VJC H2 Chem 2013 Prelim P3 Soln

 VJC 2013 9647/03/PRELIM/13 [Turn over 1 Victoria Junior College 2013 H2 Chemistry Prelim Exam 9647/3 Suggested Answers 1 During strenuous exercise, pyruvic acid, CH 3COCOOH, is converted to lactic acid, CH3CH(OH)COOH, which builds up in our muscles due to limited oxygen availability. (a) Such anaerobic respiration is catalysed by the enzyme lactate dehydrogenase, with concomitant conversion of NADH to NAD +. NADH plays a key role in the production of energy through redox reactions. NADH and NAD+ are cofactors, substances that act with and are essential to the activity of an enzyme. (i) Write a balanced equation for anaerobic respiration. Using the data given in the table below, calculate the value of E cell for this reaction. C C H 3COCOOH + NADH + H+  CH3CH(OH)COOH + NAD+ Eo = +0.89 – (-0.32) = +1.21V (ii) It is not possible to use Eo values reliably to decide whether a chemical reaction will occur. Suggest a reason for this. The reaction may be kinetically unfeasible due to high activation energy, even though it is thermodynamically feasible indicated by a positive Eo cell value. (iii) The activity of lactate dehydrogenase hinges on the active site Fe 2+ contained in a heme group which can be oxidized to Fe 3+. Suggest a mechanism for the catalysis of anaerobic respiration by lactate dehydrogenase. Support your answers with relevant E values given above and from the Data Booklet. You may represent the reduced and oxidised forms of the enzyme as LDH Fe2+ and LDHFe3+. Step 1: 2LDHFe2+ + CH3COCOOH + 2H+  2LDHFe3+ + CH3CH(OH)COOH E cell = +0.89 – (+0.77) = +0.12V (> 0, hence feasible) Step 2: 2LDHFe3+ + NADH  2LDHFe2+ + NAD+ + H+ E cell = +0.77 – (-0.32) = +1.09V (> 0, hence feasible) (iv) After exercising, we do not stop panting immediately because we need to breathe in additional oxygen so that lactic acid can be reconverted back to pyruvic acid. Both breathing and heart rates increase until the excess lactic acid level normalizes. Write a balanced equation for the conversion of lactic acid back to pyruvic acid. Calculate the value of E  cell for this reaction, with the help of the Data Booklet. O2 + 2CH3CH(OH)COOH  2H2O + 2CH3COCOOH Eo = +1.23 – (+0.89) = +0.34V [8] Electrode reaction E /V 2e- + H+ + NAD+ NADH –0.32 2e- + 2H+ + CH3COCOOH CH 3CH(OH)COOH +0.89 1528

 VJC 2013 9647/03/PRELIM/13 [Turn over 2 (b) (i) Using bond energy data obtained from the Data Booklet, calculate the enthalpy change of reaction, ∆H, for the conversion of pyruvic acid into lactic acid. CH3COCOOH(l) + H2(g)  CH3CH(OH)COOH(l) ∆H = 740 + 436 – 410 – 360 – 460 = –54 kJ mol-1 (ii) Given that the actual enthalpy change of this reaction is 110 kJ mol1, suggest a reason for the discrepancy between your calculated ∆H value in (b)(i) and the actual ∆H v a l u e .