Page 65 - Chemistry_Form_2

Chemistry
for Secondary Schools

All oxidation numbers must be consistent with the conservation of charge. This
means that for all neutral molecules, the oxidation number of all the atoms must
add up to zero. For example, in H O, two hydrogen atoms each of charge +1
2
combine with one oxygen atom of charge –2. The charge of the H O molecule is
2
+2 – 2 = 0.
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Note: There is a close relationship between valency and oxidation state. However,
they are not the same. Valency is a fixed value, but oxidation state is an
arbitrary value (it may vary).

Example 3.1

Find the oxidation state of chlorine in KClO
3
Solution

The oxidation number of potassium is +1

The oxidation number for oxygen is –2
For the three oxygen atoms, the oxidation number is (–2 × 3) = –6

KClO is a neutral compound. Therefore, the oxidation number of the compound
3
is zero.

Therefore, + 1 + Cl – 6 = 0
Cl = 6 – 1 = +5

The oxidation number of chlorine in KClO is +5.
3

Example 3.2

Find the oxidation number of sulfur in SO 2–
4
Solution

The total charge on the sulfate ion is –2

The oxidation number of oxygen is –2
Therefore, S + (–2 × 4) = –2
S – 8 = –2

S = 8 – 2

S = +6
The oxidation state of sulfur in SO is +6.
2–
4

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