Oxidising & Reducing Agents
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
- This means that an oxidising agent gains electrons
- Therefore, the oxidation number of the oxidising agent decreases
- For example:
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
- 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
Worked example
In which of the following reactions is the species in bold acting as an oxidising agent?
- Cr2O72– + 8H+ +3SO32– → 2Cr3+ + 4H2O +3SO42–
- Mg + Fe2+ → Mg2+ + Fe
- Cl2 + 2Br– → 2Cl– + Br2
- Fe2O3 + 3CO → 2Fe + 3CO2
Answer
- Oxidising agents are substances that oxidise other species, gain electrons and are themselves reduced.
- Cr2O72– + 8H+ +3SO32– → 2Cr3+ + 4H2O +3SO42–
- The SO32– has an oxidation number of –2
- This becomes SO42–, 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
- 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
Worked example
Give the full systematic names of the following compounds:
- FeCl2
- HClO4
- NO2
- Mg(NO3)2
- K2SO4
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
