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An electrochemical cell converts chemical energy into electrical energy using a redox reaction.
Since, metals can be oxidised or reduced depending upon their chemical environment, then such an arrangement as shown below may be set-up.
The Daniell cell, specifically, uses Zn(s)/Zn2+(aq) and Cu(s)/Cu2+(aq) reactions. Note that the two rods in the diagram are called electrodes.
The zinc rod is the negative electrode and the copper rod the positive electrode.
Each metal in contact with a solution of its ions is called a half-cell. Half-cells are often represented by half-cell equations, which show the electrode processes:
At the negative electrode: Oxidation
Zn(s) → Zn2+(aq) + 2e-
At the positive electrode: Reduction:
Cu2+(aq) + 2e- → Cu(s)
So the overall reaction equation is:
Remember this is a REDOX reaction.
Note in the diagram:
- The electrons flow in a clockwise direction, from the zinc rod to the copper.
- The salt bridge, completes the circuit by allowing the passage of ions from the copper sulphate solution to the zinc sulphate solution. This salt bridge is usually a strip of filter paper soaked in saturated potassium sulphate.
Electrochemical cells can be made, as long as you pair up two half-cells of different potential so that an electrical current can be produced.
How do we find the potential of any half-cell individually? The answer is that we use a standard hydrogen electrode (s.h.e) and give it a potential of zero.
Therefore, when it is connected to another half-cell, the electromagnetic field (e.m.f) between the s.h.e and the second half-cell is equal to the potential of the second half-cell.
The diagram below outlines the main characteristics and conditions required to establish the s.h.e:
The reaction that takes place is:
2H+(aq) + 2e- → 2H2 (g)