How does pKa change with environment?

1 Answer
anor277
Jan 31, 2016

Good question. #pK_a#'s should DECREASE with INCREASING temperature. I will confine my discussion to the water molecule, but the argument should be generalizable.

Explanation:

Now autoprotolysis of water is a well-known (and well documented) reaction:

#2H_2O rightleftharpoons H_3O^+ + HO^-#

At #298# #K# , #K_w# #=# #10^-14#; i.e #pK_w# #=# #14#.

Because the reaction AS GIVEN is BOND-BREAKING (i.e we have to break a strong #O-H# bond), at higher temperatures, the equilibrium should lie farther to the right. So at #T# #># #298# #K#, #pK_w# #<# #14#.

Does this make sense?

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