AQA Chem5: Redox Equilibria

Revision notes on redox equilibria from the physical section of AQA Chem5

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  • Created by: anna
  • Created on: 10-06-13 10:34

Redox Equations

  • Oxidation is the loss of electrons
  • Reduction is the gain of electrons
  • In a redox equation something is oxidised while something else is reduced
  • An oxidation state is a charge and number assigned to atoms in molecules or ions to show how many electrons the atom has used in bonding with a different element
  • Many elements have variable oxidation states in their compounds
  • Some elements have fixed oxidation states in their compounds e.g. halides are -1, group I are +1 and group II are +2
  • To make a redox equation you have to balance and then combine the half equations 
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Electrochemical Cells

  • convert chemical energy into electrical energy
  • involve redox reactions
  • a metal in contact with a solution of its ions is known as a half cell
  • when two half cells are connected an electrochemical cell is obtained
  • transfer of electrons between two half cells produces an electrical current
  • electrons flow from the negative half cell to the positive half cell in the external circuit
  • ions conduct electricity in the internal circuit
  • a salt bridge has to be present to allow the flow of ions and to complete the circuit
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Cell Diagrams


  • they aren't actually a diagram but a shorthand way of representing an electrochemical cell
  • the cell undergoing reduction (the positive one) is assigned to the right hand side electrode
  • generally:   M (s) | Mn+ (aq) || M1n+ (aq) | M1 (s)
  • the  single solid line represents the boundary between the solid metal and the metal ions
  • the double line represents the salt bridge
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Measuring Electrochemical Cell Potentials


  • the electrode potential of a half cell is a measure of electromotive force (e.m.f) of the cell
  • the standard electrode potential has the symbol EѲ
  • when two half cells are combined the cell potential or standard e.m.f of the cell is given by:                             EѲ cell = EѲ R.H.S - EѲ L.H.S
  • EѲ values are determined using a standard reference electrode, this is usually the standard hydrogen electrode (S.H.E)
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The Standard Hydrogen Electrode

The S.H.E consists of:

  • a Pt electrode
  • H2 gas at 100kPa
  • a 1moldm-3 solution of H+ ions (HCl)
  • a mercury contact
  • a copper wire
  • (it is all at 298K)

The half cell diagram is: Pt(s) | H2(g) | 2H+(aq) ||

  • the EѲ is 0.00V
  • it is difficult to set up so a secondary reference electrode is used
  • this is the calomel electrode which is more convenient to use
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The Electrochemical Series


  • the reactions are written as reductions
  • positive potentials accept electrons and are good oxidising agents e.g. Au3+(aq) + 3e- <-> Au(s)
  • negative potentials donate electrons and are good reducing agents e.g. Li+(aq) + e- <-> Li(s)
  • it can be said that those with positive potentials are 'electron grabbing' and those with negative potentials are 'electron giving'
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Calculating EѲ cell using Electrode Potentials


Use EѲcell = EѲ R.H.S - EѲ L.H.S

  • the more positive electrode is on the right hand side (R.H.S)
  • if the EѲcell value is negative the cells are in reverse
  • when calculating EѲcell do not change the signs of the values (i.e. keep them negative or positive)
  • the more positive half cell is always reduced
  • electrons flow from the less positive to the more positive
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