Redox Reactions
Chemical reactions often involve the transfer of electrons or changes in oxidation numbers. Redox reactions help explain combustion, batteries, corrosion, and many biological processes.
1. What Is a Redox Reaction?
A redox reaction is a chemical reaction in which oxidation and reduction occur simultaneously. The word redox combines reduction and oxidation.
In this reaction, zinc loses electrons and becomes a zinc ion. Copper ions gain those electrons and become copper metal.
Oxidation
Loss of electrons, increase in oxidation number, or gain of oxygen in the traditional definition.
Reduction
Gain of electrons, decrease in oxidation number, or loss of oxygen in the traditional definition.
Remember: oxidation and reduction always occur together in a redox reaction because electrons lost by one species must be gained by another.
2. Oxidising and Reducing Agents
An agent is the substance that causes oxidation or reduction in another substance.
Oxidising agent
Accepts electrons and causes another species to be oxidised. The oxidising agent itself gets reduced.
Reducing agent
Donates electrons and causes another species to be reduced. The reducing agent itself gets oxidised.
- Zinc is the reducing agent because it donates electrons.
- Cu²⁺ is the oxidising agent because it accepts electrons.
3. Oxidation Number
The oxidation number, or oxidation state, is a bookkeeping value used to track electron changes in compounds and ions. It is not always the same as the actual charge on an atom.
Important rules
- An element in its free form has oxidation number 0. Examples: Na, O₂ and Cl₂.
- A monatomic ion has an oxidation number equal to its charge. For example, Mg²⁺ is +2.
- Group 1 metals are usually +1 in compounds; Group 2 metals are usually +2.
- Oxygen is usually −2, with exceptions such as peroxides.
- Hydrogen is usually +1 with non-metals and −1 in metal hydrides.
- The sum of oxidation numbers in a neutral compound is 0.
- The sum in a polyatomic ion equals the ion's overall charge.
Hydrogen is +1 and oxygen is −2. Let sulfur's oxidation number be x.
2 + x − 8 = 0
x = +6
Therefore, sulfur has oxidation number +6.
4. Identifying Redox Reactions
One useful method is to compare the oxidation numbers of elements before and after a reaction.
| Change | Meaning |
|---|---|
| Oxidation number increases | Oxidation |
| Oxidation number decreases | Reduction |
| Both changes occur | Redox reaction |
| No oxidation numbers change | Not a redox reaction by this test |
Magnesium changes from 0 to +2, so it is oxidised. Oxygen changes from 0 to −2, so it is reduced. Therefore, this is a redox reaction.
5. Balancing Redox Equations
A balanced chemical equation must conserve both atoms and electric charge. Redox equations can be balanced using the half-reaction method.
Half-reaction method
- Separate the reaction into oxidation and reduction half-reactions.
- Balance all elements except hydrogen and oxygen.
- Balance oxygen by adding H₂O.
- In acidic solution, balance hydrogen by adding H⁺.
- Balance charge by adding electrons.
- Multiply the half-reactions so the electrons cancel, then add them together.
- Check that atoms and total charge balance.
Unbalanced reaction:
Balanced equation:
The atoms and total charge are balanced on both sides.
Balancing in basic solution
First balance the equation as appropriate for acidic solution. Then add OH⁻ ions to both sides to neutralise each H⁺ ion, combine H⁺ and OH⁻ to form water, cancel excess water, and check atoms and charge again.
6. Disproportionation Reactions
A disproportionation reaction occurs when the same element in one reactant is simultaneously oxidised and reduced.
Oxygen in hydrogen peroxide has oxidation number −1. It changes to −2 in water and 0 in oxygen gas. Thus, some oxygen is reduced while some is oxidised.
7. Redox Reactions in Everyday Life
Corrosion
Metals such as iron react with substances in their surroundings. Rusting involves redox processes and is encouraged by moisture and oxygen.
Batteries
Electrochemical cells use oxidation and reduction reactions to convert chemical energy into electrical energy.
Combustion
Many fuels react with oxygen through redox processes, releasing energy as heat and often light.
Living organisms
Cellular respiration involves a series of electron-transfer reactions that help cells obtain usable energy.
8. Quick Revision
- Oxidation means electron loss; reduction means electron gain.
- Oxidation number increases during oxidation.
- Oxidation number decreases during reduction.
- The oxidising agent is reduced; the reducing agent is oxidised.
- A balanced redox equation conserves atoms and charge.
- Corrosion, batteries and combustion involve redox chemistry.