Does anyone know what effect temperature has on pKa for carboxylic acids?
You want to be careful about just memorizing the answer to this problem. Most general chem exams will have a question that relates temperature and acids, because it's a great way to also test your understanding of Le Chatelier and pKa. You want to be able to work through why
raising the temperature will decrease pKa, because that way you will never get mixed up or confused on an exam.
The best way to think of this is to remember that breaking a hydrogen bond, even a weak one, requires energy. This means that the reaction described by a Ka value is endothermic
. Now, what does Le Chatelier tell us about an endothermic reaction when heat is "added"?
HA + heat
Note that as you add heat, the reaction is "pushed" to the right, and so more products will be formed, ie, more H+
. Now, Ka is defined as ([H+
]/[HA]). An increase in H+
the value of Ka.
Thus, Ka increases with temperature, and the acid will become stronger. Since the -log(x) decreases as x gets large, an increasing Ka will have a correspondingly lower pKa.