1. A loss of electrons is called oxidation. A gain in electrons is called reduction.
2. Reduction and oxidation happen simultaneously - hence the term "redox" reaction.
3. An oxidisng agent accepts electrons and gets reduced.
4. A reducing agent donates electrons and gets oxidised.
Oxidation state rules
There are lots of rules. Take a deep breath...
1. All atoms are treated as ions for this, even if they're covalently bonded.
2. Uncombined elements have an oxidation state of 0.
3. Elements just bonded to identical atoms, like O2 and H2, also have an oxidation state of 0.
4. The oxidation state of a simple monatomic ion, e.g. Na+, is the same as its charge.
5. In compounds or compound ions, the overall oxidation state is just the ion charge.
6. The sum of the oxidation states for a neutral compound is 0.
7. Combined oxygen is nearly always -2, except in peroxides where it's -1 (an in fluorides OF2, where its +2 and O2F2, where its +1 and in O2 where it's 0).
8. Combined hydrogen is +1, except in metal hydrides where it is -1 (and H2 where it's 0).
Roman Numerals Give Oxidation States
Sometimes, oxidation states aren't clear from the formula of a compound.
If you see Roman numerals in a chemical name, it's an oxidation number.
E.g. copper has oxidation state +2 in copper (II) sulphate and manganese has oxidation state +7 in a manganate (VII) ion (MnO4-)
Writing half equations
1. Ionic half equations show oxidation or reduction.
2. You can combine half-equations for different oxidising or reducing agents. Together to make full equations for redox reactions.
E.g. Magnesium burns in oxygen to form magnesium oxide.
1. Oxygen is reduced to O2-: O2 + 4e- --> 2O2-
2. Magnesium is oxidised to Mg2+: Mg---> Mg2+ + 2e-
3. You need both equations to contain the same number of electrons. So double everything in the second equation: 2Mg --> 2Mg2+ + 4e-
4. Combining the half equations makes: 2Mg + O2---> 2MgO
The electrons aren't included in the full equation. There are four on each side - so they cancel.
Writing half equations part 2
E.g.2. Aluminium reacts with chlorine to form aluminium chloride.
1. Aluminium is oxidise to Al3-: Al ---> Al3+ + 3e-
2. Chlorine is reduced to Cl-: Cl2 + 2e- ---> 2Cl-
3. Now make sure the equations each contain the same number of electrons:
Al---> Al3++ 3e- (x2) 2Al ----> 2Al3+ + 6e-
Cl2 + 2e- ---> 2Cl- (x3) 3Cl2 + 6e- ---> 6Cl-
4. Combining the half equations makes: 2Al + 3Cl2---> 2AlCl3