Oxidising & Reducing Agents (Cambridge (CIE) AS Chemistry)
Revision Note
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
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