Group 7: Halogens
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On page 117 of the Nuffield Chemistry students book a question is asked, under the 'in your notes:' section: 'Why do you think the halogens are more soluble in hydrocarbon solvent than water?'
What is the hydrocarbon solvent? I thought it was acetone (propanone), but the halogens are more soluble in water the acetone. Also what are the colours of the halogens in their standard state and in compounds?
Jenkin writes
The halogens consist of covalent molecules. As they are non-polar, their intermolecular forces are van der Waals forces. These will be of similar magnitude to the intermolecular forces in the hydrocarbon solvent, so solvent-solute attractions are likely to take place readily, leading to the formation of a solution.
Water, on the other hand, has hydrogen bonding as its most significant intermolecular attraction. The hydrogen bonds between water molecules are much stronger than the van der Waals forces between iodine molecules. They will also be stronger than any attractive forces between iodine molecules and water molecules, so solution does not occur easily. The hydrogen bonding in water 'keeps out' the halogen molecules.
A common ’hydrocarbon solvent’ is a mixture of liquid hydrocarbons commonly called ‘petroleum spirit’. This contains compounds such as heptane and octane as well as branched-chain isomers of them.
Acetone (propanone), CH3COCH3 is not a hydrocarbon but a ketone. Hydrocarbons by definition are compounds formed from the elements hydrogen and carbon only.
For colours of halogens in their standard states refer to the Nuffield Book of Data, Table 5.2, pages 56-59. The column headed ‘St’ tells you the state and the final column, ‘Notes’ gives colours.
As for colours of the compounds, many of the common ones are colourless unless they contain a transition metal. If you are doing the Nuffield course you will have met the silver halides in experiment 6.4, part 1 and noted that their different colours can be used as the basis of a test distinguishing between chlorides, bromides and iodides (AgCl – white, AgBr – cream, AgI – yellow).
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updated: 11 June 2004
