Define oxidation in terms of change in oxidation number - Leaving Cert Chemistry - Question 11 - 2005

When chlorine gas (Cl₂) is bubbled into an aqueous solution of sodium bromide (NaBr), the solution turns a reddish-brown color. This color change occurs due to the reaction of bromide ions (Br⁻) with chlorine gas.

In this reaction, bromide ions (Br⁻) are oxidized to bromine (Br₂) while chlorine gas (Cl₂) is reduced to chloride ions (Cl⁻). The half-reactions can be represented as:

• Oxidation: ( ext{2Br}^- \rightarrow \text{Br}_2 + 2 e^-)
• Reduction: (\text{Cl}_2 + 2 e^- \rightarrow 2\text{Cl}^- )

This illustrates that bromide ions lose electrons (are oxidized) and chlorine gains electrons (is reduced).

Oxidation occurs at the anode. In this scenario, if electrode A is the anode and electrode B is the cathode, oxidation will take place at electrode A.

In the electrolysis of acidified water, the species that is oxidized is water (H₂O). It can be represented in the overall reaction.

The balanced half-equation for the oxidation of water at the anode can be represented as:

$\text{2H}_2\text{O} \rightarrow \text{O}_2 + 4H^+ + 4e^-$

This equation shows that water is oxidized to produce oxygen gas along with protons and electrons.