Halide Ions as Reducing Agents and Trends in Reducing Strength of Halide Ions

Ability of Halide Ions as Reducing Agents

This section discusses the ability of halide ions to act as reducing agents, particularly in reactions with concentrated sulfuric acid. A reducing agent donates electrons to another substance, reducing it in the process. The reducing strength of halide ions increases as you move down the group.

With Bromide Ions

Bromide ions (Br^–) are strong enough to reduce concentrated sulfuric acid, being oxidized to bromine (Br₂):

Half-reaction: 2Br^– → Br₂ + 2e^–

In this reaction, bromide ions reduce sulfuric acid (H₂SO₄) to sulfur dioxide (SO₂):

Half-reaction:  H₂SO₄ + 2H⁺ + 2e^– → SO₂ + 2H₂O

Combining these two half-equations gives:

The overall ionic equation combining these half-reactions is:

Overall reaction: H₂SO₄ + 2H⁺ + 2Br^– → Br₂ + SO₂ + 2H₂O

With Iodide Ions

Iodide ions (I^–) are even stronger reducing agents than bromide ions. They are oxidized to iodine (I₂):

Half-reaction: 2I^– → I₂ + 2e^–

The iodide ions reduce sulfuric acid to a variety of products, including sulfur dioxide (SO₂), sulfur (S), and hydrogen sulfide (H₂S):

• Sulfur dioxide formation: H₂SO₄ + 2H⁺ + 2e^– → SO₂ + 2H₂O
• Sulfur formation: H₂SO₄ + 6H⁺ + 6e^– → S + 4H₂O
• Hydrogen sulfide formation: H₂SO₄ + 8H⁺ + 8e^– → H₂S + 4H₂O

The most significant of these reduction products is hydrogen sulfide. The combined equation for iodide ions reducing sulfuric acid is:

Overall reaction: H₂SO₄ + 8H⁺ + 8I^– → 4I₂ + H₂S + 4H₂O

Summary of the Trend in Reducing Ability

• Fluoride (F^–) and chloride (Cl^–) ions do not reduce concentrated sulfuric acid.
• Bromide ions (Br^–) reduce sulfuric acid to sulfur dioxide and are oxidized to bromine.
• Iodide ions (I^–) reduce sulfuric acid to a mixture of products, including hydrogen sulfide, and are oxidized to iodine.

The reducing ability of halide ions increases as you move down the Group in the periodic table.

Explaining the Trend

• When a halide ion acts as a reducing agent, it donates electrons to another substance, meaning the halide ion itself loses electrons.
• As you move down the Group, the outer electrons of halide ions are further from the nucleus and are more shielded by inner electrons. This makes it easier for the halide ions to lose electrons because the attraction between the outer electrons and the nucleus is weaker.