Edexcel Jan 2015 (IAL) Paper 4 Q23 (with explained solutions) |

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Edexcel Jan 2015 (IAL) Paper 4 Q23

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23 Ethanoic acid, CH3COOH, is a weak acid found in vinegar. Successive replacement of the methyl hydrogen atoms by chlorine atoms gives chloroethanoic acid, CH2ClCOOH dichloroethanoic acid, CHCl2COOH trichloroethanoic acid, CCl3COOH (a) (i) Explain the term weak acid. (2) WeakAcid(ii) Write the expression for the acid dissociation constant, Ka, of dichloroethanoic acid, CHCl2COOH. (iii) Use the Ka data from page 18 in the Data Booklet to place the four acids, named in the introduction to the question, in order of increasing strength. State how the order that you have given can be deduced from the Ka data. weakest acidstrongest acidReason (1) (2)18 *P45044A01828*<br />
(b) (i) Calculate the pH of a solution of 0.10 mol dm3 ethanoic acid. State clearly any assumptions you have made. (4) Assumptions*P45044A01928* 19 Turn over<br />
(ii) Draw the titration curve, showing the change in pH when 0.10 mol dm3 sodium hydroxide solution is added to 25 cm3 of 0.10 mol dm3 ethanoic acid, until 40 cm3 of sodium hydroxide has been added. (4) 1413121110pH 98765432100 10 15 25 Volume of NaOH(aq) / cm3 20 *(iii) Select a suitable indicator for this titration. Justify your selection. 30 35 40 (2)20 *P45044A02028*<br />
(c) Ethanoic acid and trichloroethanoic acid react together in an acid-base reaction. Complete the equation by writing the formulae of the acid and base produced in this reaction. Identify the conjugate acid-base pairs in the spaces under the formulae. (2) CH3COOH + CCl3COOH (cid:111)+(Total for Question 23 = 17 marks) TOTAL FOR SECTION B = 47 MARKS *P45044A02128* 21 Turn over<br />

