Displacement Reactions with Halogens

Displacement Reactions with Halogens

Finding the reactivity of halogens are done by reacting the elements with potassium halides. Colour change will indicate a reaction.

Note: Colour changes are due to the element being displaced. For example, the colour change from yellow to brown when chlorine reacted with potassium bromide was due to the fact that the bromine was displace. It was the brown of the Bromine that turned the solution brown.

Potassium is only a spectator ion. It does not change.

But now we have a problem. To distinguish whether bromine or iodine has been displaced is difficult, as both elements produce very similar shades of brown. What do we do? We add an organic solvent such as Volasil. When Volasil is added, the iodine turns pink while the bromine stays brown. Pretty neat huh?

These reactions are known as redox reactions, where oxidation and reduction are occurring (not just one of them).

Explaining the Trend in Reactivity of Halogens

As you go down the group, the oxidizing ability of the halogens falls due to the decreasing reactivity. When a halogen oxidizes something, it does so by removing electrons from it. Chlorine is a strong oxidizing agent because its atoms readily attract an extra electron to make chloride ions. Bromine is less successful. Why? This relates to electron shells again. In Chlorine, there are three shells which shield the nucleus’ attraction force from attracting another electron to gain a full outer electron shell. Bromine however, has a lot more shells to shield the attraction, therefore, the force is much weaker.

Halogens: Quick Notes

• Diatomic molecules
• Seven electrons on outer shell
• Highly reactive – only need one electron to fill outer shell
• Form hydrogen halides when reacting with hydrogen
• Reaction increases as you go up the group
• Halogens can displace each other
• Volasil turns iodine pink