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.

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