• 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

Answer

• 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

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

Answer

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

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

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

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

Answer 3:
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

Answer

• 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