# How can pKa values be changed by the environment?

Feb 20, 2016

Consider the following reaction:

$2 {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + H {O}^{-}$

#### 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?

Feb 20, 2016

The $\text{p"K_"a}$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 ${K}_{\text{a}}$ decreases and $\text{p"K_"a}$ increases.

Lowering the temperature will is favorable for the dissociation, thus ${K}_{\text{a}}$ increases and $\text{p"K_"a}$ decreases.

Vice versa if the dissociation is endothermic.