Oxidation and Reduction Occuring Simultaneously

Remark

When a substance is oxidized, another should be reduced as illustrated below:
Acid and Bases

Question: How do we determine whether a chemical reaction is an oxidation-reduction reaction?

Answer:
By following the changes in the oxidation numbers of each element in the reaction.

Example

Zn(s) + 2HCl(l) → ZnCl₂(aq) + H₂(g)
Here, during oxidation, Zn(s) lost 2e to become Zn²⁺ whereas H⁺ gained 2e to become H₂(g)
Zn (ON is zero) is oxidized to Zn²⁺ (ON is +2 ) , while H(ON is +1 ) is reduced to H₂ (ON is zero).

H caused Zn to be oxidized → H is an "oxidixing agent"
Zn caused H to be reduced → Zn is an "reducing agent"

Balancing Equations

Both mass and charge is reserved in chemical reactions.
Electrons transfer from one compound to another, but not lost in a chemical reaction.
A convenient way to balance an oxidation-reduction reaction is to separate the reaction into half reactions,
and deal with each half reaction separately then recombine them in a one final balanced reaction.

Examples

Cr₂O₇²⁻(aq) + CH₃OH(aq) → Cr³⁺(aq) + HCO₂H(aq)
NO₂⁻(aq) + Cr₂O₇²⁻(aq) → Cr³⁺(aq) + NO₃⁻(aq)

Balancing Equations in a Basic Solution

Here OH⁻ and H₂O are used instead of H⁺ and H₂O
The same method is used, but here, OH⁻ is added to "neutralize" the H⁺ used, forming H₂O

Examples:

MnO₄⁻(aq) + Br⁻(aq) → MnO₂(s) + BrO₃⁻(aq)
Cr(OH)₃(s) + ClO⁻(aq) → CrO₄²⁻(aq) + Cl₂(g)

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Date of last modification: Summer , 2019