Introduction to Electron Transfer Reactions
Redox reactions are among the most important reaction types in chemistry , they power your phone battery, drive cellular respiration, cause metals to rust, and underpin vast industrial processes. Understanding them is essential not just for passing exams, but for making sense of the chemical world around you.
Redox reaction: A redox reaction is a chemical reaction involving the simultaneous transfer of electrons between species , one species loses electrons (oxidation) and another gains electrons (reduction). These two processes always occur together.
The word "redox" is a portmanteau of reduction and oxidation. In every redox reaction, you can identify:
- A species that is oxidised (loses electrons, becomes more positive)
- A species that is reduced (gains electrons, becomes more negative)
Use the classic mnemonic OIL RIG to remember which process is which:
- Oxidation Is Loss (of electrons)
- Reduction Is Gain (of electrons)
Because oxidation and reduction always occur simultaneously, if you know one half of the reaction, you can always deduce the other.
Defining Oxidation and Reduction
Oxidation and reduction can be defined in three complementary ways. The electron transfer definition is the most versatile and universally applicable, but the others are useful in specific contexts.
1. In terms of electron transfer (most important)
- Oxidation: loss of electrons
- Reduction: gain of electrons
2. In terms of oxygen
- Oxidation: gain of oxygen
- Reduction: loss of oxygen
3. In terms of hydrogen
- Oxidation: loss of hydrogen
- Reduction: gain of hydrogen
Electron transfer example:
Sodium reacting with chlorine:
Oxygen gain/loss example:
The reduction of copper(II) oxide by hydrogen:
CuO loses oxygen → reduced. H₂ gains oxygen → oxidised.
Hydrogen gain/loss example:
Hydrogenation of ethene:
Ethene gains hydrogen → reduced.
The oxygen and hydrogen definitions are traditional and useful in organic chemistry, but they do not apply in all situations (e.g., reactions with no oxygen or hydrogen). Always default to the electron transfer definition when in doubt.