Potential Difference, Equilibrium and Concentration

Potential difference, equilibrium and concentration

Again we can use the zinc-copper cell as an example.

When the chemical reaction between the zinc and the copper slows, the increase in product concentration, and the decrease in reactant concentration, slow too. This means that the electron transfer rate will decrease.

When the chemical reaction in the cell stops:

• The reaction is no longer converting chemical potential energy to electrical potential energy.

• The concentrations of the reactants and products have become constant and equilibrium has been reached.

• There is no excess or deficit of electrons on the electrodes, and the potential difference of the cell is \(\text{0}\).

• A potential difference of \(\text{0}\) means that the current is \(\text{0}\).

So the potential difference across a cell is related to the extent to which the cell reaction has reached equilibrium. When equilibrium is reached, the potential difference of the cell is zero and the cell is said to be 'flat'. There is no longer a potential difference between the two half-cells, and no more current will flow.