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Edexcel Jun 2016 (IAL) Paper 4 Q
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![Section B Question Number 11(a) Question Number 11(b)(i) Mark (2) Mark (2) Acceptable Answers Reject [H3O+]2 numerator [H3O+]2 numerator (aq)] (Ka1 =) [H3O+(aq)] [HCO3 (1) [H2CO3(aq)] (Ka2 =) [H3O+(aq)][ CO3 (1) [HCO3 ALLOW H+(aq) for H3O+(aq) IGNORE state symbols, even if incorrect 2(aq)] (aq)] Acceptable Answers Reject H2CO3(aq)H3O+(aq) + HCO3 (aq) ] = 2.04 x 10-4 / 0.000204 (mol dm- Initially 0.100 0 0 At eqm 0.100 x x Ka1 = x2 0.100 M1 x2 = 4.17 x 10-8 (mol2 dm-6) (1) M2 (x = 2.0421 x 10-4) [HCO3 3) (1) Final answer for M2 must be to 3 SF Correct final answer without working scores (2) ALLOW M2 TE on candidates value for M1, as long as final answer to 3 sf IGNORE units even if incorrect Acceptable Answers Question Number 11(b)(ii) (pH = -log 2.04 x 10-4 =) 3.69 / 3.7 Reject pH = 4 Mark (1)<br />
TE on answer to (b)(i), provided pH <7 pH = 3.19 / 3.2 from a [HCO3 6.46 x 10-4 (mol dm-3) ALLOW any SF except 1 SF ] value of pH = 3 Question Number *11(b)(iii) Max 2 if HA and Aused for H2CO3 Acceptable Answers ] OR [H+] = [HCO3 Any THREE from: Assumption 1 [H2CO3]equilibrium = [H2CO3]initial OR The dissociation of H2CO3/the acid is negligible OR 2.04 x 10-4 is (very) small compared to the initial concentration of H2CO3/0.100 (hence a valid assumption), or reverse argument Assumption 2 [H3O+] = [HCO3 OR Negligible H+ from (the dissociation of) water / H+ only from H2CO3 Assumption 3 Negligible dissociation of HCO3 HCO3 OR Ka2 very much smaller than Ka1 ALLOW Stage 2 does not occur (significantly) Assumption 4 Measurements at 298 K / standard temperature IGNORE References to the concentration of water References just to standard conditionsdoesnt (significantly) dissociate further ]/ Reject Mark (3)<br />
Mark (5) Reject pH rising by half a square or more at any point Acceptable Answers Question Number 11(c) M1 General shape of a weak base-strong acid curve with pH decreasing and either one or two vertical sections shown. Penalise vertical section8 pH units (1) M2 (Approximately) vertical section at 10 cm3 (1) M3 (Approximately) vertical section at 20 cm3 (1) M4 Any (mid-point of a) vertical section identified as an equivalence point (1) M5 Initial pH = 11-12 and line becomes asymptotic to pH range 1-2 ALLOW a recognisable plateau in the pH range 12 If curve is the wrong way round the starting pH should be pH 1-2 and the asymptote or plateau should be pH 11-12 Curve does not need to reach 40 cm3 (1)<br />
ALLOW Two vertical sections not at 10/20 cm3 scores (1) if M2 and M3 not awarded (Total for question 11 = 13 Marks) Reject Mark Acceptable Answers Question Number 12(a)(i) Effervescence / bubbles / fizzing IGNORE gas evolved / temperature increase Acceptable Answers Question Number *12(a)(ii) A statement that entropy is positive needs to be made once only and can be used to award M1 and M2 Penalise omission of statement that entropy is positive once only M1 Entropy (of the system) positive and solid and liquid reactants form (a solid, a liquid and) a gas ALLOW gas formed / gas is a product (1) M2 Entropy (of the system) positive and EITHER 3 moles4 moles OR more moles of products (than reactants) ALLOW molecules for moles OR More ways of distributing energy OR (1) Mark (2) Reject If entropy of system is negative / decreases scores (0) particles<br />](https://learnah.org/wp-content/uploads/2018/08/June_2016_IAL_MS_-_Unit_4_Edexcel_Chemistry_11-0.jpg)
