Factors Affecting the Electrode Potential
Half-cell reactions are reversible, and like any other reversible reaction the equilibrium position is affected by changes in temperature, pressure and concentration.
Changing the equilibrium position changes the cell potential. To get around this, standard conditions are used to measure electrode potentials.
Using standard conditions means you always get the same value for the electrode potential and you can compare values for different cells.
The Standard hydrogen Electrode
We measure the electrode potential of a half-cell against a standard hydrogen electrode.
In the standard hydrogen electrode, hydrogen gas is bubbled into a solution of aqueous H,+ ions.
A platinum electrode is used as a platform for the oxidation/reduction reactions.
When measuring electrode potentials using the standard hydrogen electrode it is important that everything is done under standard conditions:
1. Any solutions of ions must have a concentration of 1.00 moldm^-3.
2. The temperature must be 298 K (25 C).
3. The pressure must be 100 KPa.
Measuring Standard Electrode Potentials
The standard electrode potential of a half-cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.
An example of an electrochemical cell with a standard hydrogen electrode:
- Pt | H2(aq) || Zn,2+(aq) | Zn(s)
The standard hydrogen electrode is always shown on the left - it doesn't matter if the other half-cell is more negative.
Standard hydrogen electrodes can be used to calculate standard electrode potentials because the standard hydrogen electrode half-cell has an electrode potential of 0.00 V.
The whole cell potential is: E*cell = (E*right - E*left)
The E*left is zero so the voltage reading will be equal to E*right = E*cell.