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Electrode potentials are used to quantify the tendency of a redox reaction to move in the direction of oxidation (positive), or reduction (negative). |
Electrode potential
As we have already seen earlier in the book, there is a tendency for electricity to flow from one half cell in which a redox reaction can take to another half cell in which a different redox reaction is possible. Such electrochemical cells are called Voltaic cells.
By measuring the direction of flow of electricity it is possible to arrive at a list of half-cells ordered according to how easily they lose (or gain) electrons. As each half-cell is basically a redox half-equation, we can arrange these half-equations in order of tendency to move in the direction of oxidative or reductive change.
For example, through experiment we can see that when a zinc half cell is connected to a copper half cell, the electricity always flows from zinc to copper around the external circuit. However, when a silver half cell is connected to a copper half cell, the electrons flow from copper to silver around the external circuit.
This suggest that the equation:
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Zn2+(aq) + 2e
Zn(s) |
Is more easily able to move to the left (releasing electrons) than:
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Cu2+(aq) + 2e
Cu(s) |
It seems that zinc is better at providing electrons than copper, and copper in turn is better at providing electrons than silver. This gives us a simple order of reducing capability (remember that species which provide electrons are reducing agents)
Zinc is the strongest reducing agent (releaser of electrons), followed by copper, followed by silver.
Each of the cells constructed provides a voltage, called the cell potential. This 'cell potential' is found to depend on the concentration of the aqueous solutions as well as the temperature.
In order to be quantitative in our dealings with cell potentials all measurements should be carried out under standard conditions, which are 25ºC and 1 mol dm-3 concentrations. Even so, it is difficult to use the values of the cell potentials above to decide on the order of reactivity as they are comparative values.
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half cells
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cell potential
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Zn - Cu
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1.10V
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Zn - Ag
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1.56V
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Cu - Ag
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0.46V
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That is, the potential of the Zinc|Zn2+(aq) half cell is more negative than the Cu|Cu2+(aq) half cell by 1.10 V. The Zn|Zn2+(aq) half cell is more negative than the Ag|Ag+(aq) half cell by 1.56 V.
Clearly, as there are hundreds of possible half-cells there are many different comparative cell potentials. It makes sense to have a standard cell to which everything can be compared in order to standardise all of this data. The standard half cell chosen is the standard hydrogen electrode.
