Part B correct As H+ ions don’t feature in the net ionic equation, either as reactants or products, the change in pH must be due to the acids themselves Part C at least one of each type of reaction As H3AsO3 solution has a higher pH than H3AsO4 solution it must react to a lesser extent with water discussed correctly with values included. E so H3AsO3 is a weaker acid than H3AsO4 (c) 5 6 marks – The precipitation reactions are in the form (where = 3 or 4) x an understanding of principles of 3Ag+(aq) + AsOx3–(aq) Ag3AsOx(s) chemistry by: On addition of acid Part A Completes (a) and some progress on (b) AsOx3–(aq) + H3O+ HAsOx2–(aq) + H2O and (c). The weaker acid has the stronger base so AsO33– reacts with ethanoic acid sufficiently to lower the Part B correct with one arithmetical error concentration of AsO33– to the extent that is not exceeded. Ks(Ag3AsO3) This does not happen with the weaker base (of the stronger acid), AsO43–, and the weak acid and it acceptable requires a strong acid to dissolve the precipitate. Part C Shows an understanding of the Part B 2Al + 3O2 Al2O3 requirements and at least one equation correct. 0.70% of 46.0 g Al = 0.322 g (assuming 0.70% by mass) n(Al) = 0.322/27 mol 1 4 marks = 0.0119 mol – Al Al3+ + 3e Shows an understanding of principles of = 3 x 0.0119 mol n(electrons) chemistry by: = 0.0358 mol Part A completes (a) but little progress with Which is equal to 0.0358 F or parts (b) and (c) 3452 C Part B Correct method but incomplete or 3452 = 0.850 x t t = 3452/0.850 incorrect answer. = 4061 seconds Part C Shows some understanding of what is or 1.13 hours (67.68 min) required
(c) As H+ ions don’t feature in the net ionic equation, either as reactants or products, the change in pH must be due to the acids themselves As H3AsO3 solution has a higher pH than H3AsO4 solution it must react to a lesser extent with water so H3AsO3 is a weaker acid than H3AsO4 The precipitation reactions are in the form (where x = 3 or 4) 3Ag+(aq) + AsOx3–(aq) Ag3AsOx(s) On addition of acid AsOx3–(aq) + H3O+ HAsOx2–(aq) + H2O The weaker acid has the stronger base so AsO33– reacts with ethanoic acid sufficiently to lower the concentration of AsO33– to the extent that Ks(Ag3AsO3) is not exceeded. This does not happen with the weaker base (of the stronger acid), AsO43–, and the weak acid and it requires a strong acid to dissolve the precipitate. Part B correct Part C at least one of each type of reaction discussed correctly with E values included. 5 – 6 marks an understanding of principles of chemistry by: Part A Completes (a) and some progress on (b) and (c). Part B correct with one arithmetical error acceptable Part C Shows an understanding of the requirements and at least one equation correct. 1 – 4 marks Shows an understanding of principles of chemistry by: Part A completes (a) but little progress with parts (b) and (c) Part B Correct method but incomplete or incorrect answer. Part C Shows some understanding of what is required Part B 2Al + 3O2 Al2O3 0.70% of 46.0 g Al = 0.322 g (assuming 0.70% by mass) n(Al) = 0.322/27 mol = 0.0119 mol Al Al3+ + 3e n(electrons) = 3 x 0.0119 mol = 0.0358 mol Which is equal to 0.0358 F or 3452 C 3452 = 0.850 x t t = 3452/0.850 = 4061 seconds or 1.13 hours (67.68 min)
nzic_2014_schol_answers.docx