Primary and secondary cells
This section covers the distinction between non-rechargeable primary cells and rechargeable secondary cells.
Commercial sources of electrical energy
Electrochemical cells can provide electrical energy for practical applications. Commercial cells may be non-rechargeable, rechargeable or fuel cells.
A primary cell is non-rechargeable because the reactions occurring during discharge are effectively irreversible. A secondary cell uses reactions that can be driven in the reverse direction so that the cell can be recharged.
Primary cell
- Used until its reactants have been substantially consumed.
- The discharge reaction is not practically reversed by electrical charging.
- Zinc-carbon cells are an example of a non-rechargeable cell.
Secondary cell
- The discharge reaction can be reversed.
- Electrical energy supplied during charging regenerates the reactants.
- Lead-acid, nickel-cadmium and lithium cells are examples of rechargeable systems.
A primary-cell example
In a zinc-carbon cell, zinc is oxidised at the negative electrode. Its electrode reactions can be combined to give the discharge reaction.
This cell has an EMF of about .
The term battery properly refers to a collection of cells. A car battery, for example, contains several cells connected together.
Exam Tip: If electrode data are given for a commercial cell, identify the more positive reduction reaction, reverse the other half-equation for oxidation, combine the equations and calculate the cell EMF from the values given.