Oxidation & Reduction

Oxidising agent

An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
An oxidising agent itself gets reduced – gains electrons
Therefore, the oxidation number of the oxidising agent decreases

H₂O₂ acting as the oxidising agent

Balanced chemical equation identifying an oxidising agent

Example of an oxidising agent in a chemical reaction

Reducing agent

A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons
A reducing agent itself gets oxidised – loses/donates electrons
Therefore, the oxidation number of the reducing agent increases

H₂O₂ acting as a reducing agent

Balanced chemical equation identifying an reducing agent

Example of a reducing agent in a chemical reaction

For a reaction to be recognised as a redox reaction, there must be both an oxidising and reducing agent
Some substances can act both as oxidising and reducing agents
Their nature is dependent upon what they are reacting with and the reaction conditions

Oxidising & Reducing Agents Table

Oxidising agents....	Reducing agents....
.... oxidise other species	.... reduce other species
.... accept electrons	.... donate electrons
.... are themselves reduced	.... are themselves oxidised

Applying the definitions of oxidising and reducing agents allows you to identify them in chemical equations
By deducing the oxidation numbers of the species you can determine whether it has been oxidised or reduced

Oxidation number of redox line

Oxidation number and redox

A visual reminder of oxidation numbers and redox. This is like an elevator in a building going up to higher floors is oxidation and going down to the basement is reduction

Worked example

Four reactions are shown. In which reaction is the species in blue acting as an oxidising agent?

A. Cr₂O₇^2- + 8H⁺ + 3SO₃^2-→ 2Cr³⁺ + 4H₂O+ 3SO₄^2-

B. Mg + Fe²⁺ → Mg²⁺ + Fe

C. Cl₂ + 2Br^- → 2Cl^-+ Br₂

D. Fe₂O₃ + 3CO → 2Fe + 3CO₂

Answer:

The correct option is B.
- Oxidising agents are substances that oxidise other species, gain electrons and are themselves reduced.
- Write down the oxidation numbers of each species in the reaction

0		+2		+2		0
Mg	+	Fe²⁺	→	Mg²⁺	+	Fe

- In equation B, Fe²⁺ oxidises Mg(0) to Mg²⁺(+2) and is itself reduced from Fe²⁺(+2) to Fe(0)

Examiner Tip

Don't forget:
- Oxidation is the gain of oxygen or the loss of hydrogen
- Reduction is the loss of oxygen or the gain of hydrogen
Using these other definitions can make it quicker / easier to spot which species is being reduced or oxidised

Oxidation & Reduction (HL IB Chemistry)

Revision Note