Chemical Reactions of the Halogens

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Chemical Reactions of the Halogens

The halogens can act as oxidising agents by gaining electrons to form halide ions.

X2 + 2e- → 2X-

The oxidising ability decreases down the group with fluorine being the strongest oxidising agent. In simple terms the formation of X- occurs in two steps.

Chemical Reactions of the Halogens

A strong oxidising agent will readily form X-. It will therefore, have a low bond energy and a high electron affinity. Both bond energies and electron affinities decrease down the group as the nuclear pull on the outer electrons decrease.

The change in electron affinities is most significant and so oxidising ability decreases down the group as the nuclear pull on the outer electrons decreases down the group.

Fluorine has anomalous properties due to its small size. It has a lower electron affinity than Chlorine due to electron repulsions in the overcrowded outer shell. However, it is a stronger oxidising agent than Chlorine due to its unusually low bond energy. This is due to the repulsions between outer electrons in the F2 molecule.

Halide ions act as reducing agents by losing electrons.

2X- - 2e- → X2

The reducing ability increases from F- to I-.

In all their reactions the halogens act as oxidising agents. Hence, their reactivity decreases down the group.

Reaction with water

a) F2 oxidises H2O to O2 gas in a very exothermic reaction.

2F2(g) + 2H2O(l) → O2(g) + 4HF(g)

b) Cl2 dissolves in H2O and some hydrolysis occurs. A yellow solution of 'chlorine water' is formed which is a mixture of two acids. No O2 is evolved.

Cl2(g) + H2O(l) → HCl(aq) + HOCl(aq)

c) Br is only slightly soluble in H2O and there is less hydrolysis.

Br2(l) + H2O(l) → HBr(aq) + HOBr(aq)

d) I2 is virtually insoluble in H2O. It is however soluble in KI solution due to the formation of the triiodide anion.

I2(s) + I-(aq) → I3-(aq)

Note: All halogens are more soluble in non-polar solvents such as CCl4. Cl2 gives a colourless solution. Br2 a red solution and I2 a violet one.

Displacement reactions

If a more reactive halogen is placed into a solution containing a less reactive halide a displacement reaction is seen.

For example: Cl2(g) + KI(aq) → KCl(aq) + I2(s)

  • F2 will displace Cl- Br- I-
  • Cl2 will displace Br- I-
  • Br2 will displace I-
  • I2 will displace none

Reaction with alkali ( NaOH soln)

This reaction depends on the conditions:

cold NaOH solution.

2OH-(aq) + Cl2(g) → Cl-(aq) + ClO-(aq) + H2O(l)

hot NaOH solution.

6OH-(aq) + Cl2(g) → 5Cl-(aq) + ClO3- + 3H2O(l)

In hot solution the ClO- ions are disproportionating

3ClO-(aq) → 2Cl-(aq) + ClO3-(aq)

Reactions with other halogens are similar.