The (forward) reaction is endothermic. Hence, we can increase the temperature to shift the equilibrium position right, so as to absorb the excess heat, which allows more NH3 to be converted.

7 [Please Turn Over (v