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
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
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
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. Cr2O72- + 8H+ + 3SO32- → 2Cr3+ + 4H2O+ 3SO42-
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
