# 1.6.2 Redox Reactions

### Balancing Redox Reactions

• Oxidation numbers can be used to balance chemical equations
• Roman numerals between brackets are used to show the ox. no. of an atom that can have multiple oxidation states, eg:

Fe(III) = iron with ox. no. +3

#### Worked example: Writing overall redox reactions

• Step 1: Write the unbalanced equation and identify the atoms which change in ox. no.

• Step 2: Deduce the ox.no. changes

• Step 3: Balance the ox.no. changes

• Step 4: Balance the charges

• Step 5: Balance the atoms

### Redox & Disproportionation Reactions

#### Oxidation

• Oxidation is the gain of oxygen, eg:

Cu + H2O → CuO + H2

(Cu has gained an oxygen and is oxidised)

• Oxidation is also the loss of a hydrogen, eg:

2NH3 + 3Br2 → N2 + 6HBr

(N has lost a hydrogen and is oxidised)

• Oxidation is also the loss of electrons, eg:

Cu2+ → Mg → Mg2+ + Cu

(Mg has lost two electrons and is oxidised)

• Oxidation causes an increase in ox. no., eg:

Cu2+ + Mg → Mg2+ + Cu

(change in ox. no. of Mg is +2 thus Mg is oxidised)

#### Reduction

• Reduction is the loss of oxygen, eg:

Cu+ H2O → 2CuO + H2

(O has been reduced)

• Reduction is also the gain of a hydrogen, eg:

2NH3+ 3Br2 → N2 + 6HBr

(Br has been reduced)

• Reduction is also the gain of electrons, eg:

Cu2+ + Mg → Mg2+ + Cu

(Cu has been reduced)

• Reduction causes a decrease in oxidation number, eg:

Cu2+ + Mg → Mg2+ + Cu

(change in ox. no. of Cu is -2 thus Cu is reduced)

Use the acronym “Oil Rig” to help you remember the definitions of oxidation and reduction

#### Redox reactions

• Redox reactions are reactions in which oxidation and reduction take place together
• While one species is oxidising, another is reducing in the same reaction, eg:

Cu2++ Mg → Mg2+ + Cu

(Cu has been reduced and Mg has been oxidised)

#### Worked example: Oxidation and reduction

Oxidised: Cl as the ox. no. has increased by 1

Reduced: Na+ as the ox. no. has decreased by 1

Oxidised: Mg as the ox. no. has increased by 2

Reduced: Fe2+ as the ox. no. has decreased by 2

Oxidised: C as it has gained oxygen

Reduced: Ag as it has lost oxygen

#### Disproportionation reactions

• A disproportionation reaction is a reaction in which the same species is both oxidised and reduced

Example of a disproportion reaction in which the same species (chlorine in this case) has been both oxidised and reduced

#### Worked example: Balancing disproportionation reactions

• Step 1: Write the unbalanced equation and identify the atoms that change in ox. no.

• Step 2: Deduce the ox. no. changes

• Step 3: Balance the ox. no. changes

• Step 4: Balance the charges

• Step 5: Balance the atoms

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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