3.1.3 The Halogens

Halogens

Group 7 elements.

Properties of halogens

Low melting and boiling points; Exist as diatomic molecules.

Trend in boiling point down group 7

Increases down the group because the size of the atom increases as more occupied electron shells lead to stronger London forces of attraction between molecules, requiring more energy to break.

Trend in reactivity down group 7

Reactivity decreases because atomic radius increases, electron shielding increases, and the ability to gain an electron and form 1- ions decreases.

Trend in oxidising ability down the group

Decreases down group (Cl strongest, I weakest) because Cl has the fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus, making it easiest to gain electrons and be reduced.

Trend in reducing ability of the halides down the group

Increases down the group (Cl- weakest, I- strongest) because I- has the most occupied electron shell, so outer electrons are further from the nucleus, making it easiest to be oxidised.

Displacement reaction

When a more reactive halogen displaces a less reactive halide.

Colour of chlorine in water

Pale green.

Colour of bromine in water

Orange.

Colour of iodine in water

Brown.

Colour of chlorine in cyclohexane

Pale green.

Colour of bromine in cyclohexane

Orange.

Colour of iodine in cyclohexane

Violet.

Halides oxidised by chlorine

Br- & I- ions.

Equation for chlorine oxidising bromide ions

Cl₂ (aq) + 2Br- (aq) → 2Cl- (aq) + Br₂ (aq).

Equation for Cl2 oxidising 2I-

Cl₂ (aq) + 2I- (aq) → 2Cl- (aq) + I₂ (aq).

Halides oxidised by bromine

I- ions.

Equation for bromine oxidising iodide ions

Br₂ (aq) + 2I- (aq) → 2Br- (aq) + I₂ (aq).

Halides oxidised by iodine

Does not oxidise Cl- or Br-.

Disproportionation

The oxidation and reduction of the same element in a redox reaction.

Equation for the reaction of Cl2 with water

Cl₂ (g) + H₂O (l) → HClO (aq) + HCl (aq).

Type of reaction of chlorine with water

Disproportionation; chlorine is both oxidised and reduced.

Why chlorine is added to drinking water

It kills the bacteria in the water and makes it safer to drink.

Forms of the chlorate ion

ClO- is chlorate (I); ClO₃- is chlorate (V).

Equation for forming bleach

Cl₂ (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H₂O (l).