Oxidation Numbers

• Oxidation numbers show how many electrons are gained or lost by an element when atoms turn into ions

Oxidation Number Rules

1.                  The oxidation number of un-combined elements is zero.

2.                  In simple ions the oxidation number of the element is the charge on the ion

3.                  The sum of the oxidation numbers in a neutral compound is zero.

4.                  The sum of the oxidation numbers for an ion is the charge on the ion.

5.                  Some elements have fixed oxidation numbers in all their compounds

Metals                            Non-metals

Group I metals              +1        hydrogen          +1 (except in metal hydrides, H^-)

Group II metals            +2        fluorine -1

Aluminium                    +3        oxygen             -2 (except in peroxides O₂^2-                                          and compounds with fluorine).

   chlorine            -1 (except in compounds                  with  oxygen / fluorine).

Problems

1.                  Calculate the oxidation state of sulphur in the sulphate ion (SO₄^2-)

Step 1: Allocate known charges

x = charge on sulphur

overall charge = -2

charge on each oxygen = -2

Step 2: Write equation

x + (4×-2)        =  -2

x – 8                = -2

x                      = -2+8

x                      = +6

Oxidation state of sulphur in SO₄^-2 = +6

Always remember to include the fact that these are positive or negative – DON’T Assume!

2.                  Calculate the charge on the manganese in the MnO₄^- ion.

3.                  Calculate the charge on the chromium in Cr₂O₇^2-.

Calculate the oxidation states of the starred ions in these compounds.

1. Br^*O^-           2. Br^*O₃^-          3. HBr              4. N^*H₄⁺

5. P^*Cl₅            6. S^*O₃^2-          7. N^*O₂^-           8. S^*₂O₃^2-

Some formulae have roman numerals e.g. iron (II) bromide. This number gives the oxidation states of the iron.

The compound CuSO₄½H₂O is commonly called copper (II) sulphate. Its correct name is tetraaquocopper (II) tetraoxosulphate (VI)-1-water. However very few chemists call is this and copper (II) sulphate is acceptable.

Write the formulae of these compounds.

a)                  tin (II) oxide

b)                  tin (IV) oxide

c)                  sodium chlorate (III)

d)                  iron (III) nitrate (V)

e)                  potassium chromate (VI)

Use the concept of oxidation states to help you balance these equations, state what is oxidised and which is reduced.

a)                  Fe + Br₂ ? FeBr₃

b)                  F₂ + H₂O ? HF + O₂

c)                  IO₃^- + H⁺ + I^- ? I₂ + H₂O

d)                  S₂O₃^2- + I₂ ? S₄O₆^2- + I^-

Answers