Oxidation and Reduction Reaction – SS2 Chemistry Lesson Note (First Term)

Oxidation and reduction are processes defined in terms of oxygen, hydrogen, electronegative elements or electron transfer. In terms of electrons, oxidation is the loss of electrons and reduction is the gain of electrons. In a redox reaction, the oxidation number of an element changes between reactants and products.

Behavioural objectives

By the end of the lesson, students should be able to:

• define oxidation and reduction in terms of oxygen, hydrogen, electronegative elements and electron transfer;
• state the rules for determining oxidation numbers;
• calculate the oxidation number of an element in a compound;
• give the IUPAC name of a compound using the oxidation number of its metal;
• identify a redox reaction using oxidation numbers.

What are oxidation and reduction?

The following definitions are used for the oxidation and reduction processes.

1. In terms of oxygen: Oxidation is the addition of oxygen to a substance, while reduction is the removal of oxygen from a substance.
   Example: 2Mg(s) + O₂(g) → 2MgO(s)
2. In terms of hydrogen: Oxidation is the removal of hydrogen from a substance, while reduction is the addition of hydrogen to a substance.
   Example: H₂S(g) + Cl₂(g) → 2HCl(g) + S(s)
3. In terms of electronegative elements: Oxidation is the addition of an electronegative element to a substance, while reduction is the removal of an electronegative element from a substance.
   Example: 2Na(s) + Cl₂(g) → 2NaCl(s)
4. In terms of electron transfer: Oxidation is the loss of electrons from a substance, while reduction is the gain of electrons by a substance.
   Example: 2FeCl₂(s) + Cl₂(g) → 2FeCl₃(s)

What is an oxidation number, and how does it relate to IUPAC naming?

The oxidation number (or oxidation state) of an element in a neutral compound or radical (ion) is the numerical electrical charge it appears to have, as determined by a set of arbitrary rules.

Rules for determining oxidation numbers

1. The oxidation number of all uncombined elements is zero, e.g. Na, Cl.
2. The oxidation number of an ion consisting of a single element is the same as the ionic charge on it. For example, the oxidation number of Na⁺ is +1 and that of Cl^– is -1.
3. The oxidation number of an ion consisting of more than one element (a radical) is the algebraic sum of the oxidation numbers of all the elements in the ion.
4. The oxidation number of a neutral compound is zero. It is the algebraic sum of the oxidation numbers of all the elements in the compound.

Worked example

Question: Find the oxidation number of the manganese atom in KMnO₄ and give the IUPAC name of the compound.

Solution:

KMnO₄ = 0
(+1) + Mn + 4(-2) = 0
+1 + Mn – 8 = 0
Mn – 7 = 0
Mn = +7

The oxidation number (state) of manganese in KMnO₄ is +7.

Hence the IUPAC name of the compound is potassium tetraoxomanganate(VII).

How are redox reactions identified using oxidation numbers?

Redox reactions can be identified with the aid of oxidation numbers. In a redox reaction, the oxidation number of an element on the reactant side differs from its oxidation number on the product side. There must be changes in oxidation numbers.

Evaluation

1. Define oxidation and reduction in terms of electron transfer.
2. State the oxidation numbers of Na⁺ and Cl^–.
3. What is the oxidation number of an uncombined element?
4. How can oxidation numbers show that a reaction is a redox reaction?

Assignment

1. Write the definitions of oxidation and reduction in terms of oxygen, hydrogen, electronegative elements and electron transfer.
2. State the rules for determining oxidation numbers.