Experiment: Reactivity of Group 17 elements

Where Group 1 shows reactivity increasing down the group, Group 17 shows the opposite trend, and displacement reactions are the neatest way to prove it. By seeing which halogen can push another out of its salt solution, you can rank chlorine, bromine and iodine in order of reactivity. This experiment is a core practical of the Periodic Table chapter. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.

Aim

To compare the reactivity of the Group 17 elements chlorine, bromine and iodine by carrying out displacement reactions with halide solutions, and to relate the trend in reactivity to their position in the group.

Apparatus and materials

• Chlorine water, bromine water and iodine solution
• Potassium chloride, potassium bromide and potassium iodide solutions
• A colourless organic solvent, for example 1,1,1-trichloroethane
• Test tubes and a test-tube rack
• Droppers
• Stoppers for the test tubes
• Access to a fume cupboard, safety goggles

Procedure

1. Label test tubes for each halogen–halide pairing to be tested.
2. To a test tube of potassium bromide solution, add a little chlorine water.
3. Add a small volume of the organic solvent, stopper the tube, shake, and let the layers settle; observe the colour of the lower organic layer.
4. Repeat with chlorine water added to potassium iodide solution.
5. Repeat with bromine water added to potassium iodide solution.
6. As controls, add bromine water to potassium chloride solution and iodine solution to potassium bromide and potassium chloride solutions.
7. In each case shake with the organic solvent and record the colour of the organic layer.
8. Use the pattern of which halogen appears to rank the three halogens in order of reactivity.

Expected observations

When chlorine water is added to potassium bromide solution, the solution turns orange and the organic layer becomes orange–brown, showing that bromine has been displaced. When chlorine water is added to potassium iodide, a brown colour appears and the organic layer turns purple, showing that iodine has been displaced. When bromine water is added to potassium iodide, iodine is again displaced and the organic layer turns purple. In the control tubes, bromine with chloride, and iodine with bromide or chloride, there is no colour change, because no displacement occurs.

Inference and conclusion

A more reactive halogen displaces a less reactive halogen from its halide solution. Chlorine displaces both bromine and iodine; bromine displaces iodine but not chlorine; iodine displaces neither. The order of reactivity is therefore chlorine, then bromine, then iodine, so reactivity decreases down Group 17. This is because, going down the group, the atoms are larger and the outer shell is further from the nucleus, so the atom attracts an incoming electron less strongly. Since a halogen reacts by gaining one electron, the larger, lower atoms are less reactive.

Science process skills (Paper 3 style)

Making a hypothesis. The higher a halogen is in Group 17, the more reactive it is, so it can displace a halogen below it from that halogen’s salt solution.

Identifying variables. The manipulated variable is the halogen added; the responding variable is whether displacement occurs (shown by the colour of the organic layer); the controlled variables include the halide solution used, its concentration and the volume of organic solvent.

Tabulating data. Draw a table with rows for each halogen–halide pairing and columns for the observation and the conclusion (displacement or no displacement), so the reactivity order can be read off.

Making inferences. From the fact that chlorine displaces bromine and iodine but iodine displaces neither, infer the reactivity order; from the trend, infer that the ability to gain an electron decreases down the group.

Operational definition. Displacement is operationally defined as having occurred when the colour in the organic layer is that of the halogen originally in the halide salt, not the halogen added.

Safety precautions

• Work in a fume cupboard, because chlorine and bromine vapours are toxic and irritating to the lungs.
• Keep the organic solvent away from flames, because it and its vapour can be harmful and some organic solvents are flammable.
• Wear safety goggles and avoid skin contact, because bromine water and the halogens are corrosive.
• Stopper the tubes before shaking, so the contents and vapours are not released.

Common errors

• Not using the organic solvent. The colours in water can be hard to tell apart; the organic layer makes bromine (orange) and iodine (purple) clear.
• Cross-contaminating droppers. Using one dropper for several solutions gives false displacement results; use a clean dropper for each.
• Adding too much halogen water. Excess colour masks the result; add a small amount and shake.
• Ignoring the controls. Without the no-reaction controls, the reactivity order is not fully justified.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we set this beside the Group 1 experiment so students see the two opposite trends and can explain both in terms of atomic size and electron transfer. A student who can predict each displacement and justify the reactivity order can answer the halogen questions that recur across SPM Chemistry. Because displacement and periodic-trend reasoning are common Paper 2 and Paper 3 tasks (Paper 3 is a practical test assessing science process skills), a firm grasp of this experiment protects marks throughout the Periodic Table topic.