Chemguide: Support for CIE A level Chemistry

Group 17 (previously Group 7)

Learning outcomes 11.2.2 and 11.2.3

These statements are about the reactions of the halogens with hydrogen, and the thermal stability of the hydrogen halides.

Before you go on, you should find and read the statements in your copy of the syllabus.

Statement 11.2.2

This is about the reactions between the halogens and hydrogen.

Fluorine combines explosively with hydrogen even in the cold and dark to give hydrogen fluoride gas.

Chlorine and hydrogen explode if exposed to sunlight or a flame to give hydrogen chloride gas. Alternatively, you can make them combine more peacefully if you light a jet of hydrogen and then lower it into a gas jar of chlorine. The hydrogen continues to burn and hydrogen chloride gas is again formed.

Bromine vapour and hydrogen combine with a mild explosion if you put a flame in. Hydrogen bromide gas is formed.

Iodine and hydrogen only combine partially even on constant heating. An equilibrium is set up between the hydrogen and the iodine and hydrogen iodide gas.

This shows the fall in reactivity of the halogens as you go down Group 17.

Statement 11.2.3

This is about the thermal stability of the hydrogen halides.

Hydrogen fluoride and hydrogen chloride are very stable to heat. They don't split up into hydrogen and fluorine or chlorine again if heated to any normal lab temperature.

Hydrogen bromide splits slightly into hydrogen and bromine on heating, and hydrogen iodide splits to an even greater extent.

The reason for this lies in the strengths of the hydrogen-halogen bonds. Using bond energy values from the CIE Data section (which you will find towards the end of the syllabus):

You will see that as you go down the Group, the bonds become weaker. This is because the halogen atoms are getting bigger, and so the bond length is longer, and the bonding pair of electrons is getting further from the halogen nucleus.

The weaker the bond, the less heat energy you need to supply to break it.

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