[lsc04361]

In writing the equilibrium quotient(or constant), solid are usually excluded, for example
                S(s) + R(aq) --> P(aq)

              Q = [P]/[R]

The reason is the concentration of the solid is determined by its density which is a constant. So the solid will not affect the rate of reaction.
Is it the answer?


Considering the reaction equilibrium(with solid as one of the reactant), it should be the forward reaction rate = backward reaction rate.
The forward reaction rate, besides the concentration of reactant, also depending on the surface area of the solid.

As the reaction taking place, the solid is smaller and smaller, so the total surface area will be gradually decreasing.
Then the reaction rate should be decreased due to the change of size of the solid.
So the solid isn't a constant factor anymore, but the equilibrium do not take this in account.

For example,

S(s) + R(aq) --> P(aq)

At equilibrium, there is S, R and P in the reaction mixture, the forward and backward rate are equal.
The solid S is then broken into smaller piece by spatula. The forward reaction rate should be increase since the total surface area of the solid is increased, while the backward reaction should remain unchanged since it does not involve reaction surface.




Thanks.