Predict, giving a reason, a difference between the reactions of the same concentrations of hydrochloric acid and ethanoic acid with samples of calcium carbonate.

Dissolved carbon dioxide causes unpolluted rain to have a pH of approximately 5, but other dissolved gases can result in a much lower pH. State one environmental effect of acid rain.

Write an equation to show ammonia, NH₃, acting as a Brønsted–Lowry base and a different equation to show it acting as a Lewis base.

Determine the pH of 0.010 mol dm⁻³ 2,2-dimethylpropanoic acid solution.

K_a (2,2-dimethylpropanoic acid) = 9.333 × 10⁻⁶

Explain, using appropriate equations, how a suitably concentrated solution formed by the partial neutralization of 2,2-dimethylpropanoic acid with sodium hydroxide acts as a buffer solution.

slower rate with ethanoic acid

OR

smaller temperature rise with ethanoic acid

[H⁺] lower

OR

ethanoic acid is weak

OR

ethanoic acid is partially dissociated

Accept experimental observations such as “slower bubbling” or “feels less warm”.

[2 marks]

Any one of:

corrosion of materials/metals/carbonate materials

destruction of plant/aquatic life

«indirect» effect on human health

Accept “lowering pH of oceans/lakes/waterways”.

[1 mark]

Brønsted–Lowry base:

NH₃ + H⁺ → NH₄⁺

Lewis base:

NH₃ + BF₃ → H₃NBF₃

Accept “AlCl₃ as an example of Lewis acid”.

Accept other valid equations such as Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺.

[2 marks]

[H⁺] «$=\sqrt{{\text{K}}_{\text{a}}\times \left[{\text{C}}_5{\text{H}}_{10}{\text{O}}_2\right]}=\sqrt{9.333\times {10}^{-6}\times 0.010}$» = 3.055 × 10^–4 «mol dm^–3»

«pH =» 3.51

Accept “pH = 3.52”.

Award [2] for correct final answer.

Accept other calculation methods.

[2 marks]

(CH₃)₃CCOOH(aq) + OH^–(aq) → (CH₃)₃CCOO^–(aq) + H₂O(l)

OR

(CH₃)₃CCOOH(aq) + OH^–(aq) $\rightleftharpoons$ (CH₃)₃CCOO^–(aq) + H₂O(l) AND addition of alkali causes equilibrium to move to right

(CH₃)₃CCOO^–(aq) + H⁺(aq) → (CH₃)₃CCOOH(aq)

OR

(CH₃)₃CCOO^–(aq) + H⁺(aq) $\rightleftharpoons$ (CH₃)₃CCOOH(aq) AND addition of acid causes equilibrium to move to right

Accept “HA” for the acid.

Award [1 max] for correct explanations of buffering with addition of acid AND base without equilibrium equations.

[2 marks]