Some Standard Potentials

Oxidants

Reductants

E⁰
(volts)

Key Concepts

• A more active metal will displace a least active metal from solution

• A more active metal is a stronger reductant
  (A reductant is a species that causes another species to be reduced, and is itself oxidised by losing electrons)

• An activity series lists metals in order of decreasing strength as reductants
  An example of an activity series of metals based on the Standard Potentials given would be:

Examples

1. If a strip of clean magnesium metal were placed in a copper sulphate solution, the magnesium metal would displace the copper from solution since magnesium is a more active metal than copper.

   The magnesium metal strip would disintegrate while solid copper would be deposited

   As Cu²⁺ ions are removed from solution, the solution would become a paler blue in colour.

   The magnesium metal would donate electrons.

   Mg(s) -----> Mg²⁺(aq) + 2e   E⁰ = +2.36V
   Magnesium metal is being oxidised (losing electrons) therefore it is the reductant.

   The copper ions in solution would accept electrons forming solid copper.

   Cu²⁺(aq) + 2e -----> Cu(s)   E⁰ = +0.34V
   Copper ions are being reduced (gaining electrons) therefore the copper ions are the oxidant.

   The overall equation would be:
   Mg(s) + Cu²⁺(aq) -----> Mg²⁺(aq) + Cu(s)
   E⁰ = +2.36 V + +0.34 V = +2.70V

   The positive value for E⁰ indicates a spontaneous reaction will occur.

2. If a strip of clean copper metal were placed in a magnesium chloride solution, no displacement reaction would occur since the copper is a less active metal than the magnesium.

         Cu(s) ---> Cu²⁺(aq) + 2e   E⁰= -0.34V
   Mg²⁺ + 2e---> Mg(s)               E⁰= -2.36V

   ---

   Cu(s) + Mg²⁺(aq) ---> Cu²⁺(aq) + Mg(s)
                                               E⁰=-2.70V

   The negative E⁰ value indicates that the forward reaction will not proceed spontaneously.