Topic 7 · page 13 of 14
Non-aqueous solvents
A solute dissolves in a solvent when the attractions formed between solute and solvent are about as strong as the attractions broken in each. Substances with London forces dissolve in hexane, not water. Ionic compounds do the opposite.
Read first: Alcohols and halogenoalkanes in water
Matching the forces 7.67.6(iv)
A non-aqueous solvent is a solvent that is not water. Hexane, C₆H₁₄, is a typical one: a liquid with only London forces between its molecules.
The rule is the one used for water on the last two pages. When a solute dissolves, the attractions between its particles are broken, the attractions between solvent molecules are broken, and new attractions between solute and solvent are formed. The substance dissolves if the new attractions are similar in strength to the ones broken. This is the real content of "like dissolves like": similar intermolecular forces in solute and solvent.
Take iodine, I₂, which is non-polar and has London forces only.
- Iodine in hexane: the London forces broken in iodine and in hexane are replaced by London forces between iodine and hexane, of about the same strength. Iodine dissolves (the solution is purple).
- Iodine in water: the strong hydrogen bonds between water molecules would have to be broken, and only weak London forces form with iodine in return. Iodine hardly dissolves.
| Solute | Solvent | Forces in the solute | Forces in the solvent | Result |
|---|---|---|---|---|
| iodine, I₂ | hexane | London | London | dissolves: forces broken and formed are similar |
| iodine, I₂ | water | London | hydrogen bonds | poor: hydrogen bonds lost, only weak attractions formed |
| sodium chloride | hexane | ionic lattice | London | insoluble: hexane cannot hydrate the ions |
| sodium chloride | water | ionic lattice | hydrogen bonds | dissolves: ions hydrated (page 11) |
| bromoethane | hexane | dipole–dipole, London | London | dissolves: forces of similar size |
| bromoethane | water | dipole–dipole, London | hydrogen bonds | poor: cannot form hydrogen bonds (page 12) |
| hexane | water | London | hydrogen bonds | poor (they do not mix): hydrogen bonds lost, only weak attractions formed |
Hexane and sodium chloride do not mix for a different reason from hexane and water. In sodium chloride the ionic attractions are strong, and hexane has no polar end to attract the ions, so nothing replaces the strong ionic lattice. Ionic compounds need a polar solvent like water to hydrate them.
Worked exampleWill it dissolve?
Predict whether hexane dissolves in water, and whether it dissolves in cyclohexane.
- Hexane in water. Hexane has London forces only. Water has hydrogen bonds. To mix, hydrogen bonds between water molecules must be broken. The new attractions between hexane and water are only weak London forces, so they do not make up for the loss. Hexane does not dissolve.
- Hexane in cyclohexane. Both have London forces only. The attractions broken and formed are all London forces of similar strength. Hexane dissolves.
Check yourself
- State what is meant by a non-aqueous solvent.
- Explain why iodine dissolves in hexane.
- Explain why iodine is not very soluble in water.
- Explain why sodium chloride does not dissolve in hexane.
- Hexane and water do not mix. Explain, in terms of the forces broken and formed.
- Bromoethane has permanent dipole–dipole interactions. Predict whether it dissolves in hexane, and explain.
Answers cover these until you have tried the questions
- A solvent that is not water (such as hexane).
- Iodine has London forces between its molecules, and so does hexane. The London forces broken in each are replaced by London forces between iodine and hexane of about the same strength, so iodine dissolves.
- Water molecules are held by hydrogen bonds. To dissolve iodine, these would have to be broken, and they would be replaced only by weak London forces between iodine and water, so little iodine dissolves.
- Sodium chloride is held by strong ionic attractions. Hexane has only London forces and no polar end, so it cannot hydrate (or solvate) the ions. The attractions formed are far weaker than the ones broken, so it does not dissolve.
- Mixing breaks the hydrogen bonds between water molecules. Only London forces form between hexane and water, which are weaker. The loss is not made up for, so they do not mix.
- Yes, it dissolves. The dipole–dipole interactions between bromoethane molecules are broken, as are London forces in hexane, but London forces between bromoethane and hexane form, and the forces involved are all similar in strength (the dipole–dipole forces are weak).