A zinc-carbon dry cell or battery is packaged in a zinc can that serves as both a container and negative terminal. It was developed from the wet Leclanché cell (pronounced /lɛklɑːnˈʃeɪ/). The positive terminal is usually a carbon rod or graphite rod surrounded by a mixture of manganese dioxide and carbon powder . The electrolyte used is a paste of zinc chloride and ammonium chloride dissolved in water. Zinc chloride cells are an improved version from the original ammonium chloride variety. Zinc-carbon batteries are the least expensive primary batteries and thus a popular choice by manufacturers when devices are sold with batteries included. They are commonly labeled as "General Purpose" batteries. They can be used in remote controls, flashlights, clocks, or transistor radios, since the power drain is not too heavy.
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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 negative terminal. 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 positive terminal. The manganese dioxide is mixed with carbon powder to increase the conductivity. The reaction is as follows:
- 2MnO2(s) + H2(g)→ Mn2O3(s) + H2O(l)
The H2 comes from the NH4+(aq):
- 2NH4+(aq) + 2 e- → H2(g) + 2NH3(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. When the zinc case thins enough, zinc chloride begins to leak 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 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. These cells are often marketed as "Heavy Duty" cells, to differentiate them from "General Purpose" carbon-zinc cells. This has been a source of consumer confusion after the introduction of alkaline cells, which last longer than the zinc-chloride "Heavy Duty" cell. 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)
Common Mistake
The words anode and cathode can be very confusing. In electrolytic cells, the anode is referred as the positive terminal since all the anions (negative ions) will migrate to the anode to be selectively discharged while the cathode is the negative terminal because the cations (positive ions) will move to the cathode to be selectively discharged. Meanwhile, for voltaic cells, the anode and cathode are opposite to each other. This means that the anode is the negative terminal, while the cathode is the positive terminal. This is due to the theory which states that all anodes are terminals that undergo oxidation or release of electrons, and all cathodes are terminals which undergo reduction.
References
- Eveready: Carbon Zinc Application Notes
- Rayovac: Alkaline and Heavy Duty Application Notes
- Power Stream Battery Chemistry FAQs
- Photoflash Battery
- Cell Construction
- Malaysia Chemistry Text Book for Secondary School Form 4.
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