Assessment Schedule 2017 Chemistry: Demonstrate understanding of equilibrium principles in aqueous systems (91392)

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1 NCEA Level 3 Chemistry (91392) 2017 page 1 of 5 Assessment Schedule 2017 Chemistry: Demonstrate understanding of equilibrium principles in aqueous systems (91392) Evidence Statement ONE (a)(i) HF + H 2 O F + H 3 O + HBr + H 2 O Br + H 3 O + ONE correct equation. To conduct a current, a substance requires mobile charged particles / ions. HF and HBr solutions have ions in solution, so both will conduct electricity. HBr is a strong acid, and therefore completely dissociates to produce a high [Br ] and [H 3 O + ]. In contrast, HF is a weak acid and therefore only partially dissociates to produce a lower [ions], i.e. F and H 3 O +. So, HF is a poorer electrical conductor / weaker electrolyte. Mobile charged particles / ions. Relates electrical conductivity to the degree of dissociation / ion concentration. TWO species related to concentration. Links the degree of dissociation to the amount of ions for both HBr and HF. (Correct follow on accepted from incorrect equations) Fully compares and contrasts the electrical conductivity of HBr and HF, i.e. links the degree of dissociation to the amount of ions (not only H 3 O + ), and relates this to conductivity. AgBr Ag + + Br Correct equation. Br is a common ion / added to solution An increase in [Br - ] will result in the reverse reaction being favoured, to restore equilibrium / minimise the change. This results in a decrease in [Ag + ] (since Ag + reacts with some of the added Br to help use it up) / AgBr becomes less soluble, (until [Ag + ][Br ] is again equal to K s.) Solubility of AgBr or [Ag + ] decreases. Links increase in [Br ] to reverse reaction being favoured a decrease in solubility of AgBr [Ag + ] decreasing. Full explanation linking an increase in [Br ] to a shift in equilibrium towards the reactants, and subsequent decrease in the solubility of AgBr [Ag + ] decreasing. (iii) AgBr Ag + + Br K s (AgBr) = [Ag + ] [Br ] = [Ag + ] 65 [Ag + ] = mol L 1 Correct K s expression. Correct calculation Correct calculation with unit and significant figures. No response; no relevant evidence 1a 2a 4a 5a 2m 3m 2e 3e

2 NCEA Level 3 Chemistry (91392) 2017 page 2 of 5 TWO (a)(i) [H 3 O + ] = ( K w [NH 3 ]) [H 3 O + ] = ( ) One step correct. Correct answer, with minor error e.g. significant figures. Correct answer, including [H 3 O + ] = moll 1 ph = -log ( ) = 11.1 NH H 2 O NH 3 + H 3 O + = [NH 3 ][H 3 O+ ] [NH 4 + ] = 5 [H 3O + ] 1 [H 3 O + ] = ph = 9.94 p = 9.24 Correct process for determining ph. Buffer use / concept Correct ph. Correct evaluation of the buffer. Correct ph, including AND Correct evaluation of the buffer. The solution will function as a buffer (as it contains a weak base and conjugate acid in a 5:1 ratio / within a 1:10 ratio / within a ph range of ). Since the [NH 3 ] > [NH 4 + ] / ph > p, the buffer will be more effective at neutralising added strong acid.

3 NCEA Level 3 Chemistry (91392) 2017 page 3 of 5 Cu(OH) 2 Cu OH K s = [Cu 2+ ] [OH ] 2 (i) or correct. (iii) Let s = solubility [Cu 2+ ] = s [OH ] 2 = (2s) 2 K s = 4s 3 4s 3 = So; s = mol L 1 Correct process for calculating solubility. Correct answer for solubility, with appropriate unit. (c) When copper(ii) hydroxide is dissolved in an acidic solution, the H 3 O + ions neutralise the OH ions / H 3 O + + OH 2H 2 O A decrease in [OH - ] will result in the forward reaction being favoured, to restore equilibrium / minimise the change. This causes more solid Cu(OH) 2 to dissolve, i.e. the solubility of Cu(OH) 2 increases / so that [Cu 2+ ][OH ] will again equal K s. Recognises OH ions are reacting with acid. Partial explanation for an increase in solubility. Complete explanation for the increased solubility of Cu(OH) 2 in an acidic solution. No response; no relevant evidence 1a 2a 3a 4a 3m 4m 1e 3e

4 NCEA Level 3 Chemistry (91392) 2017 page 4 of 5 THREE (a) Phenolphthalein. Correct indicator. CH 3 COOH + H 2 O CH 3 COO + H 3 O + = COO ][H 3 O + ] ( ) = Correct process with minor error. Correct calculation. = mol L 1 n(naoh) added = mol L L = mol Initial n(ch 3 COOH) = mol L L = mol n(ch 3 COOH) remaining in 30 ml = mol mol = mol n(ch 3 COO ) in 30 ml = mol Correct n(naoh) Initial n(ch 3 COOH). Two steps correct Correct ph, including = COO ][H 3 O + ] [H 3 O+ ] = [H 3 O + ] = ph = log ph = 4.47

5 NCEA Level 3 Chemistry (91392) 2017 page 5 of 5 (c)(i) CH 3 COO, Na +, CH 3 COOH, OH, H 3 O + THREE species. Both titrations produce a basic salt at the equivalence point because [OH ] > [H 3 O + ] / ph > 7 / equations. At the equivalence point the ethanoic acid is converted into the ethanoate ion and the methanoic acid is converted into the methanoate ion. The methanoic acid titration has a lower ph than the ethanoic acid titration at the equivalence point because: methanoate ions are a weaker base / produce fewer OH ions than ethanoate ions HCOOH has a larger than CH 3 COOH, as it is a stronger acid. As a result, its conjugate base, HCOO, is weaker than CH 3 COO, so the ph will be lower at the equivalence point. Both are basic HCOOH is a stronger acid than CH 3 COOH HCOO is weaker than CH 3 COO Links equivalence point ph for either, to the relative strength of the conjugate base relative concentrations of hydroxide ions. Compares and contrasts the equivalence point for both titrations. No response; no relevant evidence 1a 2a 3a 4a 2m 3m 2e (with minor error / omission) 2e Cut Scores Not Achieved Achievement Achievement with Merit Achievement with Excellence