| As we have seen, the electrical cell is a controlled form of chemical reaction. Here we examine the chemical reactions occurring at the electrodes in electrochemical cells. |
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The negative electrode - anode
This is the electrode that actually produces the electrons that would flow around the external circuit. It is always the electrode of the most reactive metal (SL), the half cell with the most negative electrode potential (HL only).
The metal ions in the electrode dissolve as ions, leaving their electrons behind on the electrode. These electrons are then able to flow around the external circuit. For example in the zinc-copper voltaic cell the zinc half cell is the negative electrode: In voltaic cells this electrode is called the anode.
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Zn(s) →
Zn2+(aq) + 2e
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The reactions at the negative electrode always involve electrons being 'dropped off' by metals in the electrodes dissolving as ions. This is a process of oxidation; the metal atoms are getting oxidised to ions (and releasing electrons).
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The positive electrode
This is electrode towards which the electrons in the external circuit flow. It is the electrode that has positive ions (from the solution) removing the electrons as they arrive. In the case of the zinc-copper voltaic cell, the copper half cell is the positive electrode:
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Cu2+(aq) + 2e →
Cu(s)
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In voltaic cells this electrode is called the cathode. It is the electrode at which the electrons are 'picked up' by the ions from the solution. The reactions occurring at this electrode are always reductions. The ions from the solution collect electrons to become atoms.
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The cell reaction
The overall cell reaction can be obtained by adding together the reactions occurring at the positive and negative electrodes. If the number of electrons involved at both electrodes is different then the equation must be manipulated by mutiplication to make the number of electrons involved in each the same.
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Example 1: The copper-zinc voltaic cell
In this case the number of electrons involved at both electrodes is the same, so the equations can be added together: Zn(s) → Zn2+(aq) + 2e Cu2+(aq) + 2e → Cu(s) Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) |
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Example 2: The copper-silver voltaic cell
In this case the number of electrons involved at both electrodes is not the same, so before the the equations can be added together the silver equation must be doubled (multiplied by 2): Cu(s) → Cu2+(aq) + 2e 2Ag+(aq) + 2e → 2Ag(s) Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s) |
Cell representation conventions
Descriptions of voltaic cells can be drawn using a standard convention. The vertical lines represent barriers between different phases and the salt bridge is represented by a double vertical line. The chemical components of the cell are written as drawn in a cell diagram. The external circuit is not drawn. By convention, the most negative half cell is written first.
The copper-zinc voltaic cell can be represented as:
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Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)
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This literally means reading from left to right: There is a solid zinc electrode dipped in a solution containing zinc 2+ ions. This zinc 2+ ion solution is connected by means of a salt bridge to a solution containing copper 2+ ions. This copper 2+ ion solution has a solid copper electrode immersed in it. The external circuit joins the solid copper electrode to the solid zinc electrode.
If two different phases are present in the same half cell, then these are usually shown with a semi-colon between them. This is the case in the standard hydrogen electrode (see section )
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Pt|H2(g);H+(aq)||Cu2+(aq)|Cu(s)
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Here, the salt bridge joins the copper 2+ ion solution to a solution containing hydrogen ions. These hydrogen ions are also in contact with hydrogen gas at the surface of a platinum electrode.
