Enthalpies of Solution

Question 1

When a solid ionic lattice dissolves in water, what two things happen?

Answer 1

1. The bonds between the ions break to give free ions - this is endothermic
2. Bonds between the ions and water are made - this is exothermic

The enthalpy change of solution is the overall effect on the enthalpy of these two things

Question 2

Why can the water molecules bond to the ions?

Answer 2

Because oxygen is more electronegative than hydrogen, so it draws electrons towards itself, creating a dipole. The dipole means that the positively charged hydrogen atoms can form bonds with negative ions, and negatively charged oxygen atoms can form bonds with positive ions

Question 3

When do substances generally only dissolve?

Answer 3

Substances generally only dissolve if the energy released is roughly the same, or greater than the energy taken in. So soluble substances tend to have exothermic enthalpies of solution

Question 4

How can you work out enthalpy of solution?

Answer 4

By using a Born-Haber cycle

Question 5

Give the steps to construct a Born-Haber cycle to work out the enthalpy change of solution

Answer 5

1. Put the ionic lattice and the dissolves ions at the top - connect them by the enthalpy change of solution. This is the direct route
2. Connect the ionic lattice to the gaseous ions by the lattice dissociation enthalpy
3. Connect the gaseous ions to the dissolved ions by the hydration enthalpies of each ion. This completes the indirect route