The equilibrium constant and solids

[whsmt85]

Why are the concentrations of solids not incorporated into the equilibrium expression? All the books say it's because the "concentration of a pure solid is constant", but that doesn't make sense to me... If the solid is dissociating to produce a given product, why doesn't the concentration change?

Thanks for your help

 

[Borek]

You drop a nail into the solution.

Does it react?

What is its "concentration"?

 

[whsmt85]

You drop a nail into the solution.

Does it react?

What is its "concentration"?

So I'm guessing that because the nail is so big, whatever ions it loses to the solution don't cause a significant change to its "concentration".

But aren't there any situations where you have a small enough amount of solid that the ions it produces does detract significantly from the solid's original mass?...

Thanks

 

[Borek]

It is not the size that matters, what is important is the fact that reaction takes place only of the surface.

 

[whsmt85]

It is not the size that matters, what is important is the fact that reaction takes place only of the surface.

Alright, but say you have a solid that dissolves 99% into solution, and you are left with only 1% of the original solid, how can one consider the solid's concentration to be constant if only 1% of it is left?..

 

[Borek]

Concentration is already an approximation. Reaction quotient is built using activities of all substances. For diluted solutions activity equals concentration, for solids activity is always 1.

There are several other ways of approaching the problem. They were discussed here on many occasions.