Oxidation, Reduction and Redox Reactions

Oxidation is a chemical process that can be defined in different ways:

1. In Terms of Oxygen Transfer – Oxidation is the gain of oxygen.

• Example: 2Mg + O₂ → 2MgO

2. In Terms of Electron Transfer – Oxidation is the loss of electrons.

3. In Terms of Hydrogen Transfer – Oxidation is the loss of hydrogen.

4. In Terms of Oxidation Number – Oxidation is an increase in oxidation state.

Examples of Oxidation Reactions:

• C + O₂ → CO₂​
• MgH₂ → Mg + H₂↑
• CH₃ − CH₃ → CH₂ = CH₂ + H₂↑

In reactions where oxygen is gained or hydrogen is lost, oxidation occurs. Some oxidation reactions require oxidising agents such as:

• Potassium dichromate (K₂Cr₂O₇​) in acid
• Potassium permanganate (KMnO₄​) in acid
• Hydrogen peroxide (H₂O₂​​)
• Ozone (O₃​)

Reduction is a chemical process that can be explained in four ways:

1. In Terms of Oxygen Transfer – Reduction is the loss of oxygen.

2. In Terms of Electron Transfer – Reduction is the gain of electrons.

• Example: O₂ + 4e⁻ → 2O^2-

3. In Terms of Hydrogen Transfer – Reduction is the gain of hydrogen.

4. In Terms of Oxidation Number – Reduction is a decrease in oxidation state.

Example of Reduction Reaction:

When hydrogen gas is passed over copper oxide (CuO), it removes oxygen, reducing CuO to copper:

CuO + H₂ → Cu + H₂O

• Copper oxide loses oxygen → Reduction
• Hydrogen gains oxygen → Oxidation

A redox reaction is a chemical reaction where oxidation and reduction occur simultaneously.

Examples of Redox Reactions:

1. Cellular Respiration: Glucose is oxidised to carbon dioxide, while oxygen is reduced to water.
2. Combustion: Methane burns in oxygen to form carbon dioxide and water.
3. Photosynthesis: Carbon dioxide and water react in sunlight to form glucose and oxygen.
4. Corrosion: Iron reacts with oxygen and moisture to form rust (Fe₂O₃​​).
5. Formation of Sodium Chloride: Sodium is oxidised from 0 to +1, while chlorine is reduced from 0 to -1.

Redox Reactions in Biological Systems:

Redox reactions also occur in living organisms. For example, during cellular respiration, the enzyme cytochrome C oxidase helps transfer electrons to drive biological energy production.

Oxidation and Reduction in Iron Compounds:

\[\ce{{Fe} <-->[Oxidation][Reduction] \overset{}{FeO} <-->[Oxidation][Reduction] \overset{}{Fe2O3}}\]

Here, iron is gradually oxidised as its oxidation state increases.

Oxidising Agent:

An oxidising agent is a substance that oxidises another substance by accepting electrons. It undergoes reduction itself while causing oxidation in another substance.

Characteristics of an Oxidising Agent:

• Gains electrons during the reaction.
• Often contains oxygen or other electronegative elements.
• Common examples include oxygen (O₂​), hydrogen peroxide (H₂O₂​​), halogens (F, Cl, Br, I), potassium permanganate (KMnO₄​), and potassium dichromate (K₂Cr₂O₇​​​).

Some oxidising agents also transfer oxygen atoms to a substrate, known as oxygenation reagents or oxygen-atom transfer (OAT) agents (e.g., MnO₄⁻, CrO₄²⁻​).

Reducing Agent:

A reducing agent is a substance that reduces another substance by donating electrons. It undergoes oxidation itself while causing reduction in another substance.

Characteristics of a Reducing Agent:

• Loses electrons during the reaction.
• Often contains hydrogen or metals with low electronegativity.
• Common examples include hydrogen (H₂​), sodium (Na), lithium aluminium hydride (LiAlH₄​), sodium amalgam ($)$), formic acid, and sulphites.

Oxidising and Reducing Agents in a Redox Reaction:

Example,

2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l)

• Aluminium (Al) is the reducing agent because it loses electrons and removes oxygen from iron oxide.
• Iron(III) oxide (Fe₂O₃​​) is the oxidising agent because it gains electrons and provides oxygen to aluminium.

In redox reactions, reducing agents always transform into their corresponding oxidised forms, and oxidising agents transform into their reduced forms.