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A Zinc-carbon dry cell or battery is packaged in a zinc can that serves as both a container and anode. It was developed from the wet Leclanché cell (pronounced /lɛklɑːnˈʃeɪ/). "Super" or "Heavy Duty" batteries, technically called zinc chloride cells, are an improved version from the cheaper "General Purpose" variety. The cathode is a mixture of manganese dioxide and carbon powder. The electrolyte is a paste of zinc chloride and ammonium chloride dissolved in water. Zinc-carbon batteries are the least expensive primary batteries and thus a popular choice by manufacturers when devices are sold with batteries included. They can be used in remote controls, flashlights, toys, or transistor radios, where the power drain is not too heavy.

Contents 1 A primary cell 2 Chemical Reactions 3 Leakage 4 The Zinc Chloride Cell 5 References

A primary cell

A Zinc-carbon dry cell is described as a primary cell because as the cell is discharged, it is not intended to be recharged and must be discarded. "Battery Rejuvenators" were once marketed to restore partially discharged Zinc-carbon cells by applying a reverse current to them. However the effects of such devices were only temporary and prone to cause the cell to leak or burst. [1] Zinc-carbon cells are more likely to leak as the anode is the container. The first ever zinc-carbon battery was invented in Selsdon, Croydon in Greater London in 1834.

Chemical Reactions

The container of the zinc-carbon dry cell is a zinc can. This contains a layer of NH₄Cl with ZnCl₂ aqueous paste separated by a paper layer from a mixture of powdered carbon & manganese (IV) oxide (MnO₂) which is packed around a carbon rod.

In a dry cell, the outer zinc container is the anode (-). The zinc is oxidised according to the following half-equation.

Zn(s) → Zn²⁺(aq) + 2 e^-

A graphite rod surrounded by a powder containing manganese(IV) oxide is the cathode(+). The manganese dioxide is mixed with carbon powder to increase the conductivity of the cathode mixture. The cathode reaction is as follows:

2MnO₂(s) + 2 H⁺(aq) + 2 e^- → Mn₂O₃(s) + H₂O(l)

The H⁺ comes from the NH₄⁺(aq):

NH₄⁺(aq) → H⁺(aq) + NH₃(aq)

and the NH₃ combines with the Zn²⁺.

In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3).

There are other possible side-reactions, but the overall reaction in a zinc-carbon cell can be represented as:

Zn(s) + 2MnO₂(s) + 2NH₄⁺(aq) → Mn₂O₃(s) + Zn(NH₃)₂²⁺(aq) + H₂O(l)

The battery has an e.m.f. of about 1.5 V. The approximate nature of the e.m.f is related to the complexity of the cathode reaction. The anode (zinc) reaction is comparatively simple with a known potential. Side reactions and depletion of the active chemicals increases the internal resistance of the battery, and this causes the e.m.f. to drop.

Leakage

When the dry cell has been used for a certain time, the zinc container becomes thinner because zinc metal is oxidised to zinc ions. Therefore Zinc Chloride Solution leaks out of the battery. The old dry cell is not leakproof. It becomes very sticky as the paste leaks through the holes in the zinc case. The service life of the battery is short, with a shelf life of around 1.5 years.

Furthermore, the zinc casing in the dry cell gets thinner slowly, even when the cell is not being used. It is because the ammonium chloride inside the battery is acidic, reacting with the zinc.

The Zinc Chloride Cell

The (heavy duty) zinc chloride cell is an improvement on the original zinc-carbon cell, using purer chemicals and giving a longer life and steadier voltage output as it is used. Instead of an electrolyte mixture containing much NH₄Cl, it is largely only ZnCl₂ paste. The cathode reaction is thus a little different:

MnO₂(s) + H₂O(l) + e^- → MnO(OH)(s) + OH^-(aq)

as is the overall reaction:

Zn(s) + 2 MnO₂(s) + ZnCl₂(aq) + 2 H₂O(l) → 2 MnO(OH)(s) + 2 Zn(OH)Cl(aq)

References

• Eveready: Carbon Zinc Application Notes
• Rayovac: Alkaline and Heavy Duty Application Notes
• Power Stream Battery Chemistry FAQs
• Photoflash Battery
• Cell Construction

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