3.2.5 Group 7(17), the Halogens - Trends in the reducing abilities of the halide ions

Specification

The reducing ability of the halide ions

From the previous page, the more reactive halogen will displace the ions of a less reactive halogen from its solution. This is a redox reaction with the halogen acting as an oxidising agent and the halide ion as a reducing agent.

Chlorine oxidises iodide ions to iodine and is itself reduced to chloride ions:

2I^- + Cl₂ 2Cl^- + I₂

The trend in reducing power of the halide ions is opposite to the trend in oxidising power of the halogen elements. This means that the iodide ion is the best reducing agent.

Iodide ions can also reduce other species. This is used in the analysis of copper(II) ions in solution. Excess iodide ions (usually potassium iodide) are added to the copper(II) solution and the iodine produced is titrated using sodium thiosulfate solution.

The titration is carried out until the colour of the iodine solution nearly disappears and then some starch indicator is added to highlight the remaining iodine. The titration is then continued until the starch/iodine colour completely disappears.

2I^- + 2Cu²⁺ 2Cu⁺ + I₂

I₂ + 2S₂O₃^2- S₄O₆^2- + 2I^-

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The reaction of NaX and H₂SO₄

Sulfuric acid is a powerful acid, dehydrating agent, sulfonating agent and a protonating agent. Hence, it is a polifacetic substance that can react, or cause reactions to take place, via several mechanisms.

With alkali metal chlorides, sulfuric acid sets up an equilibrium:

H₂SO₄ + MX MHSO₄ + HX(g)

As the hydrogen halide is a volatile gas it is removed from the equilibrium pulling the reaction to the right hand side. If the metal halide is in excess the reaction can go further and the metal hydrogen sulfate can react with more metal halide:

MHSO₄ + MX M₂SO₄ + HX(g)

The two equations can be combined to give:

H₂SO₄ + 2MX M₂SO₄ + 2HX(g)

This provides a means of preparing all of the hydrogen halides. However, in the preparation of hydrogen iodide, it is easily decomposed by heat and iodine usually contaminates the products.

2HI H₂ + I₂

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