I think without excess iron, the system could not reach a state of equilibirum.
The net equation is:
Fe
(s) + Cu
2+ Fe
2+ + Cu
(s)Originally we have only Cu2
+ ions in solutions. When we add the Fe
(s), the right hand side of the equation becomes favourable. As such, Fe
(s) dissociates into Fe
2+ ions until the ratio of (Fe
2+/Cu
2+) = k (a fixed value at that temperature).
We end up with an excess of Fe
(s) and Cu
(s) in the solution.
So what would happen if we did NOT add excess pulverised iron but only a small amount.
I suppose we could not create sufficient Fe
2+ and hence equilibirum is not attainable.