Classical Idea of Redox Reactions
Quick Notes
- Oxidation: Addition of oxygen / Removal of hydrogen
- Reduction: Removal of oxygen / Addition of hydrogen
- Oxidation and reduction always occur together in a redox reaction.
- Redox reaction = Oxidation + Reduction
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Examples:
- C + O2 → CO2 (oxidation)
- CuO + H2 → Cu + H2O (reduction of CuO, oxidation of H2)
- Common in combustion, corrosion, and metal extraction (metallurgy)
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Example:
- ZnO + C → Zn + CO
- ZnO is reduced (loses oxygen)
- C is oxidized (gains oxygen)
Full Notes
In the early development of chemistry, redox reactions were described without reference to electrons. The classical concept defines oxidation and reduction in terms of oxygen and hydrogen transfer.
Oxidation
‘Oxidation = addition of oxygen/electronegative element to a substance or removal of hydrogen/ electropositive element from a substance’
Examples Classical oxidation processes
- C + O2 → CO2
Carbon is oxidized by gaining oxygen. - H2 + ½ O2 → H2O
Hydrogen is oxidized by gaining oxygen.
This understanding arose from processes like combustion, where oxygen from the air combines with fuel.
Reduction
'Reduction = removal of oxygen/electronegative element from a substance or addition of hydrogen/ electropositive element to a substance'
Example Reduction of copper(II) oxide
CuO + H2 → Cu + H2O
Copper oxide (CuO) is reduced to metallic copper (Cu) by losing oxygen. Simultaneously, hydrogen is oxidized to water by gaining oxygen.
The term "reduction" is rooted in the Latin reductio, meaning to restore – often used in metallurgy for obtaining metals from their oxides.
Redox Reactions – Always Coupled
Oxidation and reduction always occur together (they are interdependent).
- When one substance loses oxygen (is reduced), another gains it (is oxidized), and vice versa.
- Thus, the term redox reaction (reduction–oxidation) is used to describe these coupled changes.
Application
A classic real-world application is extraction of metals:
Example Metallurgical reduction with carbon
ZnO + C → Zn + CO
- ZnO is reduced to Zn (loses oxygen)
- C is oxidized to CO (gains oxygen)
This type of redox reaction is the basis for many metal extraction techniques, such as in the reduction of metal oxides using carbon or hydrogen.
Summary
- Oxidation is addition of oxygen or removal of hydrogen.
- Reduction is removal of oxygen or addition of hydrogen.
- Every redox process has simultaneous oxidation and reduction.
- Classical redox ideas explain combustion, corrosion and metallurgical extraction.