A-LEVEL AQA ChEMISTRY NOTES 
Electrode Potentials and Electrochemicals (A-level)

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Electrode Potentials
  • Redox reactions can be used in electrochemical cells to generate electricity (a flow of charge).
  • An electrochemical cell is a device capable of generating a potential difference from redox reactions
  • Electrochemical cells consist of two half-cells. At one oxidation occurs, at the other reduction. Electrons flow between the two cells, driving the redox reaction.
Zn → Zn^2+ + 2e-
Cu^2+ + 2e^- → Cu
​An equilibrium is reached:
Cu^2+ (aq) + 2e^- ⇌ Cu (s)
  • Each one of these beakers is a half-cell. A solution in a standard half-cell will have a concentration of 1.00 mol dm^-3
  • An electrode is a solid surface which allows the transfer of electrons to and from it. 
  • In half cells involving gases, the gas is bubbled through the liquid and an electrode provides a surface for the reaction to occur on.
  • In half cells with both oxidation states present as aqueous ions, equimolar solutions of both ions are present with a platinum electrode.
  • The standard electrode potential, E^⦵  is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.
  • Standard conditions includes 298K, 100kPa and 1.00 mol dm^-3.
  • E^⦵ gives the position of equilibrium, telling us a half-cell’s tendency to accept or release electrons
  • The voltage measured is also known as the electromotive force of the cell (EMF).
  • ​An electrochemical series is a list of standard electrode potentials of all the possible half-cells.
  • The more negative the electrode potential, the more the oxidation (backwards) reaction is favoured.
  • E^⦵cell = E^⦵positive terminal - E^⦵negative terminal
Electrochemical Cells
  • Electrochemical cells can be used as a commercial source of electrical energy
  • In a rechargeable battery, when the chemicals have reacted fully, a potential difference can be applied to the cell in the opposite direction, which will regenerate the original chemicals
  • Lithium-ion batteries are rechargeable:
  • Some cells are non-rechargeable and disposed of when the chemicals have fully reacted.
  • In fuel cells the chemicals are stored externally and are fed into the cell when electricity is required.
  • An alkaline hydrogen fuel cell:
Advantages
Disadvantages
1. They are more efficient than burning fossil fuels
2. They release water, which isn’t harmful
3. They do not need to be recharged – they keep producing electricity for as long as they have fuel
1. Energy is needed to build the fuel cells and produce hydrogen – this energy comes from fossil fuels​
2. Hydrogen is highly flammable so needs to be carefully handled
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