Student practical

Displacement reactions

Aims

In this experiment, you will use the colour of a chemical to determine whether a reaction has happened. You will then use your experiment results to establish an order of reactivity down Group 7 of the periodic table.

Learning outcomes

After completing this practical, you should be able to:

  • explain what happens in a halogen displacement reaction
  • write balanced symbol equations for reactions involving halogens,
    including state symbols.

Task

In this experiment, you will observe the reaction between aqueous solutions of the halogens and solutions of the metal halide salts. You will design your own results table to record any observations you make during these reactions.

A more reactive halogen will displace a less reactive halogen from its metal halide salt. These displacement reactions can be used to establish an order of reactivity down Group 7 of the periodic table.

Safety

  • Chlorine water is harmful and bromine water is toxic. Tell your teacher if you spill either of these chemicals on your skin.
  • Cyclohexane is highly flammable and harmful, so make sure there are no naked flames around.
  • Wear chemical splash-proof eye protection when carrying out this experiment.
  • Dispose of chemicals as instructed.

Equipment and materials

  • eye protection
  • test tube rack
  • test tubes
  • rubber bungs
  • plastic dropping pipettes
  • chlorine water, 0.1% (w/v)
  • bromine water, 0.3% (w/v) – toxic and irritant
  • iodine solution, 0.5% (w/v)
  • potassium chloride solution, about 0.1 mol/dm3
  • potassium bromide solution, about 0.1 mol/dm3
  • potassium iodide solution, about 0.1 mol/dm3
  • cyclohexane – highly flammable and harmful

Method

1Pour about 2 cm3 of each of the aqueous halogen solutions (chlorine water, bromine water, iodine solution) into separate test tubes. Add 1 cm3 of cyclohexane to each tube. Stopper the tube firmly and, holding your thumb over the bung, shake the mixture by inverting the test tube a few times.

2Allow the two layers to settle. The cyclohexane will be the top layer (organic layer). Observe the colour of each layer and record this in a results table.
It may be necessary to shake the test tubes again to transfer more of the halogen from the water to the cyclohexane layer.

3Pour about 2 cm3 of each of the aqueous halogen solutions into separate test tubes. Add 2 cm3 of potassium chloride to each tube and then 1 cm3 of cyclohexane, stopper the tube and, holding your thumb over the bung, shake the mixture by inverting the test tube a few times.

4Observe the colour of the cyclohexane layer and record this in your results table.

5Repeat steps 3 and 4 with potassium bromide and potassium iodide.

Student follow up

1Suggest what the purpose is of first carrying out the experiment in steps 1 and 2 without the halide.

2Which combinations of aqueous halogen and metal halide solution reacted? Give evidence to support your answer.

3 Write a balanced symbol equation for any reaction that occurred in steps
3–5 of the practical.

4Which halogen displaced all the other halogens from solutions of their salts?

5Write a list of the halogens you used, putting them in order of reactivity with the most reactive first.

6Extend the list that you made in Question 5 to include all the Group 7 elements.

7Write an ionic equation for the reaction between chlorine and iodide ions.

© Oxford University Press 2016

This resource sheet may have been changed from the original.1