Practical Activity

Reactivity trend in group 7

It is important in this practical activity to ensure the safe use and careful handling of liquids, including careful mixing of reagents. In particular, solutions of the group 7 elements are toxic, irritating or harmful.

This outlines one way to carry out the practical. Eye protection must be worn.


To use displacement reactions to identify the trend in reactivity of the group 7 (IUPAC group 17) elements.

A more reactive halogen can displace a less reactive halogen from solutions of its salts. For example, chlorine is more reactive than iodine. A solution of chlorine can displace iodine from potassium iodide solution:

chlorine + potassium iodide → potassium chloride + iodine

Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)

The reaction mixture turns darker and iodine solution forms.


  1. use a dropping pipette to add a few drops of potassium chloride solution, KCl(aq), to a column of three wells in a spotting tile
  2. repeat step 1 with two more columns of wells, using potassium bromide solution, KBr(aq), then potassium iodide solution, KI(aq)
  3. use a dropping pipette to add a few drops of chlorine solution, Cl2(aq), to a row of three wells in the spotting tile
  4. repeat step 3 with two more rows of wells, using bromine solution, Br2(aq), then iodine solution, I2(aq)


Observe and record what happens in each well at step 3 or 4 in a suitable table. This table shows some sample results.

KCl(aq)KBr(aq)Kl(aq)Number of reactions seen
Cl2(aq)Not doneSolution darkensSolution darkens2
Br2(aq)No visible reactionNot doneSolution darkens1
I2(aq)No visible reactionNo visible reactionNot done0


Count the number of reactions seen with each halogen solution.


Use the results in the table to deduce an order of reactivity, starting with the most reactive halogen.

The order of reactivity is: chlorine > bromine > iodine. This is because chlorine could displace bromine and iodine, bromine could only displace iodine, but iodine could not displace chlorine or bromine.



Suggest an explanation for why three combinations of halogen and salt solution were not done.

A halogen cannot displace itself from a solution of one of its salts, so these combinations were not done.


Astatine is placed below iodine in group 7. Predict whether astatine will displace iodine from potassium iodide solution.

Astatine is less reactive than iodine, so it will not displace iodine from potassium iodide solution.

Example - Higher

a) Write an ionic equation for the reaction between chlorine solution and bromide ions from potassium bromide solution.

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

b) Write two half equations for this reaction.

Cl2(aq) + 2e- → 2Cl-(aq)

2Br-(aq) → Br2(aq) + 2e-

c) Explain why this displacement reaction is also a redox reaction.

Chlorine gains electrons and is reduced to chloride ions. At the same time, bromide ions lose electrons and are oxidised to bromine.

Hazards, risks and precautions

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardPossible harmPossible precaution
Chlorine solutionChlorine gas is produced, which is toxic if inhaled.Make sure the lab is well ventilated and avoid inhaling the gas.
Bromine solutionToxic bromine vapour is produced. Solution irritates the skin.Make sure the lab is well ventilated and avoid inhaling the vapour. Wear gloves.
Iodine solutionHarmful to the skin. Stains clothes.Wear gloves. Avoid contact with clothing.