Electrochemistry - Dry cell and lead accumulator

Primary Cells (Dry Cell): These are cells where the redox reaction occurs only once and the cell becomes dead after a period of use as the reaction cannot be reversed. The Leclanché cell is the most common example.

Dry Cell Components: The anode consists of a Zinc ($Zn$) container. The cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide ($MnO_2$) and carbon. The electrolyte is a moist paste of ammonium chloride ($NH_4Cl$) and zinc chloride ($ZnCl_2$).

Dry Cell Reactions: At the anode: $Zn(s) ightarrow Zn^{2+}(aq) + 2e^-$. At the cathode: $2MnO_2(s) + 2NH_4^+(aq) + 2e^- ightarrow Mn_2O_3(s) + 2NH_3(g) + H_2O(l)$.

Role of $ZnCl_2$: The $NH_3$ gas produced at the cathode combines with $Zn^{2+}$ ions from $ZnCl_2$ to form a complex ion $[Zn(NH_3)_4]^{2+}$, which prevents the buildup of gas pressure inside the cell.

Secondary Cells (Lead Accumulator): These cells can be recharged by passing current through them in the opposite direction. They can be used through several cycles of discharging and charging.

Lead Accumulator Components: The anode is a lead ($Pb$) plate. The cathode is a grid of lead packed with lead dioxide ($PbO_2$). The electrolyte is a $38\%$ solution of sulphuric acid ($H_2SO_4$) with a density of $1.30 \, g/cm^3$.

Discharging of Lead Accumulator: During discharge, chemical energy is converted to electrical energy. $H_2SO_4$ is consumed, and $PbSO_4$ is deposited on both electrodes.

Recharging of Lead Accumulator: An external source of $EMF$ is applied. The cell reactions are reversed: $2PbSO_4(s) + 2H_2O(l) ightarrow Pb(s) + PbO_2(s) + 2H_2SO_4(aq)$.