The Elements of Group 7 (Halogens)

Full Notes

Properties of Halogens (Group 7)

The halogens show trends as you go down the group.

Boiling Point Trend

Boiling points increase down the group.

Explanation:

• Halogens exist as diatomic molecules (X₂).
• Larger molecules have more electrons, leading to stronger London Dispersion forces.
• More energy is required to overcome these intermolecular forces.

Electronegativity Trend

Electronegativity is the ability of an atom to attract a bonding pair of electrons.

Electronegativity decreases down group 7.

Explanation:

• Atomic radius increases, so bonding electrons are further from the nucleus.
• Shielding increases, reducing nuclear attraction to bonding electrons.

As atoms get larger and shielding increases, their ability to attract bonding electrons decreases.

Reactivity Trend

Reactivity decreases down group 7.

Explanation:

• Halogens react by gaining electrons (forming X⁻).
• Larger atoms have weaker nuclear attraction for incoming electrons due to increased shielding and atomic size.

Displacement Reactions with Halide Ions

A more reactive halogen displaces a less reactive halide ion from solution.

For example:

Example Chlorine displaces bromide; bromine displaces iodide

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Br₂ + 2I⁻ → 2Br⁻ + I₂

We can use an organic solvent like hexane to help identify the displaced halogen by colour.

Disproportionation Reactions of Chlorine

Disproportionation reactions have been covered in detail here.

Chlorine can undergo disproportionation reactions when reacted with water and sodium hydroxide solution - you need to know these examples.

With water:

The reaction is a disproportionation reaction as chlorine atoms (from Cl₂) are both oxidized and reduced.

Their oxidation state changes from 0 (in Cl₂) to +1 (in ClO⁻) and −1 (in Cl⁻).

HClO is the active disinfecting agent in water treatment.

With cold, dilute NaOH:

Forms sodium chlorate(I) (NaClO) – the active ingredient in bleach.

With hot alkali:

Forms sodium chlorate(V) – a stronger oxidising agent.

Oxidation Reactions of Halogens

Halogens react by gaining an electron and get reduced, acting as oxidising agents.

Halides react by losing an electron and get oxidised, acting as reducing agents.

Halogens with Group 1 and 2 metals:

Halogens react with group 1 and group 2 metals, forming ionic salts:

For Example: Formation of ionic halides

2Na + Cl₂ → 2NaCl
Mg + Br₂ → MgBr₂

The halogen is reduced (oxidation number 0 → −1), while the metal is oxidised.

Reactions of NaX (sodium halide) with concentrated H₂SO₄:

Sodium Halides react with concentrated sulfuric acid. These reactions show how halide ions act as reducing agents and have different reducing abilities.

Chloride (Cl⁻):
NaCl + H₂SO₄ → NaHSO₄ + HCl (steamy fumes only)

Chloride ions are not strong enough reducing agents to reduce the sulfur in sulfuric acid.

Bromide (Br⁻):
NaBr + H₂SO₄ → NaHSO₄ + HBr
2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

Bromide ions are stronger reducing agents than chloride ions, they can reduce the sulfur from a +6 oxidation state to a +4 oxidation state.

Iodide (I⁻):
NaI + H₂SO₄ → NaHSO₄ + HI
2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O
6HI + SO₂ → H₂S + 3I₂ + 2H₂O

Iodide ions are stronger reducing agents than chloride, they can reduce sulfur from +6 to +4 to −2.

Reducing ability increases: Cl⁻ < Br⁻ < I⁻

Testing for Halide Ions Using Silver Nitrate

Acidified silver nitrate solution (AgNO₃) is used to identify halide ions in solution.

• Halide ions react with Ag⁺ to form precipitates of silver halides.
• The silver halides have different solubilities in ammonia, further enabling identification as the colours of the precipitates can be difficult to distinguish.
  • AgCl dissolves in dilute NH₃,
  • AgBr only in concentrated NH₃
  • AgI does not dissolve in either dilute or concentrated NH₃.

Note - the silver nitrate solution must be acidified with dilute nitric acid to avoid false positive tests with other anions that may form a white precipitate with Ag⁺ ions (such as carbonates).

Hydrogen Halides with Water and Ammonia

You need to know the reactions of hydrogen halides with water and ammonia.

With water:

Hydrogen halides dissolve in water to form strong acids:

For example HCl(g) → H⁺(aq) + Cl⁻(aq)

With ammonia:

Hydrogen halides react with ammonia, forming white fumes of ammonium halides:

For example HCl + NH₃ → NH₄Cl

Fluorine and Astatine Predictions

We can predict the properties of fluorine and astatine based on their positions in group 7 - following the general trends shown by the other halogens.

Fluorine (F₂)

• Top of the group, meaning:
  • Most reactive halogen
  • Strongest oxidising agent
  • Reacts violently with nearly all substances

Astatine (At₂):

• Bottom of the group, meaning:
  • Least reactive halogen
  • Follows general group trends

Summary

• Boiling point increases down Group 7 due to stronger London forces.
• Electronegativity and reactivity decrease down the group.
• More reactive halogens displace less reactive halide ions from solution.
• Chlorine shows disproportionation with water and with cold and hot alkali.
• Halide ions act as reducing agents and reducing ability increases from Cl⁻ to I⁻.
• Concentrated sulfuric acid with NaX gives different products for Cl⁻, Br⁻, I⁻.
• Silver nitrate test distinguishes halides using precipitate colour and ammonia solubility.
• Fluorine is most reactive while astatine is least reactive following group trends.