Oxidation and Reduction

When an oxidation reaction and reduction reaction occur simultaneously this is called a redox reaction. Oxidation is the loss of electrons. Reduction is the gain of electrons. An element is oxidised when its oxidation state increases, it becomes more positive. An element is reduced when its oxidation state decreases, it becomes more negative.

Atoms in elements always have an oxidation state of zero. For example Carbon's O.S.=0 oxygen O2 O.S.=0 it doesn't matter if they're diatomic. In compounds and ions oxidation states are assigned to each atom so when added together must come to zero or the overall charge of the ion. So atoms O.S. rarely change, F is -1, O is -2 (except in O2 its -1 and in OF2 its +2) H is +1 and Cl is usually -1 (except when combined with O or F). CO2 O is -2 as there are 2 O overall there's -4 thus the C must be +4 so the overall charge is 0  

Cl2 + 2I-  ->  2Cl- + I2   

0       -1         -1       0

The Oxidation State for chlorine decreases from 0 to -1 so it is reduced

The Oxidation State for Iodine increases from -1 to 0 so it is oxidised 

Some elements such as transition metals can exist in different oxidation states for example Fe 2+ and Fe 3+ when this occurs the oxidation state is included in the name of the compound, the oxidation state is written in roman numerals  

Example: 

FeO iron(II) oxide  Fe2O3 iron (III) oxide 

The names of oxyanions also include oxidation states, oxyanions are negative ions that contain oxygen and another element with the suffix -ate e.g sulphate 

ClO- chlorate(I) chlorine oxidation state = +1

ClO2- chlorate(III) chlorine oxidation state = +3

ClO3- chlorate (V) chlorine oxidation state = +5

ClO4- chlorare (VII) chlorine oxidate state = +7

2Na+Cl2 -> 2NaCl     Can be seen as 2 different equations

2Na -> 2Na+   +    2e-   Oxidised, electrons have been lost Na went from 0 to +1

Cl2 + 2e-  -> 2Cl-    Reduction, gained electrons O.S. changed from 0 to -1 therefore reduction