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. (iii) How does the trend in ∆H calculated in (c)(ii) relate to the Eo values? Hence, predict the relative reducing powers of magnesium and calcium. (iv) Account for the difference between the decomposition temperature of CaCO 3 and YCO3. [6] [Total: 20] 1518

 VJC 2013 9647/03/PRELIM/13 [Turn over 4 + 2 (a) Compound X, CH 3CH=CHCHClCH3, reacts with alcoholic KCN according to the following equation: CH3CH=CHCHClCH3 + CN–  CH3CH=CHCH(CN)CH3 + Cl– To investigate the above reaction, two separate experiments were performed using different initial concentrations of CN . In each experiment, the concentration of X in the reaction mixture was determined at different times as the reaction progresses. The same graph was obtained in both experiments, as shown below: The following results were obtained: (i) Explain why the reaction requires an alcoholic instead of an aqueous medium to take place. (ii) Referring to the graphs above, deduce the order of reaction with respect to each of the reactants, CN– and X. (iii) State, with a reason, whether the reaction proceeds by the S N1 or S N2 mechanism. (iv) Hence, sketch a well labelled reaction pathway diagram for the reaction, given that the reaction is exothermic. (v) Suggest why the reaction of X is more likely than that of CH 3CH2CH2CHClCH3 to exhibit the mechanism stated in (ii). [9] [X] /mol dm-3 time /min [CN]1 and [CN]2 1519

 VJC 2013 9647/03/PRELIM/13 [Turn over 5 (b) The enzymatic hydrolysis of glycerides in oils is recently attempted industrially as an energy-saving method due to mild reaction conditions required. An enzyme such as lipase, is commonly used to speed up the rate of the hydrolysis of glycerides for the production of three fatty acids and glycerol, as shown by the structures below: (i) Draw the structural formula of glyceride and identify the functional group in the glyceride. (ii) State the number of sigma and pi bonds surrounding the carbon atom in the functional group stated in (i) . Hence, identify the type of hybridisation of the carbon atom in the functional group. (iii) The following graph shows how the rate of enzymatic hydrolysis of glyceride varies with increasing substrate concentration, [S] with a fixed amount of lipase used under fixed temperature. Suggest an explanation for the shape of the curve. (iv) Besides using catalyst, the rate of reaction can also be increased by increasing the temperature. The rate of hydrolysis of glyceride is doubled when temperature increased by 10 oC. If the rate of reaction at T oC is r , determine the new temperature if the rate is increased to 16r. Explain your answer. [6] (c) In an experiment involving the decomposition of ammonia, nickel is used as a catalyst to speed up the rate of decomposition of ammonia at 500 oC in a reactor of volume 10 dm3. (i) State the type of catalysis which nickel exhibits and explain how its presence speeds up the rate of decomposition of ammonia. (ii) It was found that at equilibrium of decomposition for the experiment, 90% of the 6.80 g ammonia has decomposed. Calculate the total pressure of the gas mixture after reaction. [5] [Total: 20] fatty acids glycerol CH2CHCH2 OH OHOH RCOOH R'COOH R''COOH Rate [S] 1520

 VJC 2013 9647/03/PRELIM/13 [Turn over 6 3 This question concerns the transition elem ents iron and chromium, together with their compounds. (a) Iron is the fourth most common element in the Earth’s crust. It is the most common element (by mass) forming much of Earth’s outer and inner core. Some reactions of iron are given below: Fe Fe 2+(aq) Fe 3+(aq) Mg green precipitate deep red solution gas Z (i) State the reagents that could be used for Steps I to III. (ii) Identify the gas Z and write an equation for its formation. (iii) Describe what you would expect to observe when solid calcium carbonate is added to an aqueous solution of Fe3+. [6] (b) The following data are given: Iron Aluminium Atomic radius / nm 0.116 0.143 Ionic radius / nm 0.076 (Fe 2+) 0.064 (Fe3+) 0.050 Explain why the atomic radius of iron is smaller than that of aluminium but the ionic radii of iron(II) and iron(III) are larger than that of Al 3+. [2] (c) Chromium is the hardest of all known metal elements in the Periodic Table. It is also odourless and tasteless. The name of the element is derived from the Greek word "chrōma" meaning colour, because many of its compounds are intensely coloured. The metal also reacts with oxygen to form a series of oxides. One of these oxides, CrO 2, is often used to coat cassette tapes, due to its