Predicting the direction of Redox Reactions

  • Redox reactions involve the transfer of electrons.
  • The oxidising agent is reduced (i.e. it gains electrons; it is the “electron thief”).
  • The reducing agent is oxidised (i.e. it loses electrons; it is the “electron donator”).
  • Redox reactions can be written in two half reactions; one reaction producing the electrons and the other reaction accepting them.
  • For example, when Zinc is added to copper sulphate, the following reaction occurs:
  • In addition to just the colour change, the reaction is also exothermic and so results in an increase in temperature.
  • The reaction can be simplified; the sulphate ion is a spectator ion and so can be removed.
  • From the oxidation numbers, it is plain to see that the zinc is being oxidised (increase in oxidation number) and the copper is being reduced (decrease in oxidation number).
  • The two half equations are:

    Cu2+(s) + 2e- Cu(aq) (reduction)
    Zn(s) Zn2+(aq) + 2e- (oxidation)

  • The reverse reaction of this can not occur; if copper is added to zinc sulphate solution, nothing will happen.
  • However, if copper is added to silver nitrate solution, the copper does react:
  • The half equations are:

    2Ag+(s) + 2e- 2Ag(s) (reduction)
    Cu(s) Cu2+(aq) + 2e- (oxidation)

  • Once again the reverse reaction cannot occur.
  • However; as you can see from the two previous reactions, the individual half reactions are reversible:

    Cu(s) Cu2+(aq) + 2e-

  • The actual direction that the reaction takes depends upon what it is reacting with. Copper ions are reduced by zinc (the zinc supplies electrons); however copper is oxidised by silver ions (the copper provides electrons).
  • Copper atoms don’t transfer electrons to zinc ions, and silver atoms don’t transfer electrons to copper ions.
  • Something must control the direction of electron transfer; to find out which way the electrons are transferred requires study of the individual half-equations, which can be done by separating the half-reactions using an electrochemical cell.

Useful books for revision

Revise A2 Chemistry for Salters (OCR A Level Chemistry B)
Salters (OCR) Revise A2 Chemistry