Temperature Dependence of pKa

We know from previous discussions that Ka is not an absolute value for a particular substance, because it depends intimately on the surroundings; such as solvent.  Most values are quoted for water at 25ºC.  It is necessary to quote a temperature with a pKa or Ka, because just as with solvent, Ka varies with temperature.

This is, of course, obvious when we consider that Ka is an equilibrium constant; if there is a change in temperature, there will be a concomitant change in equilibrium, and thus the value for Ka must change.

However, the situation is not as simple as we may wish; the values of Ka do not vary uniformly with temperature.  That is to say; just because one acid is stronger than another at room temperature does not mean that the same will be true at a different temperature.

This means that all of our previous discussions that attempted to relate the orders of pKa values to the structure of molecules are only valid at a specific temperature.

For example, the following molecules reverse their strength trend from 25ºC to 30ºC:

Below 30ºC, ethanoic acid is weaker than 2-ethylbutanoic acid, however, above 30ºC, it is stronger.

These reversals are in fact quite common, and lead us to be cautious about making sweeping statements relating structure to acidity.

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