The Rusting of Iron Aim: To find out the special name given to the corrosion of iron. To find out the two stages involved in the rusting of iron.
Iron is the most widely used metal and the rusting of iron (and steel) costs millions of pounds each year. • The corrosion of iron is given a special name – rusting. • Only iron rusts, other metals ‘corrode’. • Salt acts as an electrolyte and so will speed up the rusting process as will the presence of any soluble ionic substance.
What happens when iron rusts? • Rusting occurs when iron loses electrons (oxidation). This happens in two stages. • 1. Iron atoms losing two electrons to form iron (II) ions;
2. Iron (II) ions losing a further electron to form the iron (III) ions: Fe2+(aq) Fe3+(aq) + e- Fe3+ forms (Fe3+)2(O2-)3 which is RUST! These are both OXIDATION reactions.
However, when an oxidation reaction occurs there must be a corresponding reduction reaction. • In the case of rusting this involves water and oxygen molecules reacting as follows:
This explains the need for water and oxygen in rusting. Remember for corrosion to occur water, oxygen and carbon dioxide or another electrolyte are required.
A Rusting Indicator Aim: To investigate the indicator for rusting Method: Iron sulphate (Fe2+ ions) + Indicator C Iron sulphate (Fe2+ ions) + Indicator D Iron sulphate (Fe2+ ions) + Indicator A Iron sulphate (Fe2+ ions) + Indicator B
Results: Indicator B gives a colour change. Conclusion: Test for Fe2+ ions: Ferroxyl indicator turns from pale yellow to blue Ferroxyl Indicator can be used to determine not only if rust is present but also the amount of rusting which has taken place. The more bluecolour there is, the more rustinghas taken place.
Other Indicators • Test for OH- ions: Ferroxyl indicator turn from pale yellow to pink • Test for Fe3+ ions: Ammonium thiocyanate turns from colourless to blood red.