(i) $\ce{HCN(aq) <=> H+(aq) + CN-(aq)}$ ✔

OR

$\ce{HCN(aq) + H2O(l) <=> H3O+(aq) + CN-(aq)}$

(ii) Any two of:

measure the pH «with a meter or universal indicator» AND the $\ce{HCN}$ solution has the higher pH ✔

measure the electrical conductivity AND the $\ce{HCN}$ solution is the poorer conductor ✔

add a reactive metal / a metal carbonate / magnesium AND the $\ce{HCN}$ solution reacts more slowly «fewer bubbles per second» ✔

titrate with a strong base AND the $\ce{HCN}$ titration has an equivalence point above pH 7 / a smaller temperature rise ✔

(iii) $[\ce{H+}] = \sqrt{K_\mathrm{a} c} = \sqrt{6.25 \times 10^{-10} \times 0.160} = 1.00 \times 10^{-5}$

pH «$= -\log_{10}(1.00 \times 10^{-5})$» = 5.00 ✔

(iv) sigmoidal curve AND equivalence point above pH 7 ✔

curve starts between pH 4 and 6 «pH 5.0, from (iii)» AND finishes between pH 11 and 14 «about 12.7» ✔

steep, almost vertical rise at 16.0 $\mathrm{cm^3}$ «$n(\ce{HCN}) = 0.0250 \times 0.160 = 4.00 \times 10^{-3}$

(v) equivalence point «on the vertical section at 16.0 $\mathrm{cm^3}$» labelled "Salt" ✔

(vi) amount neutralized «$= n(\ce{HCN}) = 0.0250 \times 0.160$» $= 4.00 \times 10^{-3}$

$\Delta T = \dfrac{4.00 \times 10^{-3} \times 12\,100}{50.0 \times 4.18} = \dfrac{48.4}{209}$

(i) [1] either equation with the equilibrium arrow. (ii) [2 max] one mark per method together with its observation; award [1] for two correct methods without observations; answers must refer to rates, not amounts. (iii) [2] M1 [H+]; M2 pH; award [2] for a correct final answer. (iv) [3] M1 shape and equivalence pH above 7; M2 starting and finishing pH; M3 vertical section at 16.0 cm3. (v) [1] the label at the equivalence point of the student's own curve. (vi) [2] M1 amount of HCN neutralized; M2 temperature rise; award [2] for a correct final answer; ECF from M1.