Many metals react with oxygen to make metal oxides. For example, magnesium burns rapidly in air:
Magnesium + oyxgen → magnesium oxide
2Mg(s) + O2(g) → 2MgO(s)
The reactions are oxidation reactions because the metal gains oxygen.
Oxygen can be removed from metal oxides in chemical reactions. For example:
Copper oxide + carbon → copper + carbon dioxide
2CuO(s) + C(s) → 2Cu(l) + CO2(g)
In this reaction, carbon is oxidised because it gains oxygen. At the same time, copper oxide is reduced because oxygen is removed from it.
Oxidation is the gain of oxygen. Reduction is the loss of oxygen.
Magnesium + copper sulfate → magnesium sulfate + copper
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)
In this displacement reaction:
A reactivity series can be deduced by carrying out several displacement reactions. A piece of metal is dipped into a salt solution. Different combinations of metal and salt solution are tested. The table shows the results of one of these investigations.
|Magnesium sulfate solution||Copper sulfate solution||Iron sulfate solution||Number of reactions|
|Magnesium||Not done||Brown coating||Black coating||2|
|Copper||No visible reaction||Not done||No visible reaction||0|
|Iron||No visible reaction||Brown coating||Not done||1|
Use the results in the table to deduce an order of reactivity, starting with the most reactive metal.
The order of reactivity is: magnesium > iron > copper. This is because magnesium displaced copper and iron, iron displaced copper only, but copper could not displace magnesium or iron.
Explain why three combinations of metal and salt solution were not done in the investigation.
A metal cannot displace itself from a solution of one of its salts. There would be no reaction, so these combinations were not done.
Mg(s) + Cu2+(aq) + SO42-(aq) → Mg2+(aq) + SO42-(aq) + Cu(s)
Sulfate ions, SO42-, appear on both sides of the equation, but they do not take part in the reaction. The equation can be rewritten without them:
Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
Mg(s) → Mg2+(aq) + 2e- (oxidation)
Cu2+(aq) + 2e- → Cu(s) (reduction)
Reduction and oxidation happen at the same time, so the reactions are called redox reactions.
It helps to remember OIL RIG - oxidation is loss of electrons, and reduction is gain of electrons.
Displacement reactions are just one example of redox reactions. Electrolysis reactions are also redox reactions.