VJC H2 Chem 2013 Prelim P3 QP

 VJC 2013 9647/03/PRELIM/13 [Turn over 2 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 (ii) It is not possible to use E o values reliably to decide whether a chemical reaction will occur. Suggest a reason for this. (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+. (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. [8] (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) (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 . (iii) Acid-base neutralisation takes place when pyruvic acid reacts with barium hydroxide. Rank barium hydroxide, pyruvic acid, and its salt in increasing melting point. Explain your answer. [ 6 ] Electrode reaction E  /V 2e- + H+ + NAD+ NADH –0.32 2e- + 2H+ + CH3COCOOH CH 3CH(OH)COOH +0.89 1517

 VJC 2013 9647/03/PRELIM/13 [Turn over 3 (c) Muscles work by contracting and relaxing. Both processes are triggered by an electrical impulse. Calcium causes muscles to contract while magnesium relaxes them. Y is another element in Group II. Element Mg Ca Y E /V –2.37 –2.87 –2.95 Decomposition temperature of carbonates /K 470 1170 1770 (i) Predict the relative melting points of magnesium and calcium. (ii) With the help of the Data Booklet , calculate the enthalpy change, ∆ H when Mg and Ca ionise to Mg2+ and Ca2+ respectively.