The halogens

Introduction

The chemical and physical properties of the halogens appear under Topic 3.2 Periodic trends on the core. Although the physical properties such as atomic and ionic radii are often covered with the remainder of topic 3 it can make sense to leave the study of their chemical properties until students have covered oxidation and reduction.

If your students do not understand the concept of redox then much of the chemistry of the halogens will make little sense. This practical is worth doing as it gives students a hands-on experience of these important elements and ions. The reaction of the halide ions with silver ions is no longer on the syllabus but it a good example of precipitation, the effect of light on silver salts and solubility in non-polar solvents so I think it is still worth doing. The solubility of iodine in non-polar solvents to give a purple solution can be contrasted with its solubility in potassium iodide solution which gives a brown colour. This fits in well with Topic 4.4 Intermolecular forces.

Teacher’s notes

Chlorine, bromine and iodine are hazardous chemicals so students should not be allowed to use them in their pure state. If you are able to obtain bromine and iodine then it is helpful to demonstrate them to the students using a fume cupboard. Chlorine can be prepared freshly by reacting concentrated hydrochloric acid with crystals of potassium manganate(VII). They should be able to see the green gas being formed. Wear protective gloves when handling liquid bromine. With care you can ‘pour’ the gas from the bottle so that students can see how volatile and dense the vapour is. A few crystals of iodine can be heated carefully in a tube. Sometimes books say that it sublimes and certainly purple crystals will be seen subliming on the sides of the tube but it can also be seen to melt. If you cannot make or obtain the pure halogens then these properties can be shown using the following video clip.

Properties of the halogens  

The experiments the students do with chlorine water, bromine water and a solution of iodine in aqueous potassium iodide are safe and easy to carry out. Heptane is given here as an example of an organic solvent but other non-polar organic solvents can be used in its place. It is a good exercise in using small quantities and observing carefully. All the halogens are much more soluble in non-polar solvents than they are in water and their colour can easily be seen in the top layer. Iodine is purple in non-polar solvents as it exists as I2 molecules whereas in aqueous potassium iodide solution it is brown due to the presence of the tri-iodide ion, I3–(aq). The reaction of light with silver halides is the basis of pre-digital black and white film photography.

Standard Level Higher Level Student worksheet

GROUP 17- THE HALOGENS

Group 17 of the periodic table contains the elements fluorine, chlorine, bromine, iodine and astatine. In this practical you will examine some of the properties and reactions of chlorine, bromine and iodine and their respective ions.

ENVIRONMENTAL CARE:

The elements are all oxidising agents and can potentially have an adverse effect on natural systems. Place all halogen residues in the marked container in the fume cupboard where any organic layer can be separated and the elements can eventually be converted into halides for disposal. Do not throw halogen residues down the sink. Halides, the salts of the halogens, all occur naturally in sea water and can be disposed of safely down the sink.

SAFETY:

All the halogens are poisonous (chlorine was used as a poison gas in the First World War). Use only small quantities, handle them with care, avoid inhaling the vapours and do not spill them on your skin or clothing. Fluorine is too dangerous for you to use and astatine is radioactive.

PROCEDURE:

1. Prepare for the practical beforehand by drawing out a table and using the data book give the following information for each element. a) symbol, b) electron configuration, c) atomic mass, d) melting point and e) boiling point.

2. You will be shown pure samples of the three elements in the fume cupboard. Describe their physical appearance.

3. Solubility tests. Fill a small test tube one quarter full with aqueous chlorine solution and add a few drops of heptane. Stopper and shake well. Observe which layer appears to have the greatest concentration of chlorine. This may not be easy as chlorine solution only has a very pale green colour. Repeat this test with aqueous solutions of bromine and iodine (dissolved in aqueous potassium iodide solution), noting the colour in both layers.

4. Redox reactions. Line up four small test tubes. In the first, quarter fill with KCl(aq), the second with KBr(aq), the third with KI(aq) and the fourth with distilled water. To each tube add a few drops of chlorine solution and observe the changes. A few drops of heptane and a good shake may help you to understand what is taking place. Wash out the tubes, refill with the same four solutions and now add bromine solution to each. Repeat a third time, adding iodine solution to each.

5. Test for halide ions. Line up three small test tubes and quarter fill the first with KCl(aq), the second with KBr(aq) and the third with KI(aq). To each add a few drops of silver nitrate solution and observe. Leave each solution in bright light for several minutes and note any further changes.

QUESTIONS

1. Which is the better solvent for the halogens and why?

2. Explain the chemistry behind the redox reactions giving the relevant equations.

3. Write an ionic equation for the reaction of chloride ions with silver nitrate.

4. Explain why iodine gives a purple solution when it is dissolved in heptane but a brown solution when it is dissolved in an aqueous solution of potassium iodide.

5. Silver nitrate used to be used to test for the presence of all three halide ions. Silver chloride is white silver bromide is cream coloured and silver iodide is yellow. It can sometimes be difficult to distinguish between silver bromide and silver iodide. Can you now devise a test which would definitely enable you to distinguish between bromide and iodide ions?

This worksheet can also be downloaded from:

  The halogens

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