Types of Reduction & Oxidation (Edexcel AS Chemistry): Revision Note
Electron Transfer & Change in Oxidation Number
Oxidation and reduction in a reaction can be demonstrated in terms of electron transfer
For example:
2NH3 + 3Br2 → N2 + 6HBr
The oxidation number of N in NH3 has changed from -3 to 0
As the oxidation number has increased, nitrogen has been oxidised
The oxidation number of Br has changed from 0 to -1
As the oxidation number has decreased, bromine has been reduced
Overall, nitrogen has reduced bromine by donating electrons
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 – gains electrons
Therefore, the ox. no. of the oxidising agent decreases

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 ox. no. of the reducing agent increases

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
Worked Example
Four reactions are shown.
In which reaction 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:
The correct option is 2
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 2, Fe2+ oxidises Mg(0) to Mg2+(+2) and is itself reduced from Fe2+(+2) to Fe(0)
Examiner Tips and Tricks
Some exam questions will ask for the strongest or weakest reducing / oxidising agent
The strongest reducing agent comes from the equation with the most negative electrode potential
Remember: A reducing agent is itself oxidised
So, it will lose electrons / increase in oxidation number
The strongest oxidising agent comes from the equation with the most positive electrode potential
Remember: An oxidising agent is itself reduced
So, it will gain electrons / decrease in oxidation number
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