Question

How can pKa values be changed by the environment?

Answer 1

Consider the following reaction:

`2H_2O(l) rightleftharpoons H_3O^+ + HO^-`

Explanation:

Obviously, this reaction represents the autoprotolysis of water. This is an equilibrium reaction that has been measured at various temperatures. Normally, the equilibrium constant for this reaction, `K_w`, is quoted as `10^-14`, and the temperature is specified to be `298` `K`, i.e. standard laboratory temperature.

Now of course we can change that temperature (and in effect CHANGE the environment). We could increase the temperature to `348` `K` or even `373` `K`.

Given that the reaction as written is a BOND-BREAKING reaction, how do you think the equilibrium constant, `K_w`, would evolve? Would it become smaller or larger?

Answer 2

The `"p"K_"a"`of an acid can be changed by changing the surrounding temperature.

Explanation:

It is Le Chatelier's Principle.

If the dissociation of the acid is exothermic:

Raising the temperature will is unfavorable for the dissociation, thus `K_"a"` decreases and `"p"K_"a"` increases.

Lowering the temperature will is favorable for the dissociation, thus `K_"a"` increases and `"p"K_"a"` decreases.

Vice versa if the dissociation is endothermic.