Question

I have a midterm next week so i just wanted to make sure i understand:

When we have a solid in a equation like:

A(s)<---> B(g)+ C(g)

and we add more A why do the equilibrium not change and remain the same

Answer 1

K for the reaction you have written is
(B)(C) = Keq. A doesn't enter into the equilibrium AS LONG AS THERE IS A PINCH OF 'A' THERE. It MUST be there in order to establish the equilibrium but whether there is a pinch, or a handful, or a bucket of it, the concentration of A doesn't change. Said another way, in a solution saturated with compound A with solid A on the floor of the beaker or flask, adding more A will not change the concentration of A (you can't make more of it dissolve) and removing some solid A will not change the concentration of A either because removing it will NOT cause some of it to dissolve or ppt. In other words, the amount of A is a constant as long as there is a pinch of it there at equilibrium so we COULD write:
K =(B)(C)/(A) but since A is a constant, then
K = (B)(C)/k and
K*k = (B)(C) and
Keq = (B)(C).

Answer 2

Thank You, It helped with my question