Question

What are concentration cells?

Answer 1

A type of galvanic cell where the two half-cells are identical except for in concentration.

Explanation:

This is a concentration cell:

As we can see, the two half-cells are completely identical—the two electrodes are both made of solid `Cu`, and the solutions both contain `CuSO_4`.

The only thing that's different in these half-cells is concentration of the `CuSO_4`—in one half cell, its concentration is `0.01M`, and in the other, its concentration is `1M`.

This will cause electrons to migrate in the direction that results in the increase in concentration of the `0.01M` `CuSO_4` cell and the decrease in concentration of the `1M` cell.

So, in our case, electrons will flow from left to right.
In the left half-cell, this causes `Cu(s)` to become `Cu^(2+)`, increasing its concentration.
In the right half-cell, `Cu^(2+)` ions reduce to become `Cu(s)`, decreasing its concentration.