Oxidation & Reduction (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Oxidation & Reduction
There are three definitions of oxidation and reduction used in different branches of chemistry
The different definitions of oxidation and reduction, with an example are shown below:
Oxidation | Reduction |
|---|---|
Gain of oxygen 2Mg + O2 → 2MgO Magnesium has been oxidised | Loss of oxygen 2CuO + C → 2Cu + CO2 Copper oxide has been reduced |
Loss of electrons 2O2- → O2 + 4e- Oxide ions have been oxidised | Gain of electrons Al3+ + 3e-→ Al Aluminium has been reduced |
Loss of hydrogen CH3OH→ CH2O + H2O The CH3OH has been reduced | Gain of hydrogen C2H4 + H2 → C2H6 Ethene has been reduced |
We will focus on oxidation and reduction in terms of electrons
In order to remember the definition in terms of electrons, we can use the phrase, OIL RIG
A redox reaction is one in which oxidation and reduction occur simultaneously
E.g. The reaction between copper oxide and magnesium
CuO (s) + Mg (s) → Cu (s) + MgO (s)
Cu2+ + 2e- → Cu
Mg - 2e- → Mg2+
Copper gains electrons so has been reduced
Magnesium loses electrons so has been oxidised
Oxidising & Reducing Agents
Oxidising 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 – it gains/accepts electrons
Oxidising agents can therefore be referred to as electron acceptors
The oxidation state of an oxidising agent will decrease
Reducing agents
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
Reducing agents an be referred to as electron donors
The oxidation state of a reducing agent will increase
Worked Example
Calcium and iodine react to form calcium iodide:
Ca (s) + I2 (s) → CaI2 (s)
Which species has been oxidised?
Which species has been reduced?
What is the oxidising agent?
What is the reducing agent?
Answers:
Calcium has been oxidised because it lost/donated electrons to iodine
Iodine has been reduced because it gained/accepted electrons from calcium
Iodine is the oxidising agent because it caused calcium to lose electrons and was itself reduced by accepting electrons
Calcium is the reducing agent because it caused iodine to gain electrons and was itself oxidised by losing electrons
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