Calculating Standard Cell Potential
- Once the Eꝋ of a half-cell is known, the potential difference or voltage or emf of an electrochemical cell made up of any two half-cells can be calculated
- These could be any half-cells and neither have to be a standard hydrogen electrode
- The standard cell potential (Ecellꝋ) can be calculated by subtracting the less positive Eꝋ from the more positive Eꝋ value
- The half-cell with the more positive Eꝋ value will be the positive pole
- By convention this is shown on the right hand side in a conventional cell diagram, so is termed Erightꝋ
- The half-cell with the less positive Eꝋ value will be the negative pole
- By convention this is shown on the left hand side in a conventional cell diagram, so is termed Eleftꝋ
- The half-cell with the more positive Eꝋ value will be the positive pole
Ecellꝋ = Erightꝋ - Eleftꝋ
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- Since oxidation is always on the left and reduction on the right, you can also use this version
Ecellꝋ = Ereductionꝋ - Eoxidationꝋ
Worked example
Calculating the standard cell potential
Calculate the standard cell potential for the electrochemical cell below and explain why the Cu2+ / Cu half-cell is the positive pole. The half-equations are as follows:
Cu2+(aq) + 2e- ⇌ Cu(s) Eꝋ = +0.34 V
Zn2+(aq) + 2e- ⇌ Zn(s) Eꝋ = −0.76 V
Answer
Step 1: Calculate the standard cell potential. The copper is more positive so must be the right hand side.
Ecellꝋ = Erightꝋ - Eleftꝋ
Ecellꝋ = (+0.34) - (-0.76)
= +1.10 V
The voltmeter will therefore give a value of +1.10 V
Step 2: Determine the positive and negative poles
The Cu2+ / Cu half-cell is the positive pole as its Eꝋ is more positive than the Eꝋ value of the Zn2+ / Zn half-cell
Examiner Tip
A helpful mnemonic for remembering redox in cells
Lio the lion goes Roor!
Lio stands for 'Left Is Oxidation' and he is saying ROOR because that is the order of species in the cell:
Reduced/Oxidised (salt bridge) Oxidised/Reduced