How can pKa values be changed by the environment?

2 Answers
anor277
Feb 20, 2016

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?

Bio
Feb 20, 2016

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.

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