Group 7: Halogens
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What are the relative solubilities of the halides in water and propanone (acetone)?
Igloo writes … This is a very good question.
Generally speaking, common halides, such as those of sodium and potassium, are more soluble in water than in acetone. This is because the relevant ions (e.g. K+, Na+, Cl–, Br– and I–) can all be solvated by water molecules very much more effectively than they can by propanone (acetone) molecules.
Solvation causes these ions to be surrounded by solvent molecules, and in the case of water, which is a far more polar solvent than acetone, the attractive solvent-ion forces are stronger than they are with propanone (acetone). In addition to this, water molecules are smaller than those of propanone molecules, so more water molecules can “fit round the ions” than acetone molecules can manage.
These solvent-ion attractions cause a great deal of energy to be released (called solvation energy).
If this energy is more than that required to break apart the ionic lattice of the solid halide (the lattice energy), then solution occurs, and the net energy is released to the surroundings (exothermic dissolving).
If the energy released during solvation is a little less than the lattice energy, then solution occurs endothermically (the extra energy being supplied from the surroundings).
If the energy released during solvent-ion bond formation is much lower than the lattice energy, then little or no dissolving occurs.
This is an over-simplified picture of a topic which is best discussed in terms of entropy changes. The Nuffield Students’ Book on page 388 provides a good diagram to illustrate the sort of bonding between ions and solvents to which I have referred above.
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updated: 27 August 2003
