Everything Science Grade 11

CHAPTER 1. ATOMICCOMBINATIONS 1.11

number of− 1. The oxidation numbers for sodium and chlorine (on the left hand

side of the equation) arezero since these are elements.

Example 17: Oxidation numbers II

QUESTION

Give the oxidation numbers for all the atoms inthe reaction between hydrogen and oxygen to

produce water. The unbalanced equation is shown below:

H 2 + O 2 → H 2 O

SOLUTION

Step 1 : Determine which atomwill have a positive or negative oxidation number

Hydrogen will have a positive oxidation numberand oxygen will have a negative

oxidation number.

Step 2 : Determine the oxidation number for each atom

Hydrogen (group 1) will have an oxidation number of +1. Oxygen (group 16)

will have an oxidation number of− 2.

Step 3 : Check whether the oxidation numbers add upto the charge on the molecule

In the reaction H 2 +O 2 → H 2 O, the oxidation numbers for hydrogen and oxygen

(reactants, left hand sideof the equation) are zerosince these are elements. In the

water molecule (product), the sum of the oxidation numbers is 2(+1)− 2 = 0.

This is correct since theoxidation number of water is zero. Therefore, inwater,

hydrogen has an oxidation number of +1 and oxygen has an oxidation number

of− 2.

Example 18: Oxidation numbers III

QUESTION

Give the oxidation number of sulphur in a sulphate (SO^24 −) ion

SOLUTION

Step 1 : Determine which atomwill have a positive or negative oxidation number

Sulphur has a positive oxidation number and oxygen will have a negative oxida-

tion number.