What is a ion product constant?

1 Answer
Apr 23, 2017

It's the same thing as an equilibrium constant


An ion product constant is basically the same thing as an equilibrium constant. This is described mathematically as follows:

For the following reaction:

#aA + bB rightleftharpoons cC + dD#

#K# (i.e. the equilibrium constant) # = ([C]^c[D]^d)/([A]^a[B]^b)#

Note that if a species in the reaction has a coefficient, you raise it to the power of that coefficient.

If you need extra help with this concept, this is a great video by Bozeman Science that explains it in much more clarity.

The term "Ion Product Constant" is more specific to when you you have ions in your products. The most popular one you'll see is the Ion Product Constant of Water, which is shown below:

For the dissociation reaction of water:

#H_2O_(l) rightleftharpoons H_3O^+ + OH^-#

#K_w# (i.e. ion product of water) #= [H_3O^+][OH^-]#

Note that the water is not present in the equilibrium expression, since pure liquids and solids are omitted from it.

As you can see, you have a product of ion concentrations, which is why the term ion product constant is used.

To sum up, mathematically an ion product constant is the same thing as an equilibrium constant, but is specific to when you have ions in the products.

Hope that helps :)