Explain why a non-rechargeable alkaline battery must be replaced once it stops working, but a rechargeable cell does not need to be replaced in the same way.

In a non‑rechargeable alkaline battery the discharge reaction consumes the zinc anode and the manganese dioxide cathode. When the zinc is depleted or the manganese dioxide is exhausted, the reaction cannot proceed any further, so no more current can be produced and the battery must be replaced.

In a rechargeable cell the same reactions are reversible. When an external electric current is applied, the cathode and anode reactions are driven in the opposite direction, regenerating the original zinc and manganese dioxide. The reactants are restored, so the cell can be used again and does not need to be replaced.