A-level Chemistry/AQA/Module 2/Redox Reactions

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Oxidation and Reduction[edit]

Redox reactions

The word redox is short of reduction-oxidation reaction. An example of an oxidation reaction is that of copper becomin copper oxide with the addition of oxygen.

Cu + ½O2 → CuO

The reverse process is known as the reduction reaction which in this case would involve the removal of oxygen from copper in the following reaction.

CuO + H2 → Cu + H2O

Within these reactions there are what's known as reducing and oxidising agents. The the case of the top reaction the oxidising agent is the oxygen and the bottom equation the reducing agent would be the hydrogen.

Shopuld we describe what has happened to the above reaction in term of electrons we can say that when something loses electrons it is oxidised and when it gains electrons it is reduced. This premise can be easily remembered by the definition that Oxidation is loss and reduction is gain condensed into the acronym OILRIG.

Oxidation states[edit]

We use the idea of oxidation states to allow us to see what is being oxidised and what is being reduced. An alternative name is oxidation number meaning exactly the same thing.

To understand oxidation states the following rules must be taken into consideration.

  • Every element in its uncombines state has an oxidation state of 0
  • A positive number shows us that element has lost an electron thus has been oxidised e.g. Mg2+ has an oxidation state of +2.
  • A negative number shows us the element has gained an electron thus has been reduced e.g. Cl- has an oxidation number of -1
  • The more positive or negative number the more it has been oxidised or reduced
  • Atoms of element in compounds can lose or gain more than one electron
  • The sum of all the oxidation states in a compound equal 0 since all compounds are neutral
  • The sum of the oxidation states of an ion e.g. NH4+ add up to the charge on the ion in this case +1
  • In a compound the most electronegative element always has a negative value
Element Oxidation state in compound Example
Hydrogen +1 except when in metal hydrides e.g. NaH where it is -1 HCl
Group 1 Always +1 NaCl
Group 2 Always +2 CaCl2
Aluminium Always +3 AlCl3
Oxygen -2 except in peroxides and in compounds with F where it is -1 Na2O
Fluorine Always -1 NaF
Chlorine -1 except when in compounds with F and O where it has positive values NaCl