The redox couple
A redox reaction is always built up of two half reactions:
| The equation of couple 1: |
red 1 |
 |
ox 1 + electrons |
| The equation of couple 2: |
ox 2 + electrons |
|
red 2 |
| Total equation: |
red 1 + ox 2 |
|
ox 1 + red 2 |
The number of electrons involved must satisfy the rule:
ALWAYS THE SAME NUMBER OF ELECTRONS IS DONATED AS IS CAPTURED
Electrons do not get lost, are not just created from nothing, and so:
You must always confirm and certify the two half reactions in that way: the number of donated electrons must equal the number of captured electrons.
Example:
| The equation of couple 1: |
Al |
|
Al3+ + 3e- |
| x 2 |
| The equation of couple 2: |
I2 + 2e- |
|
2I- |
| x 3 |
| Total equaition: |
2Al + 3I2 |
|
2Al3+ + 6I- |
The total equation is only reliable if the number of electrons in half reaction 1 equals the number of electrons in half reaction 2.
In case of insolubility of the salt Aluminium Iodide, there will be a secondary reaction: the precipitation of an insoluble salt.
Att.: This is not the case in the example; check that in table XI:
solubility of salts in water.