Oxidising & Reducing Agents

An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
An oxidising agent itself gets reduced
- This means that an oxidising agent gains electrons
Therefore, the oxidation number of the oxidising agent decreases
For example:

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

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

In which of the following reactions is the species in bold acting as an oxidising agent?

Answer

Oxidising agents are substances that oxidise other species, gain electrons and are themselves reduced.

Cr₂O₇^2– + 8H⁺ +3SO₃^2– → 2Cr³⁺ + 4H₂O +3SO₄^2–
- The SO₃^2– has an oxidation number of –2
- This becomes SO₄^2–, which still has an oxidation number of –2
- There is no change in oxidation number so this equation cannot be the correct answer
Mg + Fe²⁺ → Mg²⁺ + Fe
- The Fe²⁺ has an oxidation number of +2
- This becomes Fe, which has an oxidation number of 0
- The oxidation number has decreased, which means that the Fe²⁺ has acted as an oxidising agent and this equation is the correct answer
Cl₂ + 2Br^– → 2Cl^– + Br₂
- The Br^– has an oxidation number of –1
- This becomes Br₂, which has an oxidation number of 0
- The oxidation number has increased, which means that Br^– has acted as a reducing agent and this equation cannot be the correct answer
Fe₂O₃ + 3CO → 2Fe + 3CO₂
- The CO has an oxidation number of 0
- This becomes CO₂, which still has an oxidation number of 0
- There is no change in oxidation number so this equation cannot be the correct answer

Roman numerals are used to show the oxidation states of transition metals which can have more than one oxidation number
Iron can be both +2 and +3 so Roman numerals are used to distinguish between them
- Fe²⁺ in FeO can be written as iron(II) oxide
- Fe³⁺ in Fe₂O₃ can be written as iron(III) oxide

Give the full systematic names of the following compounds:

Answer

Answer 1: FeCl₂
- The oxidation number of 2 Cl atoms = –2
- FeCl₂ has no overall charge
- So, the oxidation number of Fe is +2
- Therefore, the systematic name is iron(II) chloride
Answer 2: HClO₄
- The oxidation number of the H atom = +1
- The oxidation number of 4 O atoms = –8
- HClO₄ has no overall charge
- So, the oxidation number of Cl is +7
- Therefore, the systematic name is chloric(VII) acid
Answer 3: NO₂
- The oxidation number of 2 O atoms = –4
- NO₂ has no overall charge
- So, the oxidation number of N is +4
- Therefore, the systematic name is nitrogen(IV) oxide
Answer 4: Mg(NO₃)₂
- The systematic name is magnesium nitrate
- This is a salt of the common acid, which means that they are named without including the oxidation number of the non-metal
Answer 5: K₂SO₄
- The systematic name is potassium sulfate
- Again, this is a salt of the common acid, so it is named without including the oxidation number of the non-metal

Oxidising & Reducing Agents (CIE AS Chemistry)