Deciding which product will form, or will form first, depends up the reduction potential.
Sometimes, especially when H2 or O2 gas is being formed, a kinetic effect called overvoltage takes places at an electrode -- extra potential must be applied to make the predicted reaction go at a reasonable rate. As the result of overvoltage, Cl2 is formed during the electrolysis of aqueous chloride solutions, and not O2 as might be predicted from the standard reduction potentials.
In an electrolysis system, the cell potential and the current both are known. The product of current and potential is power, When the current is in amperes and the potential in volts, the product (power) is in watts. Power is energy/time. Since an electolytic cell is run for a known amount of time in order to bring about a certain extent of reaction, the energy required to bring about the electrolysis may be calculated.
The basic calculations of electrolysis can be made pertinent in several ways. For example, you can determine the thickness of an electrodeposited coating. This might be expressed in units of length, or in numbers of layers of atoms.
