Solubility (and factors that effect it)

Solubility (and factors that effect it)

1. Dissolving an ionic solid in water

   • ionic solids dissolve in cold water and there is no apparent energy change
   • Lattice energy is the energy required to break rhw atrong electro negative forces between positive and negative ions to cause the ionic lattice to be broken
   • The water molecules are polarised so hydrogen atoms have a &+ charge and oxygen &- charge. These charges resilt in attractive forces between the water molecules and ionsi n the lattice to break up the lattice
   • Water molecules arrange themselves around dispersed ions and this process of water molecules becoming arranged around an ion is called hydration.Hydration enthalpy is the energy released when hydration occurs. There is a balance between lattice energy and hydration enthalpy.
   • Hydration enthalpy has to be enough to balance the lattice energy e.g sodium chloride hydration enthalpy = 770kJmol-1 and lattice enthalpy = -770kJmol-1. But for lithium fluoride, hydration enthalpy = 1025kJmol-1 and lattice enthalpy = -1031kJmol-1. Therefore there is insufficient energy to balance lattice enthalpy and so lithium fluoride in insoluble in water

2. General patterns in solubility

   • High polar solids such as ionic salts dissolve in water (polar solvent) but not in hexane (non-polar)
   • Polar organic substances such as sucrose dissolve in water but not in hexane
   • Non-polar solids such as candle wax do not dissolve in water but dissolve in hexane
   • Non-polar liquids such as petrol and diesel mix with each other completely, i,e are miscible
   • Polar liquids are miscible with each other
   • Polar ad bib-polar liquids are immiscible

3. Solubility of alcohols in water

   • The -OH group on alcohols gives them polarity. The OH group of alcohols are able to form hydrogen bonds with the OH group in water molecules
   • The longer the carbon chain length of the alcohol, the less soluble it is. This is due to the smaller proportion of the molecule being polar
   • The same rules apply for carboxylic acid which are able to dissolve in water

4. Mixing two organic liquids

   • Non-polar liquids can mix together completely, e.g hexame and octane.
   • London forces are found in pure liquids and these London forces that are weak are able to extend throughout the mixture to make the substances miscible

5. Why are non polar substances insoluble in water?

   • As they form seperate layers e.g when hexane and water are mixed togather hexane floats on water as a seperate layer
   • This is because, the forces between the hexane molecule are much weaker than the hydrogen bonds between the water molecules meaning hexane bonds cannot disrupt the water structure and extend throughout the mixture to mix, but instead, hexan just forms a layer at the top
   • Even compounds with polar molecules such as halogenoalkanes have bonds that are too weak that cannot disrupt the water structure and so are insoluble. This is becaue there is insufficient polar character to disrupt the hydrogen bonding as there is only a small C-X polar character compared to the long hydrocarbon chain