The Chemical Reactions of the Halogens

This lesson covers: 

1. How halogens gain electrons to become ions
2. Trends in reactivity and oxidising ability down the group
3. Displacement reactions between halogens and halide ions
4. Identification of halogens based on colour changes in displacement reactions

Halogens form 1- ions by gaining electrons

Halogens react by gaining an electron to form negative ions with a 1- charge (known as halide ions). For example:

X + e⁻ ➔ X⁻

Where X represents a halogen atom.

This electron gain results in the halogen being reduced, as its oxidation number decreases from 0 to -1.

As the halogen gains an electron, it causes another substance to be oxidised - so halogens act as oxidising agents

Reactivity decreases down group 7

The reactivity of the halogens decreases going down the group because:

• Atomic radius increases down the group as more electron shells are added.
• The increasing size leads to the outer electrons being farther from the positive nucleus.
• The outer electrons also experience more shielding from inner electron shells.
• The electrostatic attraction between the outer electrons and nucleus gets progressively weaker.
• The increase in atomic radius and shielding outweigh the increase in nuclear charge so it becomes harder for larger halogens to attract the electron needed to form a negative ion.

Oxidising power of halogens decreases down group 7

The oxidising power of halogens refers to their ability to take electrons from other substances to form halide ions. Oxidising power is directly related to reactivity.

The oxidising power of halogens decreases down group 7, as does reactivity. This is because the increasing size and shielding makes it harder for larger halogens to remove electrons from (i.e. oxidise) other substances.

Therefore, fluorine is the strongest oxidising agent and iodine is the weakest.

Halogens displace less reactive halide ions

The relative reactivity and oxidising ability of the halogens can be seen in displacement reactions.

If an aqueous halogen solution is added to a solution containing halide ions, a more reactive halogen will displace a less reactive halide from the solution.

• The displacing halogen is reduced as it gains an electron to form the halide ion
• The displaced halide is oxidised as it loses an electron to form the halogen molecule

The rule is: a halogen will displace any halide ion below it in group 7.

Here is a summary table including the ionic equations for the displacement reactions:

This table shows that oxidising ability decreases down group 7 because:

• Chlorine can oxidise both bromide and iodide, showing it is the strongest oxidising agent.
• Bromine can oxidise iodide but not the more reactive chloride, so bromine has intermediate oxidising strength.
• Iodine cannot oxidise either chloride or bromide, so it has the weakest oxidising power.

Colour changes in halogen displacement reactions

When a more reactive halogen displaces a less reactive halide ion in solution, visible colour changes occur that allow the halogen product to be identified.

Colour changes in aqueous solution

When halogens displace less reactive halide ions in aqueous solution, visible colour changes occur if a reaction takes place:

• If bromide (Br^-) is displaced, forming bromine (Br₂), the aqueous solution turns yellow
• If iodide (I^-) is displaced, forming iodine (I₂), the aqueous solution turns orange/brown
• If no reaction occurs, the aqueous solution remains colourless

These colour changes allow the halogen present to be identified.