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Electrical cells are made up of two half-cells which, when connected together, allow an electrical current to flow around an external circuit. They are an essential source of electrical energy in the modern world, powering watches, mp3 players etc.. Syllabus referenceReactivity 3.2.6 - A primary (voltaic) cell is an electrochemical cell that converts energy from spontaneous redox reactions to electrical energy.
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The nature of electricity
Direct current (d.c.) electricity is a flow of electrons from a region of negative potential to a region of positive potential. The negative electrons are attracted by the positive charge. This force of attraction is called the electromotive force (E.M.F.), and is measured as a voltage. It is also called the potential difference between the negative and positive terminals, or 'ends' of the voltage source.
The amount of current that can flow depends on how much 'push' or energy it's given by the voltage, as well as the resistance of the circuit. The relationship is a very simple one, called 'Ohms Law':
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Voltage = Current x Resistance
V = IR |
The resistance of any given circuit is effectively constant, hence the current is directly proportional to the voltage applied.
| Note In physics the convention is to describe a flow of electrical current in the opposite direction to chemistry, i.e. from + to -. For this reason the physics version of current is often called 'conventional current', it flows from positive to negative. This arose historically because the laws of physics, as regards electricity, emerged before the discovery of the electron. In chemistry we know that electrons actually make up the current, so we've got it right! |
Electrical cells
As we have seen, certain species lose electrons (reducing agents) and other species gain electrons (oxidising agents) when reacting. If these species are not mixed together, but connected electrically by means of an external circuit, then these electrons will flow around the external circuit producing an electric current.
Each of the reacting species is then called a half cell and the whole set up is called an electrochemical cell. It is the basis behind the electrical battery.
In this cell the zinc metal has a tendency to dissolve as ions, leaving its electrons on the electrode. The copper, which is a weaker reducing agent, is forced to accept the electrons and use them to turn the copper ions into copper at the copper electrode. These electrons flowing around the outer (external) circuit constitute an electric current.
The 'salt bridge' is usually a filter paper soaked in potassium nitrate solution (neither of these ions react with any other ions in the experiment). This 'salt bridge' then allows ions to move in both directions, equalising any build up of electrical charge in the beakers.
Reactions in cells
The zinc forces the copper ions to accept electrons and the overall cell equation can be constructed by adding together the two 'half-equations' above.
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Zn(s)
Zn2+(aq) + 2e |
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Cu2+(aq) + 2e
Cu(s) |
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overall: Zn(s) + Cu2+(aq)
Zn2+(aq) + Cu(s) |
This type of cell can be constructed using any pair of reducing and oxidising agents. The greater the difference in the reactivity of one type of species (i.e. the reducing species) the greater the cell potential (voltage)
Consequently a cell constructed from zinc ¦ zinc sulfate in one half cell and silver ¦ silver nitrate solution in the other half cell will have a greater voltage that the cell above (there is a greater difference in reactivity between zinc and silver than between zinc and copper)
Example voltaic cells
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