# 1.9.3 Redox Equations

### Balancing Redox Reactions

• Balancing equations using redox principles is a useful skill and is best illustrated by following an example
• It is important to follow a methodical step-by-step approach so that you don’t get lost:

#### Worked Example

Writing overall redox reactions

Manganate(VII) ions (MnO4) react with Fe2+ ions in the presence of acid (H+) to 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 oxidation state

Step 2: Deduce the oxidation state changes

Step 3: Balance the oxidation state changes

Step 4: Balance the charges

Step 5: Finally, 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 hydrogen, eg:

2NH3 + 3Br2 → N2 + 6HBr

(NH3 has lost 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 oxidaiton state, 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

(H2O has been reduced)

• Reduction is also the gain of 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

(the change in oxidation state of Cu is -2 thus Cu is reduced)

#### Redox reactions

• Redox reactions are reactions in which oxidation and reduction take place simultaneously
• 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

In each of the following equations, 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 oxidation state has increased by 1
• Reduced: Na+ as the oxidation state has decreased by 1

• Oxidised: Mg as the oxidation state has increased by 2
• Reduced: Fe2+ as the oxidation state 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

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 oxidation state:

Step 2: Deduce the oxidation state changes:

Step 3: Balance the oxidation state changes:

Step 4: Balance the charges

Step 5: Balance the atoms

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