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) Calculate the temperature for which the reaction is spontaneous. [6] (b) When hydrogen chloride gas is added to organic compound P, C6H12, two products Q and R , which are isomer s of each other, C 6H13Cl, are formed in unequal proportion. When Q and R are separately boiled with aqueous sodium hydroxide, S is formed from Q, while T is formed from R. S and T are also isomers of each other, with the same molecular formula, C 6H14O. When S and T are separately boiled with acidified potassium manganate( VII), no decolourisation is observed in the test tube containing S; while decolourisation is ob served in the test tube containing T which results in the formation of U, C 6H12O2. When some sodium carbonate powder is added to U, effervescence of carbon dioxide gas is observed. When P is boiled with acidif ied potassium manganate ( VII), compound V, C5H10O and CO 2 are formed. V gives an orange precipit ate when warmed with 2,4-dinitrophenylhydrazine. When V is heated with lithium aluminium hydride in dry ether, W, C5H12O, which does not exhibit optical activity, is formed. Deduce the structures fo r each lettered compound, P to W . Explain the chemistry of the reactions involved. Balanced equations are not required. [12] (c) Using relevant information from the Data Booklet, compare and explain the relative acid strength of HCl and HF. [2] [Total: 20]

6 3 AlCl3 is a compound of alumin ium and chlorine. The solid has a low melting and boiling point and it is covalently bonded. It finds widespread application in the chemical industry as the classic catalyst for the Friedel-Crafts alkylation reaction. Typically, Al Cl3 can be used to catalyse the reaction between benzene and chloromethane. (a) Describe the mechanism for t he reaction between benzene and chloromethane. [3] (b) C6H5CH2Cl can be obtained by reacting C 6H5CH3 with Cl 2 via a free radical substitution. (i) By making use of relevant bond energies from the Data Booklet, calculate the enthalpy change of this reaction. (ii) Give two reasons why the actual enthalpy change of this reaction deviates from the above value. [4] (c) Another metal chloride A, which has the formula MC lx cannot be used to catalyse the reaction between benzene and chloromethane. When 1.0 x 10 –3 moles of aqueous sodium hydr oxide is reacted completely with 5.0 x 10−4 moles of A, 0.0292 g of a white precipitate, B is formed. When B is heated, water is lost leaving a white residue C. C is of high melting point and it is very slightly soluble in cold water. (i) Deduce the identities of A, B and C . Write equations whereve r applicable. (ii) Predict the pH of the solution formed when A is dissolved in water. [6] [Turn Over

7 (d) The reaction below is an example of nucleophilic substitution. C6H5CH2Cl + CN –  C6H5CH2CN + Cl – The reaction kinetics of this reaction is determined by monitoring the change in the concentration of CN− with time. The results are shown below: (i) Determine the rate equation for the above reaction. (ii) With the aid of Maxwell-Boltzmann distribution curve, predict and explain the effect on the rate of this reac tion when the reaction temperature is decreased. [7] [Total: 20] [CN – ] 0.01 –3Cl] = 1.0 mol dm[C H CH6 5 2 0.005 –3Cl] = 2.0 mol dm[C H CH6 5 2 Time / min x 2x [Turn Over

8 4 The oldest church in Paris named Saint-Germain I'Auxerrois, is well-known for its splendid stained glass. The components of the stained glass used in the church windows is known to contain lead( II) oxide, silicon( IV) oxide and traces of a chromium-containing oxide. (a) Explain why transition metals are denser than s-block elements. [3] (b) Describe the bonding in the followi ng substances and explain why i t contributes to the specified property in glass: substance property (i) silicon (IV) oxide hardness (ii) lead (II) oxide Electrical conductivity (if any) [4] (c) The window frames enclosing the stained glass is made of aluminium due to its low density and high melting point. Account for the high melting point of aluminium. [2] (d) Construct a Born-Haber cycle for the formation of lead( II) oxide from its elements. Use the relevant data from the table below as well as from the Data Booklet, calculate the lattice energy of lead(II) oxide. enthalpy term H / kJ mol –1 standard enthalpy change of formation of PbO –219 standard enthalpy change of