How can pKa values be changed by the environment?
Consider the following reaction:
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,
Now of course we can change that temperature (and in effect CHANGE the environment). We could increase the temperature to
Given that the reaction as written is a BOND-BREAKING reaction, how do you think the equilibrium constant,
It is Le Chatelier's Principle.
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.