Rusting and corrosion
Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. can Chemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons. in air. They react with oxygen and form metal oxides. Sodium, for example, is a very The tendency of a substance to undergo a chemical reaction. metal. When sodium is cut or scratched, its freshly exposed shiny surface rapidly turns dull as a thin layer of sodium oxide forms:
sodium + oxygen 鈫 sodium oxide
4Na(s) + O2(g) 鈫 2Na2O(s)
Other metals may oxidise more slowly. Gold and other very unreactive metals do not oxidise in air at all.
The destruction of a metal by oxidation or chemical action, eg rusting. happens when a metal continues to oxidise. The metal becomes weaker over time, and eventually all of it may become metal oxide.
Rusting
Rusting occurs when iron or steel reacts with oxygen and water. The overall reaction is:
iron + oxygen + water 鈫 hydrated iron(III) oxide
Hydrated iron(III) oxide is the orange-brown substance seen on the surface of rusty objects.
A rusting experiment
The experiment in the diagram shows that both oxygen and water are needed for rusting to happen.
The nail only rusts in the left-hand test tube. It does not rust:
- in the middle test tube, where there was water but no oxygen (because there was no air in the water)
- in the right-hand test tube, where there was oxygen (air) but no water
Question
Explain whether iron is Chemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons. or A substance is reduced if it loses oxygen or gains electrons. when it forms rust.
Iron is oxidised because it gains oxygen during rusting.
Redox reactions - Higher
In terms of electrons:
- The gain of oxygen, or loss of electrons, by a substance during a chemical reaction. is loss of Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.
- The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. is gain of electrons
Rusting is a complex process. The example below show why both water and oxygen are needed for rusting to occur. They are interesting examples of oxidation, reduction and the use of half equations:
- iron loses electrons and is oxidised to iron(II) ions: Fe 鈫 Fe2+ + 2e-
- oxygen gains electrons in the presence of water and is reduced: 陆翱2 + 2e- + H2O 鈫 2OH-
- iron(II) ions lose electrons and are oxidised to iron(III) ions by oxygen: 2Fe2+ + 陆翱2 鈫 2Fe3+ + O2-
Question
One of the stages in rusting involves the Oxidation and reduction always take place together. The combined reaction is called a redox reaction. of iron(II) ions with oxygen:
2Fe2+ + 陆翱2 鈫 2Fe3+ + O2-
(a) Write a balanced half equation for the change that happens to iron(II) ions, and explain whether these ions are oxidised or reduced.
(b) Write a balanced half equation for the change that happens to oxygen, and explain whether oxygen is oxidised or reduced.
(a) Fe2+ 鈫 Fe3+ + e-
Iron(II) ions lose electrons, so they are oxidised.
(b) 陆翱2 + 2e- 鈫 O2-
Oxygen gains electrons, so it is reduced.