Leclanché Cell

The Leclanché cell, more familiarly known as the ordinary dry cell, is the classic primary battery used in torches, remote controls and countless small devices. Its construction is:

• Anode: the zinc container itself, which also serves as the cell's outer casing.
• Cathode: a graphite rod in contact with manganese dioxide (MnO₂).
• Electrolyte: a moist paste of ammonium chloride and zinc chloride in water.
• Emf: approximately 1.5 V.

At the anode, zinc is oxidised: $\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^{-}$. At the cathode, ammonium ions are reduced, releasing hydrogen gas: $2\text{NH}_4^{+}\text{(aq)} + 2e^{-} \rightarrow 2\text{NH}_3\text{(aq)} + \text{H}_2\text{(g)}$. This hydrogen gas is then oxidised by the manganese dioxide packed around the cathode, $\text{H}_2\text{(g)} + 2\text{MnO}_2\text{(s)} \rightarrow \text{Mn}_2\text{O}_3\text{(s)} + \text{H}_2\text{O(l)}$, which is essential to the cell's operation because it prevents hydrogen gas from simply accumulating at the cathode and polarising the cell (a build-up of gas would insulate the electrode and stop the reaction). Summing all three steps gives the overall cell reaction: $\text{Zn(s)} + 2\text{NH}_4^{+}\text{(aq)} + 2\text{MnO}_2\text{(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Mn}_2\text{O}_3\text{(s)} + \text{H}_2\text{O(l)} + 2\text{NH}_3\text{(aq)}$. …