Displacement of halogens

The displacement of halogens is the Group 17 partner to metal displacement, and it is a favourite Paper 3 practical because the colours tell the whole redox story. In the Form 5 Redox Equilibrium chapter it shows that a more reactive halogen is a stronger oxidising agent, and it lets you rank chlorine, bromine and iodine by experiment rather than by memory. Follow the method, then rehearse the Paper 3 answers below.

Aim

To investigate the displacement of a halogen from an aqueous solution of its halide by a more electronegative (more reactive) halogen, and to explain the change as a redox reaction in terms of electron transfer.

Apparatus and materials

• Test tubes with stoppers, and a test tube rack
• Dropper and measuring cylinder
• Chlorine water, bromine water and iodine solution
• Potassium chloride, potassium bromide and potassium iodide solutions
• An organic solvent such as 1,1,1-trichloroethane
• Safety goggles; access to a fume cupboard

Procedure

1. Pour about 2 cm³ of potassium bromide solution into a test tube.
2. Add a few cm³ of chlorine water to the same test tube.
3. Stopper and shake the test tube gently, then observe the colour of the mixture.
4. Add about 1 cm³ of the organic solvent, stopper again, shake, and let the two layers separate.
5. Observe the colour of the lower organic layer.
6. Repeat steps 1 to 5 with other combinations, chlorine water with potassium iodide, bromine water with potassium iodide, bromine water with potassium chloride, and iodine solution with potassium bromide, using a clean test tube each time.
7. Record every observation in a table, noting the colour of the aqueous mixture and the colour of the organic layer.

Expected observations

• Chlorine water added to potassium bromide solution: the mixture turns yellow to brown, and the organic layer becomes orange-brown, showing bromine has been displaced.
• Chlorine water added to potassium iodide solution: the mixture turns brown, and the organic layer becomes purple, showing iodine has been displaced.
• Bromine water added to potassium iodide solution: the mixture turns brown, and the organic layer becomes purple, showing iodine has been displaced.
• Bromine water added to potassium chloride solution: there is no change, because bromine cannot displace chlorine.
• Iodine solution added to potassium bromide or chloride: there is no change.

Report the colours you actually see, and the colour of the organic layer, rather than inventing any numerical measurement.

Inference and conclusion

A more electronegative (more reactive) halogen displaces a less electronegative halogen from a solution of its halide. When chlorine displaces bromine, each chlorine molecule gains electrons and is reduced, so chlorine acts as the oxidising agent, while the bromide ions lose electrons and are oxidised to bromine molecules. Because reduction and oxidation happen together, the change is a redox reaction. Where no change is seen, bromine with chloride, the added halogen is less reactive than the halogen already combined as the halide, so it cannot displace it. Ranking the halogens by which displaces which gives the order of reactivity and oxidising strength: chlorine is more reactive than bromine, and bromine is more reactive than iodine.

Science process skills (Paper 3 style)

• Hypothesis. “A halogen higher in Group 17 displaces a halogen that is lower in the group from a solution of its halide.” State a testable relationship.
• Variables. The manipulated variable is the halogen added; the responding variable is whether displacement occurs, judged by the colour change and the colour of the organic layer; the controlled variables are the concentration and volume of the halide solution, the volume of halogen water, the volume of organic solvent and the temperature.
• Tabulating data. Use columns for the halogen added, the halide solution, the colour of the aqueous mixture, the colour of the organic layer, and the inference.
• Making an inference. From a purple organic layer after adding chlorine water to potassium iodide, infer that iodine has been displaced and that chlorine is more reactive than iodine.
• Operational definition. “Halogen X is more electronegative, and a stronger oxidising agent, than halogen Y if X displaces Y from a solution of a Y halide.”

Safety precautions

• Carry out the experiment in a fume cupboard, because chlorine and bromine vapours are toxic and irritate the lungs.
• Wear safety goggles, because chlorine water, bromine water and the organic solvent are harmful to the eyes.
• Keep the organic solvent away from any flame, because it is flammable and its vapour can ignite.
• Stopper the test tube before shaking, so that the toxic solutions do not splash out.

Common errors

• Not shaking the tube enough, so the organic solvent fails to extract the halogen and the confirming colour does not develop.
• Mistaking the aqueous layer for the organic layer; the organic solvent used here is denser and forms the lower layer.
• Using an old or dilute chlorine water that has lost much of its chlorine, so a real displacement appears not to happen.
• Contaminating a dropper between solutions, which carries one halogen into another test and gives a false colour.
• Reading the colour against a dark bench rather than a white background, so a pale bromine colour is missed.

How we help

Our online one-to-one SPM Chemistry lessons, taught in English, drill the colour code, brown for bromine, purple for iodine, alongside the electron-transfer explanation, so you can name the displaced halogen and write the oxidising agent in the same sentence. Lessons start from RM50 per hour, with a paid one-hour trial lesson so you can judge the teaching first. Because the halogen and metal displacement experiments share one idea, reactivity decides the direction of electron transfer across SPM Chemistry, practising them together makes the whole Redox Equilibrium chapter and Paper 3 far more secure.