QUESTION 24 (5 marks)
This table shows the effect of temperature on the pH of pure water.
Temperature (°C) | pH |
10 | 7.27 |
15 | 7.17 |
20 | 7.08 |
25 | 7.00 |
30 | 6.92 |
50 | 6.63 |
a) Analyse the data to explain whether the self‐ionisation of water is endothermic or exothermic. Explain your reasoning.
[3 marks]
b) Calculate the Kw of pure water at 50 °C. Show your working.
[2 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
24 | a) | As the temperature increases, the [H3O+] increases. 2H2O(l) ⇌ H3O+(aq) + OH−(aq) Therefore, the equilibrium shifts towards the products. Increasing temperature shifts equilibrium in the endothermic direction, therefore the self-ionisation of water is endothermic. | • identifies [H3O+] increases as temperature increases [1 mark] • identifies equilibrium shifts towards the products and the endothermic direction [1 mark] • determines self-ionisation of water is endothermic [1 mark] | Allow FT error for: equilbrium shifts to the reactants self-ionisation of water decreases. |
b) K | pH = –log [H+] = 6.63[H+] = 10−6.63= 2.34 x 10−7KKw = [H+] [OH−] = (2.34 x 10−7)2KKw = 5.48 x 10−14(to 3 significant figures) | • determines[H+] = 2.34 x 10−7[1 mark]• determines consequentially correct Kw[1 mark] | Allow FT error from [H+]. Do not penalise for incorrect decimal places/significant figures. |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.