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Acceptable Answers Question Number 23(a)(i) ALLOW answers written on either set Reject not easily dissociated / few H+ ions of dotted lines Weak: dissociates/ionizes to a small extent / partially /incompletely ALLOW does not ionise completely (1) Acid: proton donor ALLOW produces/forms /releases H+ ions / H3O+ ions / oxonium ions / hydroxonium ions / hydronium ions ALLOW electron pair acceptor (1) IGNORE just accepts electrons IGNORE contains H+ ions IGNORE reference to typical acid reactions Mark (2) Mark (1) WCH0401 1501 Acceptable Answers Reject No /round brackets (Ka =) [CHCl2COO][ H+] [CHCl2COOH] OR [ H3O+] for [ H+] ALLOW [CHCl2CO2 IGNORE []eq and state symbols, even if incorrect ] / [CHCl2CO2H] Question Number 23(a)(ii)<br />
Acceptable Answers Question Number 23(a)(iii) weakest ethanoic acid Mark (2) chloroethanoic acid dichloroethanoic acid strongest trichloroethanoic acid OR correct formulae all four correct (1) Reason the weakest acid has the lowest Ka/ acid dissociation constant OR the weakest acid has the highest pKa OR the strongest acid has the highest Ka/ acid dissociation constant OR the strongest acid has the lowest pKa OR Ka increases/pKa decreases from ethanoic acid to trichloroethanoic acid/ weakest to strongest acid ALLOW acid that dissociates least has the smallest Ka/highest pKa ORA (1) IGNORE references to the effect of the chlorine atoms on Ka/stability of anion/strength of the O-H bond IGNORE references to pH Question Number 23(b)(i) Acceptable Answers Reject Mark (4) IGNORE SF except 1 SF throughout FIRST CHECK THE FINAL ANSWER, IF answer pH = 2.88/2.9, award 2 marks IF pH = 2.89, decide which route has been followed and award 1 mark for routes 1 and 2 (rounding error) and 3 marks for route 3 IF answer is not correct, award the following marks: Route 1 [H+] = Ka x [CH3COOH] = 1.7 x 10-5 x 0.1 = 1.3038 x 10-3 (mol dm-3) (1) pH = 2.8848 (1) consequential on their [H+], provided pH is less than 7 Route 2 [H+] = Ka x [CH3COOH] pH =pKalog[CH3COOH] (1) = 2.88 (1) consequential on their expression for pH WCH0401 1501<br />
no dissociati on OR partial/ incomple te dissociati on Assumption 1 [H+] = [CH3COO-] OR no H+ from the (ionization of) water OR H+ all comes from the acid (1) Assumption 2 Ionization of the (weak) acid is negligible / very small / insignificant OR [CH3COOH]initial = [CH3COOH]eqm OR [CH3COOH]eqm = 0.1 mol dm-3 OR [CH3COOH] remains constant (1) Route 3 using [CH3COOH]eqm = 0.1 - [H+] (1) [H+]= 1.2954 x 10-3 (mol dm-3) (1) pH = 2.8876 (1) Assumption [H+] = [CH3COO-] OR no H+ from the (ionization of) water OR H+ all comes from the acid (1) ALLOW [HA]/[HX]/[acid]/[A-]/[X-]/[base] for formulae of acid and base WCH0401 1501<br />
Acceptable Answers Question Number 23(b) (ii) Mark (4) Graph starting at 2.9 (allow 2.8-3.0) or answer to (b)(i) (1) Initial rise and buffering region to 25 cm3 (1) Vertical rise at 25 cm3, starting from pH 5.5-7 and ending at pH 10-12 (1) Finishing (asymptotically) between pH 12-13 inclusive and reaching at least 38 cm3 (1) Note: If graph is sketched as if ethanoic acid is added to NaOH, the only mark available is the vertical jump down at 25 cm3, starting from 10-12 and ending at 5.5-7 WCH0401 1501<br />
Question Number 23(b)(iii) Mark (2) Acceptable Answers Reject If no titration curve (0) litmus/azolitmin universal indicator any correct indicator that has the complete pH range within the vertical jump on their titration curve Note: expected indicators numbers 14 to 17 from Data Booklet ie phenol red (6.8-8.4) thymol blue ((base)) (8.0-9.6) phenolphthalein (8.2-10.0) thymolphthalein (8.3-10.6) ALLOW bromothymol blue (6.0-7.6) if their vertical range starts at or below 6.0 (1) Justificationconditional on a correct indicator pH range (of indicator) lies (completely)within the vertical jump (on the titration curve) OR indicator will change colour in the vertical section of the graph OR pH range of indicator and pH range of vertical section of the graph stated as long as they overlap ALLOW pKin (1) is in the mid-point of the vertical jump ALLOW pKin is nearest to the pH at the end/equivalence point ALLOW indicator will change colour at the end/equivalence point (1) IGNORE (because it is a) titration of a weak acid with strong alkali WCH0401 1501<br />
Question Number 23(c) Acceptable Answers Reject Mark (2) HCH3COOH+ for first mark only Just acid and base with no link + + CCl3COO (CH3COOH + CCl3COOH) base (2) acid (1) CH3COOH2 conjugate acid conjugate base / acid 2 /base 2 First marking point both formulae correct (1) Second marking point both conjugate acid-base pairs correctly identified (1) ALLOW any indication of the correct pairs they may be linked together eg lines or arrows, provided they have been labelled correctly as acid or base Note: If equation is CH3COOH + CCl3COOH) acid (2) base (1) CH3COO- + CCl3COOH2 conjugate base conjugate acid / base 2 /acid 2 ALLOW 1 mark for the consequential acid/base pairs (Total for Question 23 = 17 marks) WCH0401 1501<br />

Question 21 22 23 24

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