Redox ReactionsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is oxidation and reduction in terms of oxygen?
The original, simplest definitions of oxidation and reduction are based on oxygen:
- Oxidation is the gain of oxygen by a substance
- Reduction is the loss of oxygen from a substance
For example, when iron oxide reacts with carbon, the iron oxide loses oxygen (it is reduced) and the carbon gains oxygen (it is oxidised).
Section 2
What is oxidation and reduction in terms of electrons?
A more general, modern definition of oxidation and reduction is based on electron transfer:
- Oxidation is the loss of electrons
- Reduction is the gain of electrons
A useful way to remember this is the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). This definition works for reactions that do not involve oxygen at all, such as metal displacement reactions.
When asked to define oxidation or reduction, always give both the oxygen definition and the electron definition unless the question specifies one.
Section 3
What is a redox reaction?
When oxidation and reduction happen at the same time in the same reaction, it is called a redox reaction (a combination of REDuction and OXidation). One species is oxidised while another is reduced simultaneously — neither can happen without the other.
Section 4
How are redox reactions described in this specification?
Redox ideas apply across several topics in this course:
- Metal displacement reactions — a more reactive metal is oxidised (loses electrons) while a less reactive metal ion is reduced (gains electrons)
- Extraction of metals — for example, iron oxide is reduced by carbon in the blast furnace
- Electrolysis — at the cathode, positive ions are reduced (gain electrons); at the anode, negative ions are oxidised (lose electrons)
In each case, you should be able to identify which species is oxidised and which is reduced, using either the oxygen or the electron definition.
In the displacement reaction Zn + CuSO4 → ZnSO4 + Cu, zinc atoms lose electrons to form Zn2+ ions (oxidation), while Cu2+ ions gain electrons to form copper atoms (reduction).
Must Know
- Oxidation = gain of oxygen OR loss of electrons
- Reduction = loss of oxygen OR gain of electrons
- OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
- A redox reaction is one where oxidation and reduction happen simultaneously
- Redox ideas apply to displacement reactions, metal extraction, and electrolysis
- Both oxygen-based and electron-based definitions can be used to describe the same reaction
That's the notes covered.
Carry on to the next subtopic.