DHS_H2_CHEM_P3

2 BLANK P AGE [Turn Over

3 Answer any four questions. 1(a) Pyruvic acid, CH 3COCOOH, occurs naturally in the body and is an end product o f the metabolism of sugar. When 10.0 cm 3 of a solution of pyruvic acid is titrated against 0.01 mol dm –3 sodium hydroxide, with a data logger, the following curve is obtained. pH Volume of NaOH / cm3 7 Equivalence point 2.5 30 (i) Suggest a suitable indicator, if the titration is to be repeated without the use of a data logger. Explain your reasoning. (ii) Calculate the value of Ka for pyruvic acid. (iii) Explain, with the aid of an appropriate equation, why the pH at equivalence point is greater than 7. [7] (b) Pyruvic acid is converted to the amino acid , alanine, in just one single step in the body by the enzyme alanine transaminase. However, to achieve the same conversion in a laborator y, multiple steps are re quired. Given the chemical structure of alanine below, show how pyruvic acid can be converted to alanine in not more than 4 steps. Indi cate the reagents, conditions and all intermediates in your answer. The alanine formed from y our suggested synthesis steps may be in its cationic, anionic or zwitterionic forms. C C H 3 H NH2 C O OH Alanine [5] [Turn Over

4 Alanine, together with the 19 oth er standard amino acids are the building blocks of proteins. State the two factors that cause protein denaturation and explain how the (c) two stated factors denature the protein. [4] (d) When 0.200 g of alanine is subjected to complete combustion, carbon dioxide, nitrogen dioxide and water is formed. The gases produced are absorbed by 20 cm3 of 1.5 mol dm –3 aqueous sodium hydroxide. The resultant solution is then titrated with 0.3 mol dm–3 H SO . Calculate the volume of H SO required for the mixture to reach equivalence point. [4] 2 4 2 4 Hint: 2NO2 + 2NaOH  NaNO3 + NaNO2 + H2O [Total: 20] [Turn Over

5 [Turn Over 2(a) Hydrogen chloride gas and ammonia gas react to form ammonium chloride. NH3(g) + HCl(g) NH4Cl(s) (i) By constructing a Hess’ Law Cycle, calculate the standard enthalpy change of the reaction, given the following information:  ) NH ( f3 H = –46.1 kJ mol–1  ) HCl ( fH = –92.3 kJ mol–1  ) Cl NH ( f4 H = –314.4 kJ mol–1 (ii) The standard entropy change of the reaction is –284 J K –1 mol–1. Explain why this value is negative. (iii) State and explain whether the reaction is spont aneous at high or low temperatures. (iv