# 1.9.3 Redox Equations

### 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

#### Writing overall redox reactions

Manganate(VII) ions (MnO4) react with Fe2+ ions in the presence of acid to (H+) form Mn2+ ions, Fe3+ ions and water

Write the overall redox equation for this reaction

• 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)

#### Oxidation and reduction

In each of the following equation, state which reactant has been oxidised and which has been reduced.

1. Na++ Cl →  NaCl
2. Mg + Fe2+  →  Mg2+ + Fe
3. CO + Ag2O  →  2Ag + CO2

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

#### Balancing disproportionation reactions

Balance the disproportionation reaction which takes place when chlorine is added to hot concentrated aqueous sodium hydroxide

The products are Cland ClO3ions and water

• 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

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