The effect of temperature on Kw and pH
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Your students may have met equilibrium constants previously in their study of chemical equilibria as another topic and they may know, or need reminding, that the equilibrium constant varies with temperature. Similarly, Kw varies with temperature.
Consider the equilibrium of the self-ionisation of water once again:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
Try the following activity showing some examples where the temperature is increased or decreased. What are the pH values and why?
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[encouraging thinking about the temperature dependence of pH and why this occurs. Equilibria shifts.]
Why is the pH value lower than 7 when the temperature is increased above 25 °C?
As temperature of pure water is increased above 25 °C, the equilibrium shifts slightly to the right, favouring more H3O+ ions in solution. A higher concentration of H3O+ ions gives a lower pH value.
Consider the above examples and equilibrium. Is the forward reaction exothermic or endothermic?
The forward reaction is endothermic. As heat is added to the system, the pH is lowered, ie more H3O+ ions are favoured, therefore the equilibrium must have shifted to the right.
In the above examples, pure water is shown to have a range of pH values different from pH 7. What can be said about the relative number of H3O+ and OH– ions?
In pure water, each water molecule dissociates to produce one H3O+ ion and one OH– ion. Therefore, it is still neutral, even if the pH is not 7. The condition of neutrality is [H+] =[OH–].
How might this concept be introduced to the class in order to get students to critically engage with the topic and think creatively?
The class could be given a lateral thinking question, based on, for example, 'a neutral salt solution is pH 6.5', giving them the opportunity to discuss and explore their own ideas creatively.