Halogen Reactivity Trends

Why it matters

A more reactive halogen kicks a less reactive one out of its salt. Add pale chlorine water to colourless potassium iodide and the solution turns brown — chlorine has thrown the iodine out. Which halogen is the stronger one?

The idea in plain words

Match the macroscopic yellow precipitate to the atomic displacement: Cl₂ + H₂S → S + 2HCl.

Predict first

You add chlorine water to potassium iodide and the solution turns brown. What has happened?

Chlorine is more reactive than iodine, so it takes the place of iodine in the salt, setting free iodine (the brown colour) plus potassium chloride. Reactivity falls down the group: F > Cl > Br > I, and a halogen can only displace the ones below it.

• The chlorine just dissolved and coloured the water
• Chlorine displaced iodine — the brown colour is free iodine (I₂) — correct
• The potassium reacted with the water
• Nothing happened — brown is just the normal colour

What you do

Add chlorine water to a bromide, then to an iodide: chlorine displaces both. Then add bromine water to an iodide. From what displaces what, work out the reactivity order.

Check yourself

Chlorine water is added to potassium bromide solution. What do you see?

Chlorine sits above bromine in the group, so it displaces it. The freed bromine turns the solution orange.

Bromine water darkens potassium iodide, but does nothing to potassium chloride. Why?

A halogen only displaces the halogens below it in the group. Bromine is above iodine, so it frees iodine; but it is below chlorine, so it cannot touch a chloride.

What is the trend in halogen reactivity going down Group 17?

Down the group the atoms get bigger and gain an electron less easily, so reactivity falls — fluorine most reactive, iodine least. That is why chlorine displaces bromine and iodine, and bromine displaces iodine.

• The solution turns orange as bromine is set free — correct
• A white precipitate forms
• Nothing — bromine is more reactive than chlorine
• Bromine displaces iodine (below it) but cannot displace chlorine (above it) — correct
• Bromine reacts with every salt
• Potassium chloride is insoluble
• It increases down the group
• It decreases down the group: F > Cl > Br > I — correct
• It stays the same all the way down