The Chemistry of Common Cells

Outcome 9: Understand electrochemical cells as a source of energy, including the

constituents of commercial cells.

Conventional v’s Electron Flow

• When we draw the flow of electricity in a circuit diagram we show the electricity flowing from the positive to negative terminal.

• In reality it is the electrons that move from the negative to the positive terminal.

• Be aware of these notations when looking at battery chemistry.

http://chemistry.about.com/library/weekly/aa082003a.htm

Zinc-Carbon Dry Cell

The ordinary dry cell is the most common and the cheapest of the commercially available cells. It is widely used in torches, radios and calculators.

The technical name for it is the Leclanché cell, named after its inventor.

It consists of a zinc outer casing, which is the source of electrons (anode). An aqueous paste of ammonium chloride, and a mixture of powdered carbon, manganese dioxide and ammonium chloride around a carbon rod (which receives the electrons to reduce the manganese) is the positive terminal.

At the negative terminal, the half reaction is:Zn(s) → Zn2+(aq) + 2e–

At the carbon rod (the positive electrode), the reduction half reaction is:

NH4+(aq) + MnO2(s) + H2O(l) + e– → Mn(OH)3(s) + NH3(aq)

Initially no zinc chloride is present, but asthe cell is used zinc ions are formed and ammonium ions are discharged.

Manganese is reduced from an oxidation state of +4 to +3.

This cell is relatively cheap to manufacture. It was the first commercial battery and therefore had a big impact upon society because it made things such as torches (flashlights), portable radios and battery-operated clocks and toys possible.

Disadvantages of the cell are that it does not contain a very large amount of electricity for its size, it cannot deliver very high currents and it can develop leaks when it goes flat (the zinc casing gets eaten away during operation).

Mercury Cell

The mercury oxide-zinc battery system was known more than 100 years ago but did not become widely used until 1942 when it found use in military applications

The battery has a long shelf life of 10. After the war it was used for small electronic devices such as pacemakers and hearing aids.

The overall reaction is: Zn + HgO ZnO + Hg

The electrolyte for the cell is potassium hydroxide or sodium hydroxide.

This cell is no longer in use due to the toxicity of mercury. The best replacement cell is the silver oxide cell however this is more expensive than the mercury cell.

Silver Oxide Cell

The silver oxide cell is a ‘button’ cell meaning it is small and looks roughly like a button.

It is widely used in miniature appliances such as watches, hearing aids and calculators.

Quite small cells can provide considerable amounts of electricity at a very constant voltage over a long period of time.

At the negative electrode: Zn(s) + 2OH–(aq) → ZnO(s) + H2O(l ) + 2e–

At the positive electrode: Ag2O(s) + H2O(l) + 2e– → 2Ag(s) + 2OH–(aq)

Overall reaction: Zn(s) + Ag2O(s) → ZnO(s) + 2Ag(s)

This cell delivers a very constant voltage throughout its lifetime, because as it operates there is no change in the concentration of the electrolyte solution (potassium hydroxide).