ASRJC 2022 J2 H2Prelim P2 QP

ASRJC JC2 PRELIM 2022 9729/02/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2022 JC 2 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 22 / _____ CHEMISTRY 9729/02 Paper 2 Structured Questions 14 September 2022 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and register number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Paper 2 1 / 10 2 / 12 3 / 15 4 / 10 5 / 18 6 / 10 s.f./units Total / 75 This document consists of 21 printed pages and 3 blank pages.

2 ASRJC JC2 PRELIM 2022 9729/02/H2 Answer all the questions. 1 (a) Calcium, magnesium, and zinc are three minerals that are vital to several bodily processes. Though they occur naturally in a variety of food, many people take supplements that contain these minerals usually found in the +2 oxidation state. (i) State and explain how the second ionisation energy of calcium compares with that of magnesium. ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[2] (ii) By quoting relevant data from the Data Booklet, suggest how the thermal stability of ZnCO3 compares with that of MgCO3 and CaCO3. Explain your answer. ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[2] (b) Beryllium exhibits physical and chemical properties that are anomalous to the rest of the Group 2 alkaline earth metals. Due to the similarities in electronegativity, beryllium shares many physical and chemical properties with aluminium in Group 13 instead. (i) Beryllium forms a complex ion in aqueous solutions with the following structure, [Be(H2O)4]2+. Suggest an equation to show the hydrolysis of beryllium ions in water. ………………………………………………………………………………………………...[1] (ii) Beryllium oxide is amphoteric. Write two chemical equations, with state symbols, showing its reaction with aqueous NaOH and dilute HCl respectively. ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[2] (iii) Explain why beryllium chloride is not ionic. ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[1]

3 ASRJC JC2 PRELIM 2022 9729/02/H2 [Turn over (iv) When beryllium chloride is heated, it sublimes and forms gaseous Be2Cl4 dimers. Be Cl Be Cl ClCl By considering the numbers and type of electron pairs, explain the change in shape and bond angle about the Be atom during the dimerisation of beryllium chloride. ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[2] [Total: 10]

4 ASRJC JC2 PRELIM 2022 9729/02/H2 2 (a) Both lactic acid, CH3CH(OH)CO2H, and ethanoic acid are colourless liquids at 298 K. Lactic acid dissociates in water to form its conjugate base, lactate ion CH3CH(OH)CO2–. CH3CH(OH)CO2H + H2O CH3CH(OH)CO2– + H3O+ An intramolecular hydrogen bond can be formed in the lactate ion. (i) Draw the structure of the lactate ion showing the intramolecular hydrogen bond formed. [1] (ii) Hence, suggest how the Ka of lactic acid would differ from that of ethanoic acid. Explain your answer. ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[1] (b) Nicotinamide adenine dinucleotide (NAD+) is a coenzyme responsible for carrying electrons from one reaction to another in living cells. The electrode potential for the reduction of NAD + in a biological system, E(pH 7), in which the conditions are at 1 mol dm–3, 25 oC and pH 7, is as shown. The reduced form of NAD+ is represented as NADH. NAD+ + H+ + 2e– NADH E(pH 7) = –0.320 V The Nernst equation can be used to calculate the electrode potential of the above system when [NAD+] and [NADH] change. E = E(pH 7) – ( n 0592.0 ) log10 ]NAD[ ]NADH[ + where n is the number of moles of electrons transferred in the system. (i) Using the Nernst equation given, determine the ratio ]NAD[ ]NADH[ + when E = –0.350 V. [1]

5 ASRJC JC2 PRELIM 2022 9729/02/H2 [Turn over (ii) Hence, calculate the percentage of NAD + in the NAD +–NADH mixture when E = –0.350 V. [1] During intense exercise where there is insufficient supply of oxygen, pyruvic acid, CH3COCO2H, is converted to lactic acid. This process involves the conjugate bases of the two acids and the conversion of coenzyme NADH to NAD+, as shown below. The standard cell potential of this reaction is measured at 25 oC and pH 7. CH3COCO2– + NADH + H+ → CH3CH(OH)CO2– + NAD+ Ecell(pH 7) = +0.135 V pyruvate lactate (iii) Write the half-equation for the reduction of pyruvate to lactate in an acidic condition and calculate its standard electrode potential at 25 oC and pH 7. ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………....... ………………………………………………………………………………………………...[2] (iv) Calculate G for the anaerobic metabolism of pyruvate at pH 7. [1]

6 ASRJC JC2 PRELIM 2022 9729/02/H2 (c) The standard enthalpy change of combustion of lactic acid can be represented as shown. CH3CH(OH)CO2H(l) + 3O2(g) → 3CO2(g) + 3H2O(l) (i) What do you understand by the term standard enthalpy change of combustion of lactic acid? …………………………………………………………………………………………………… …………………………………………………………………………………………………… ……………………………………………………………………………………………..….[1] (ii) Using the data in Table 2.1, calculate standard enthalpy change of combustion, ∆Hc o, of lactic acid. Table 2.1 Species ∆Hf o / kJ mol−1 CH3CH(OH)CO2H(l) −483.2 CO2(g) −393.5 H2O(l) −285.8 [1] (iii) Use the bond energies given in the Data Booklet to calculate another value for the standard enthalpy change of combustion of lactic acid. [2] (iv) Apart from average bond energy values found in the Data Booklet, suggest one other reason for the difference in your answers in (c)(ii) and (c)(iii). ………………………………………………………………………………………………....... …………………………………………………………………………………………….......[1] [Total: 12]

7 ASRJC JC2 PRELIM 2022 9729/02/H2 [Turn over BLANK PAGE

8 ASRJC JC2 PRELIM 2022 9729/02/H2 3 (a) Methionine is an essential amino acid in humans. It plays a critical role in the metabolism and health of humans because it is the precursor of other amino acids such as cysteine and taurine. H2N CO2H S Methionine The pKa values associated with methionine are 2.28 and 9.21. (i) Calculate the pH of 0.15 mol dm –3 solution of methionine, ignoring the effect