Group 7(17), the halogens revision notes

Group 7: The Halogens

Trends in Electronegativity and Boiling Point

As you move down Group 7 in the periodic table, electronegativity decreases. This is due to the increasing atomic radius, which means that the attraction between the nucleus and the bonding electrons weakens. For boiling points, there is an increase down the group, attributed to the greater van der Waals forces resulting from larger molecular sizes.

Oxidising Ability

The oxidising ability of halogens decreases down the group. Fluorine is the strongest oxidising agent, while iodine is the weakest. This trend can be explained by the decreasing electronegativity and increasing atomic size, which make it harder for larger atoms to attract electrons.

Halogen Displacement Reactions

Halogens can displace less reactive halogens from their compounds. For example, chlorine can displace bromine from potassium bromide. The general reaction can be represented as:

Cl2 + 2KBr → 2KCl + Br2

Ionic Equations

When writing ionic equations for displacement reactions, focus on the ions involved. For the example above, the ionic equation is:

Cl2 + 2Br⁻ → 2Cl⁻ + Br2

Tests for Halide Ions

To test for halide ions, add silver nitrate solution. A precipitate forms:

• Chloride ions produce a white precipitate of silver chloride.
• Bromide ions produce a cream precipitate of silver bromide.
• Iodide ions produce a yellow precipitate of silver iodide.

Ammonia can be used to differentiate between the precipitates, as silver chloride dissolves in dilute ammonia, while silver bromide dissolves in concentrated ammonia.

Chlorine in Water Treatment

Chlorine is used to disinfect water, killing bacteria and other pathogens. However, it can form harmful by-products, which raises health concerns.

Disproportionation Reactions

Chlorine undergoes disproportionation when added to water, forming hydrochloric acid and hypochlorous acid:

Cl2 + H2O → HCl + HOCl

In alkaline conditions, chlorine reacts to form chloride ions and chlorate ions:

Cl2 + 2OH⁻ → Cl⁻ + ClO⁻ + H2O

Evaluation of Chlorine Use

While chlorine is effective in water treatment, its use must be balanced with potential health risks from by-products. Consider both the benefits of pathogen removal and the risks of chemical exposure when evaluating chlorine's role in public health.