Standard Cell Potential: Calculations, Electron Flow & Feasibility (CIE A Level Chemistry)

• Once the E^ꝋ 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 (E_cell^ꝋ) can be calculated by subtracting the less positive E^ꝋ from the more positive E^ꝋ value
  • The half-cell with the more positive E^ꝋ value will be the positive pole
  • By convention this is shown on the right-hand side in a conventional cell diagram, so is termed  E_right^ꝋ
• The half-cell with the less positive E^ꝋ value will be the negative pole
  • By convention this is shown on the left-hand side in a conventional cell diagram, so is termed  E_left^ꝋ

E_cell^ꝋ = E_right^ꝋ - E_left^ꝋ   

• Since oxidation is always on the left and reduction on the right, you can also use this version

E_cell^ꝋ = E_reduction^ꝋ - E_oxidation^ꝋ

• The E^ꝋ values of a species indicate how easily they can get oxidised or reduced
• In other words, they indicate the relative reactivity of elements, compounds and ions as oxidising agents or reducing agents
• The electrochemical series is a list of various redox equilibria in order of decreasing E^ꝋ values
• More positive (less negative) E^ꝋ values indicate that:
  • The species is easily reduced
  • The species is a better oxidising agent
• Less positive (more negative) E^ꝋ values indicate that:
  • The species is easily oxidised
  • The species is a better reducing agent

An example electrochemical series

In this example electrochemical series the equilibria are arranged in order of decreasing E^ꝋ values. These values can then be used to identify the strongest and weakest reducing / oxidising agents.