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CIE Jun 2017 v2 Paper 4 Q6
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![PMT 6 The table lists some organic acids and their pKa values. 14 acid ethanoic acid chloroethanoic acid aminoethanoic acid (glycine) formula CH3CO2H Cl CH2CO2H H2NCH2CO2H pKa 4.76 2.86 9.87 (a) (i) State the relationship between pKa and the strength of an acid.[1] (ii) State the mathematical relationship between pKa and the acidity constant Ka.[1] (iii) Give reasons for why the pKa value for chloroethanoic acid is smaller than that for ethanoic acid.[2] (b) (i) Use the zwitterionic structure for aminoethanoic acid (glycine) in aqueous solution to write an equation for its dissociation giving H+(aq) ions.[1] (ii) Calculate the pH of a 0.100 mol dm3 solution of aminoethanoic acid. pH =[2]UCLES 2017 9701/42/M/J/17<br />
15 A 10.0 cm3 sample of 0.100 mol dm3 aminoethanoic acid (glycine) was titrated with 0.100 mol dm3 NaOH. After 20.0 cm3 of NaOH, an excess, had been added, the pH was found to be 12.5. (iii) Using the following axes, sketch a graph showing how the pH changes during this titration. PMT 14 pH 10 15 20 25 volume of NaOH added / cm3 [3] [Total: 10]UCLES 2017 9701/42/M/J/17 [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_v2_QP_-_Paper_4_CIE_Chemistry_A-level_6-0.jpg)
![9701/42 Question 5(c)(iii) 5(c)(iv) 5(c)(v) Cambridge International AS/A LevelMark Scheme PUBLISHED Answer Since (Geq2) is more negative (than Geq1) AND equilibrium 2 is more feasible H(3) = 8 (kJ mol1) ligand exchange / replacement / substitution / displacement Question Answer 6(a)(i) 6(a)(ii) the lower / smaller the pKa, the stronger the acid pKa = log(Ka) or pKa = lg(Ka) or Ka = 10pka 6(a)(iii) (stronger than ethanoic acid because) Cl is electron-withdrawing and so stabilises the RCO2 or weakens O-H bond (so H+ is more easily released)anion / conjugate base 6(b)(i) 6(b)(ii)+ H2O (cid:127)(cid:127) NH2CH2CO2+ H3O++ H+ +CH2CO2(cid:127)(cid:127) NH2CH2CO2 +CH2CO2 NH3 OR NH3 Ka = 109.87 = 1.351010 [H+] = (Ka.c) = 3.67106 pH = 5.4 (5.435.44) min 2sfUCLES 2017 Page 9 of 13 PMT May/June 2017 Total: Marks 1 1 1 17 Marks 1 1 1 1<br />
9701/42 Question Cambridge International AS/A LevelMark Scheme PUBLISHED Answer 6(b)(iii) curve starts at 5.4 and continuous vertical portion (end point) at vol added = 10.0 cm3 finishes at pH = 12.5 at 20 cm3 (and does not increase in pH) Question 7(a) Answer acyl chloride / COCl methyl ketone / CH3CO group aryl chloride aldehyde / CHO chloro(alkane) / RCl 01 [0]; 2 [1]; 3 [2]; 4 [3]; 5 [4]; 68 [5] Z Alkene / C=C phenol / C6H5OH aryl chloride PMT May/June 2017 Marks 1 1 1 10 Marks 5 Total:UCLES 2017 Page 10 of 13<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_v2_MS_-_Paper_4_CIE_Chemistry_A-level_6-0.jpg)
