Study notes for Unit 2 Topic 6 - Solubility for Chemistry Edexcel A Level 2015. Focusing on solubility, the bonding which affects it, the two main types of solvent and the solubility of certain organic compounds in different solvents.
Solubility:
Solubility is affected by bonding - for one substance to dissolve, three things must happen.
The bonds (both in the substance and the solvent) have to break, and new bonds
have to form between the substance and the solvent.
There are 2 main types of solvent:
● Polar solvents - made of polar molecules. One example is water, which bonds with
hydrogen bonds. Another example is propanone, which doesn’t form hydrogen
bonds.
● Non-polar solvents - hexane for example has molecules bonded to each other by
London forces.
Solubility of ionic compounds in water:
● Water is a polar solvent - the molecules have slightly positive hydrogen and slightly
negative oxygen.
● When an ionic substance is mixed with water, the ions in the ionic substance are
attracted to the oppositely charged ends of the water molecules.
● The ions of the substance are pulled away from the ionic lattice by the water
molecules which surround the ions.
● This process is called hydration.
(some ionic substances don’t dissolve because the bonding between their ions is too strong)
Solubility of simple alcohols in water:
● Alcohols are covalent but they dissolve in water.
● The polar O-H bond in an alcohol is attracted to the polar O-H bonds in water.
Hydrogen bonds form between the lone pairs on the slightly negative oxygen atoms
and the slightly positive hydrogen atoms.
● The carbon chain part of the alcohol isn’t attracted to water, so the more carbon
atoms there are, the less soluble the alcohol will be.
Insolubility of halogenoalkanes in water:
● Not all molecules with polar bonds can dissolve in water.
● Halogenoalkanes for example contain polar bonds but their dipoles aren't strong
enough to form hydrogen bonds with water.
● The hydrogen bonding between water molecules is stronger than the bonds that
would be formed with halogenoalkanes so they don’t dissolve.
Non-aqueous solvents:
● Any solvent that isn’t water is known as non-aqueous.
● Substances usually dissolve best in substances that have similar
intermolecular forces.
● This means that non-polar substances dissolve best in non-polar solvents - non-polar
substances have London forces between their molecules and they form similar bonds
with non-polar substances so they usually dissolve in them.
● Water molecules are less strongly attracted to non-polar substances so they don’t
dissolve in water.
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