Chemical Reactions - Redox

Reduction

In a chemical reaction, a reduction occurs when a species gains electrons. This is a fundamental concept in redox (reduction-oxidation) chemistry.

Here's a more detailed breakdown:

• Gain of Electrons: A substance that accepts electrons is said to be reduced. The number of electrons gained by the substance increases.
• Decrease in Oxidation Number: The oxidation number of the reduced substance decreases. This is because gaining electrons is associated with a more negative charge, hence a lower oxidation state.

The term "reduction" is often paired with "oxidation" because they always occur together. If one substance is being reduced, another substance must be oxidized (losing electrons).

For example, in the reaction $Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$, zinc (Zn) is reduced. Zinc atoms lose two electrons to become zinc ions ($Zn^{2+}$), resulting in a decrease in their oxidation number from 0 to +2.

$$Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$$

The copper ion ($Cu^{2+}$) is oxidized in this reaction, meaning it loses electrons and its oxidation number increases from +2 to 0.

$$Cu^{2+}(aq) \rightarrow Cu(s) + 2e^-$$

This demonstrates the principle that reduction and oxidation are always coupled processes.