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Chemistry · Ch 5 — Electrochemistry

Dry cell (Leclanche' cell)

5.10.1

Dry cell (Leclanche' cell)

The dry cell is a cell without a liquid component — but the electrolyte is not completely dry: it is a viscous aqueous paste.

Construction : The container of the cell is made of zinc, which serves as the anode (-). It is lined from inside with a porous paper to separate it from the other material of the cell. An inert graphite rod in the centre of the cell, immersed in the electrolyte paste, serves as the cathode (+). It is surrounded by a paste of manganese dioxide (MnO2\mathrm{MnO_2}) and carbon black. The rest of the cell is filled with the electrolyte — a moist paste of ammonium chloride (NH4Cl\mathrm{NH_4Cl}) and zinc chloride (ZnCl2\mathrm{ZnCl_2}). Some starch is added to the paste to make it thick, so that it cannot be leaked out. The cell is sealed at the top to prevent drying of the paste by evaporation of moisture. See Fig. 5.8.

Figure 5.8Cutaway of a Leclanche dry cell: zinc container anode lined with a paper spacer, central graphite rod cathode capped with brass, surrounded by manganese dioxide-carbon paste inside the ammonium chloride-zinc chloride electrolyte paste.
Fig. 5.8 — Cutaway of a Leclanche dry cell: zinc container anode lined with a paper spacer, central graphite rod cathode capped with brass, surrounded by manganese dioxide-carbon paste inside the ammonium chloride-zinc chloride electrolyte paste.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this figure shows. The nested construction of the dry cell, from the centre outward: the graphite rod cathode topped by the brass cap (+); the paste of MnO₂ + carbon packed around the rod; the paste of NH₄Cl + ZnCl₂ electrolyte filling the rest; the paper spacer lining; and the **Zn contain …

Cell reactions:

i. Oxidation at anode : When the cell operates, the current is drawn from the cell and metallic zinc is oxidised to zinc ions:

Zn (s)⟶Zn2+ (aq)+2e−\mathrm{Zn\,(s) \longrightarrow Zn^{2+}\,(aq) + 2e^-}

ii. Reduction at cathode : The electrons liberated in the oxidation at the anode flow along the container and migrate to the cathode. At the cathode, NH4+\mathrm{NH_4^+} ions are reduced:

2NH4+ (aq)+2e−⟶2NH3 (aq)+H2 (g)\mathrm{2NH_4^{+}\,(aq) + 2e^- \longrightarrow 2NH_3\,(aq) + H_2\,(g)}

The hydrogen gas produced in the reduction reaction is oxidised by MnO2\mathrm{MnO_2}, which prevents its collection on the cathode:

H2 (g)+2MnO2 (s)⟶Mn2O3 (s)+H2O (l)\mathrm{H_2\,(g) + 2MnO_2\,(s) \longrightarrow Mn_2O_3\,(s) + H_2O\,(l)}

The net reduction reaction at the cathode is the combination of these two reactions:

2NH4+ (aq)+2 MnO2 (s)+2e−⟶Mn2O3 (s)+2 NH3 (aq)+H2O (l)\mathrm{2NH_4^{+}\,(aq) + 2\,MnO_2\,(s) + 2e^- \longrightarrow Mn_2O_3\,(s) + 2\,NH_3\,(aq) + H_2O\,(l)}

iii. Net cell reaction : The net cell reaction is the sum of the oxidation at the anode and the reduction at the cathode:

Zn (s)+2 NH4+ (aq)+2 MnO2 (s)⟶Zn2+ (aq)+Mn2O3 (s)+2 NH3 (aq)+H2O (l)\mathrm{Zn\,(s) + 2\,NH_4^{+}\,(aq) + 2\,MnO_2\,(s) \longrightarrow Zn^{2+}\,(aq) + Mn_2O_3\,(s) + 2\,NH_3\,(aq) + H_2O\,(l)}

The ammonia produced combines with Zn2+\mathrm{Zn^{2+}} to form a soluble compound containing the complex ion:

Zn2+ (aq)+4 NH3 (aq)⟶[Zn(NH3)4]2+ (aq)\mathrm{Zn^{2+}\,(aq) + 4\,NH_3\,(aq) \longrightarrow [Zn(NH_3)_4]^{2+}\,(aq)}

Note

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