Describe the chemical reactions in a zinc-carbon cell.

In a zinc-carbon cell, zinc oxidises to form zinc ions and electrons, while manganese dioxide reduces to form manganese ions.

A zinc-carbon cell, also known as a Leclanché cell, is a type of primary cell (non-rechargeable battery) that uses zinc and manganese dioxide as its electrodes. The chemical reactions that occur in this cell are fundamental to its operation and are a classic example of redox (reduction-oxidation) reactions.

The anode of the cell is made of zinc. When the cell is in use, the zinc anode oxidises, meaning it loses electrons. This process can be represented by the following half-equation: Zn(s) → Zn2+(aq) + 2e-. The zinc atoms on the surface of the anode change into zinc ions (Zn2+) and enter the electrolyte, leaving behind two electrons in the metal.

The cathode of the cell is a rod of carbon surrounded by a mixture of manganese dioxide and carbon powder. The manganese dioxide acts as a depolariser, accepting electrons from the carbon rod and being reduced in the process. The reduction half-equation is: MnO2(s) + H2O(l) + e- → MnO(OH)(s) + OH-(aq). This reaction produces manganese hydroxide and hydroxide ions.

The overall reaction in the cell can be represented as: Zn(s) + 2MnO2(s) + 2NH4Cl(aq) → Zn(NH3)2Cl2(aq) + Mn2O3(s). This shows that the zinc and manganese dioxide react with ammonium chloride in the electrolyte to produce a complex zinc-ammonium ion, manganese(III) oxide and water.

The electrons produced at the anode travel through the external circuit to the cathode, providing the electrical energy that powers the device connected to the battery. The flow of ions in the electrolyte completes the circuit, allowing the cell to continue operating until one or both of the reactants are used up. The zinc-carbon cell is a reliable and cost-effective source of power, although it has been largely superseded by alkaline batteries, which offer better performance and longer shelf life.