5.3 Electrochemistry and batteries  (Page 3/4)

Cathode: $2{\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+2{\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{e}}^{-}\to {\mathrm{Mn}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+2{\mathrm{NH}}_3\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

The anode half reaction is as follows:

Anode: $\mathrm{Zn}\left(\mathrm{s}\right)\to {\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}$

The overall equation for the cell is:

$\mathrm{Zn}\left(\mathrm{s}\right)+2{\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{NH}}_4^{+}\to {\mathrm{Mn}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}+\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_2^{2+}\left(\mathrm{aq}\right)$ (E ${}^0$ = 1.5 V)

Alkaline batteries are almost the same as zinc-carbon batteries, except that the electrolyte is potassium hydroxide (KOH), rather than ammonium chloride. The two half reactions in an alkaline battery are as follows:

Anode: $\mathrm{Zn}\left(\mathrm{s}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+2{\mathrm{e}}^{-}$

Cathode: $2{\mathrm{MnO}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2{\mathrm{e}}^{-}\to {\mathrm{Mn}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

Zinc-carbon and alkaline batteries are cheap primary batteries and are therefore very useful in appliances such as remote controls, torches and radios where the power drain is not too high. The disadvantages are that these batteries can't be recycled and can leak. They also have a short shelf life. Alkaline batteries last longer than zinc-carbon batteries.

Environmental considerations

While batteries are very convenient to use, they can cause a lot of damage to the environment. They use lots of valuable resources as well as some potentially hazardous chemicals such as lead, mercury and cadmium. Attempts are now being made to recycle the different parts of batteries so that they are not disposed of in the environment, where they could get into water supplies, rivers and other ecosystems.

Electrochemistry and batteries

A dry cell, as shown in the diagram below, does not contain a liquid electrolyte. The electrolyte in a typical zinc-carbon cell is a moist paste of ammonium chloride and zinc chloride.

( NOTE TO SELF: Insert diagram )

The paste of ammonium chloride reacts according to the following half-reaction:

$2{\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}\to 2{\mathrm{NH}}_3\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$ (a)

Manganese(IV) oxide is included in the cell to remove the hydrogen produced during half-reaction (a), according to the following reaction:

$2{\mathrm{MnO}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)\to {\mathrm{Mn}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$ (b)

The combined result of these two half-reactions can be represented by the following half reaction:

$2{\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+2{\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{e}}^{-}\to {\mathrm{Mn}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+2{\mathrm{NH}}_3\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$ (c)

1. Explain why it is important that the hydrogen produced in half-reaction (a) is removed by the manganese(IV) oxide. In a zinc-carbon cell, such as the one above, half-reaction (c) and the half-reaction that takes place in the Zn/Zn ${}^{2+}$ half-cell, produce an emf of 1,5 V under standard conditions.
2. Write down the half-reaction occurring at the anode.
3. Write down the net ionic equation occurring in the zinc-carbon cell.
4. Calculate the reduction potential for the cathode half-reaction.
5. When in use the zinc casing of the dry cell becomes thinner, because it is oxidised. When not in use, it still corrodes. Give a reason for the latter observation.
6. Dry cells are generally discarded when 'flat'. Why is the carbon rod the most useful part of the cell, even when the cell is flat?