Group 7 - Halogens

Trend in atomic radius

• Increases down the group

• Because of additional electron shells/energy levels

Trend in reactivity

• Decreases down the group

• Because atomic radius and shielding increases

• Making it harder to gain an electron as the positive attraction of the nucleus is weakened

Trend in ionisation energy

• Decreases down the group

• Because of increased atomic radius

• Increased shielding

Trend in boiling point

• Increases down the group

• Because they are simple covalent molecules held together by v.d.W

• Strength of v.d.W increases as Ar increases

• More energy required to overcome

Trend in oxidising power

• Good oxidising agents as they accept electrons from species being oxidised and are reduced

• But oxidising power decreases down the group

• Because more shielding and a greater atomic radius decreases positive attraction from the nucleus

Relative oxidising strength

Cl2 - displaces Br- and I- ions

Br2 - displaces I- ions

I2- doesn’t displace halide ions

Halide ions reducing power

• Good reducing agents as they donate electrons and are oxidised

• Increases down the group as atomic radius and shielding increases

• Outer electron is easier to lose

Redox, fluoride and chloride ions

NaF + H2SO4 → NaHSO4 + HF

NaCl + H2SO4 → NaHSO4 + HCl

Redox, bromide ions

NaBr + H2SO4 → NaHSO4 + HBr

2HBr + H2SO4 → Br2 + SO2 + 2H2O

Redox, iodide ions

NaI + H2SO4 → NaHSO4 + HI

2HI + H2SO4 → I2 + SO2 + 2H2O

6HI + SO2 → H2S + 3I2 + 2H2O

Test for halide ions

• Acidified AgNO3 + dilute NH3 + conc. NH3

• Cl → white ppt → ppt dissolves → no visible change

• Br → cream ppt → no visible change → ppt dissolves

• I → yellow ppt → no visible change → no visible change

Chlorine and cold water

Cl + H2O → ClO- + Cl- + 2H+

• Disproportion reaction bc Cl is reduced and oxidised, 0 to +1 and -1

Chlorine and water in presence of UV

2Cl2 + 2H2O → 4HCl + O2

Use of chlorine

• Used to sterilise water

• Chlorine can be toxic so used in small amounts

Bleach production

NaOH + Cl2 → NaOCl + NaCl + H2O

Chlorine + cold, aqueous sodium hydroxide → sodium hypochlorite

Equation to show how chlorine forms an acidic solution in water

Cl2 + H2O ⇌ HCl + HOCl