Displacement reactions of non-metals

In this experiment, students test potassium sulfide, chloride, bromide and iodide using oxygen and chlorine to establish which non-metallic elements are displaced. They observe what happens when oxygen is bubbled through the solutions, and when chlorine water is added to them. Finally, the class can decide to which solutions bromine water should be added to complete the displacement series.

The experimental procedure is simple enough for this to be a student experiment, but the use of oxygen from a cylinder may cause teachers to prefer this as a demonstration, possibly with class assistance.

The practical work itself should not take more than 20 minutes as a class experiment.

Equipment

Apparatus

• Eye protection
• Access to a fume cupboard
• Test tubes, x4
• Test tube rack
• Polythene bag (see note 7 below)
• Rubber bung, one hole, fitted with 10 cm length of glass tubing

Chemicals

About 15 cm³ each of the following 0.2 M solutions:

• Potassium bromide
• Potassium chloride
• Potassium iodide
• Potassium sulfide (IRRITANT)
• Bromine water, 0.02 M (HARMFUL), 5 cm³
• Chlorine water, 0.04 M (HARMFUL), 5 cm³

For teacher use only

• Oxygen (OXIDISING), cylinder with rubber delivery tube to fit glass tubing in bung (see note 8)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout. Keep the chlorine water and bromine water well away from your face; do not inhale the fumes. 
• Potassium sulfide solution, K₂S(aq), (IRRITANT) – see CLEAPSS Hazcard HC051B. For disposal: dilute to less than 0.1 M before flushing away down a foul-water drain.
• Bromine water, Br₂(aq), (HARMFUL) – see CLEAPSS Hazcard HC015b and CLEAPSS Recipe Book RB017. Keep the solution in a fume cupboard if possible.
• Chlorine water, Cl₂(aq), (HARMFUL) – see CLEAPSS Hazcard HC022b and CLEAPSS Recipe Book RB025. Keep the solution in a fume cupboard if possible.
• Oxygen (OXIDISING) – refer to CLEAPSS Laboratory Handbook, Section 9.9, for use of gas cylinder.
• Polythene bags for distributing oxygen gas: ordinary sandwich bags should be sufficient, with the neck wrapped around the rubber bung and secured with adhesive tape or a wire tie.
• The gas will not be retained for long inside the polythene bags, so it is best to fill these as needed. If the experiment is done as a class practical, it will help the organisation of the lesson if the laboratory technician is available to fill bags as required, to allow the teacher to supervise the class as a whole.

Procedure

1. Place four test tubes in the test tube rack.
2. Add about a 3 cm depth of each of the following solutions into the row of tubes, making sure you know which test tube contains which solution:
   • Potassium sulfide 
   • Potassium chloride 
   • Potassium bromide 
   • Potassium iodide
3. Collect a polythene bag that has been filled with oxygen gas and fitted with a delivery tube.
4. Bubble a little oxygen through each solution in turn, and record any changes that you see in each solution. Decide whether the oxygen has displaced the non-metal in the compound in each solution.
5. Dispose of the mixtures as directed and rinse the test tubes well with tap water.
6. Using fresh samples of the solutions, add about 1 cm depth of chlorine solution (labelled ‘chlorine water’) to each solution, and record any changes that you see. Decide whether the chlorine has displaced the non-metal in the compound in each solution.
7. Again using fresh samples of the solutions, add about 1 cm depth of bromine solution (labelled ‘bromine water’), and record any changes that you see. Decide whether the bromine has displaced the non-metal in the compound in each solution.
8. Use your results to produce a displacement series for the five non-metal elements in the experiment.

Teaching notes

Establishing a displacement series among the halogens is a well-established routine. This experiment attempts to extend the series to include two Group 16 elements, oxygen and sulfur, as well as the halogens. In each test the displacement reaction, if it occurs, will be of the type:

Element A + potassium salt with element B → potassium salt with element A + element B

If a reaction occurs, a colour change will be observed as ‘element B’ is formed:

• Sulfur forms a cream or pale yellow precipitate (the students may report this as a ‘white cloudiness’).
• Chlorine would give a colour change too faint to observe; however, in these experiments, no chlorine will be formed.
• Bromine gives an orange-yellow colour to the solution; the colour seen will probably be fainter than that seen in the bottle of bromine water.
• Iodine gives a yellow-brown colour to the solution.

Two of the tests are in practice pointless: the additions of chlorine water to potassium chloride, and of bromine water to potassium bromide. It is worth asking students to identify these tests:

1. Which of the tests in step 6 could have been left out?
2. Which of the tests in step 7 could have been left out?

Explain why in each case.

If students are not pointed towards these reactions, they may well identify the addition bromine water to potassium chloride and potassium bromide solutions, resulting in a diluted bromine colour in the tube, as indicating a colour change, and hence evidence of reactions occurring.

The evidence for the order in the displacement series from these tests is not fully complete. Oxygen may well appear difficult to place, and class discussion could be useful, leading to a conclusion that further tests might be needed to clarify. At this stage, the situation is probably best left inconclusive, as further experiments may well only lead to further confusion!