Displacement series for non-metals

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Lesson organization

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.

Apparatus and Chemicals

The teacher or each working group will require:

Eye protection
Access to fume cupboard

Test-tubes, 4
Test-tube rack
Polythene bag (see note 1)
Rubber bung, one hole, fitted with 10 cm length of glass tubing

About 15 cm³ each of the following 0.2 mol dm^-3 solutions:

Potassium bromide (Low hazard)
Potassium chloride (Low hazard)
Potassium iodide (Low hazard)
Potassium sulfide (Irritant at this concentration) (see note 2)

Bromine water, 0.02 mol dm^-3 (Harmful at this concentration), 5 cm³ (see note 3)
Chlorine water, 0.04 mol dm^-3 (Harmful), 5 cm³ (see note 3)

For teacher use only:

Oxygen (Oxidising), cylinder with rubber delivery tube to fit glass tubing in bung (see note 4).

Technical notes

Potassium sulfide (Irritant at concentration used, Dangerous for environment) Refer to CLEAPSS Hazcard 51
Bromine water (Harmful at concentration used) Refer to CLEAPSS Hazcard 15B
and Recipe Card 28. Keep the solution in a fume cupboard if possible.
Chlorine water (Harmful) Refer to CLEAPSS Hazcard 22B and Recipe Card 28
Oxygen (Oxidising) Refer to CLEAPSS Laboratory Handbook, Section 9.9, for use of gas cylinder.

1 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.

2 Disposal should be down a fume cupboard drain, with good water flow.

3 Keep the solution in a fume cupboard if possible.

4 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 organization 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

For class practical use; adapt as appropriate for demonstration use.

HEALTH & SAFETY: Wear eye protection. Keep the chlorine water and bromine water well away from your face; do not inhale the fumes.

a Place four test-tubes in the test-tube rack.

b 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

c Collect a polythene bag that has been filled with oxygen gas and fitted with a delivery tube.

d 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.

e Dispose of the mixtures as directed and rinse the test-tubes well with tap water.

f 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.

g 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.

h 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 (Group 7) is a well-established routine. This experiment attempts to extend the series to include two Group 6 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 e could have been left out?
2 Which of the tests in f 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!

Health & Safety checked, August 2008