QUESTION 127 (5 marks)
The table gives measured pH values for pure water at two temperatures. Use the self-ionisation equilibrium to analyse the observations.
| Temperature / °C | pH of pure water |
|---|---|
| 25 | 7.00 |
| 60 | 6.51 |
(a) Calculate Kw at 60 °C. [2 marks]
(b) Determine whether self-ionisation of water is endothermic or exothermic. Explain using the data. [2 marks]
(c) Explain why the water at 60 °C is neutral despite its pH being below 7. [1 marks]
Practice marking scheme
Answer
at 60 °C. Self-ionisation is endothermic. Pure water remains neutral because .
Working
At 60 °C , and pure water has the same hydroxide concentration. Thus . Heating lowers the pH of pure water, increasing both ion concentrations and favouring ion formation; the forward ionisation is endothermic. Neutrality is equality of the two ion concentrations, not a fixed pH of 7 at every temperature.
Marking criteria
- Uses equality of H⁺ and OH⁻ concentrations for pure water. [1 mark]
- Calculates Kw = 9.55 × 10⁻¹⁴. [1 mark]
- Uses the lower pH to infer increased ion concentrations on heating. [1 mark]
- Concludes that ionisation is endothermic and connects heating to the forward shift. [1 mark]
- Explains neutrality as [H⁺] = [OH⁻]. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.