Redox Reactions - Oxidation

Definition of Oxidation

In the context of chemical reactions, oxidation can be defined in two key ways:

• (a) Loss of Electrons: Oxidation is the process of a substance losing electrons.
• (b) Increase in Oxidation Number: Oxidation is also defined as an increase in the oxidation number of an atom.

Understanding the Concepts

These two definitions are fundamentally linked. Electrons are the fundamental particles involved in oxidation, and changes in oxidation number directly reflect the gain or loss of these electrons.

Oxidation Number

The oxidation number is a way of assigning a positive or negative electrical charge to an atom in a chemical compound. It is a useful concept for tracking electron transfer in redox reactions.

Rule	Example
Elements in their elemental state	$Na^0$, $O^0$, $Cu^0$
Monoatomic ions	$Na^+$, $Cl^-$, $Mg^{2+}$
Oxygen in compounds	Oxygen is usually -2 (e.g., in $H_2O$, $O^2^-$) except in peroxides ($H_2O_2$) where it is -1 and in $O_2$ where it is 0.
Hydrogen in compounds	Hydrogen is usually +1 (e.g., in $HCl$, $H_2O$) except in metal hydrides ($NaH$) where it is -1.
Halogens in compounds	Halogens are usually -1 (e.g., $Cl^-, Br^-, I^-$) except when they are bonded to other halogens.
Noble gases	Noble gases have an oxidation number of 0.
Sum of oxidation numbers in a neutral compound	The sum of the oxidation numbers of all atoms in a neutral compound is 0.
Sum of oxidation numbers in a polyatomic ion	The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.

Example: Consider the reaction of zinc with hydrochloric acid:

$$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$$

In this reaction, zinc (Zn) loses two electrons to become a zinc ion ($Zn^{2+}$). This is an oxidation process, and the oxidation number of zinc increases from 0 to +2.

Similarly, hydrogen in hydrochloric acid gains electrons and undergoes reduction.