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What is the difference between K and K_a?

The letter K is usually used to represent the equilibrium constant for a chemical reaction. All chemical reactions eventually approach equilibrium (which is the condition where there are no changes in the concentrations of the reactants or products over time). Depending on which reaction you choose, and what the temperature is, there may be much more of the products than reactants present at equilibrium and in this case K will be a large number. If the concentrations of the reactants are much larger than that of the products at equilibrium then K will be a very small number.

All chemical reactions have a value of K associated with them. There is a class of reactions in which a molecule (or ion) splits into two fragments, one of which is a H⁺ ion. For example:

HF(aq) F^-(aq) + H⁺(aq)

These reactions are called acid dissociation reactions - they are very common and important in chemistry and the equilibrium constant for them is given the name K_a. So for the reaction above the value of the equilibrium constant is calculated from the concentrations of the reactants and products at equilibrium:

Just like a 'stop' sign is one particular type of road sign, so K_a is one particular type of equilibrium constant. The mathematical expression above could have been written with K (instead of K_a) on the left hand side. This would have been perfectly correct, but because the reaction is an acid dissociation reaction we can be more specific and replace K with K_a.

If you would like to make any comments about this article, please e-mail the author, Dr Ron Haines.