Understanding hydration and lattice enthalpies

Question 1

What does a large exothermic value for lattice enthalpy indicate about a compound?

What two things of a compound determines its lattice enthalpy value?

Answer 1

A large exothermic value for a lattice enthalpy means that there is a large electrostatic force of attraction between the oppositely charged ions and that the ionic bonds are strong.

The two factors that govern the size of lattice enthalpy:

• ionic size
• ionic charge

Question 2

In ionic compounds, what happens to the attraction between the ions and the lattice enthalpy of the compounds as the ionic size increases?

Answer 2

As the ionic radius increases:

• The attraction between the ions decreases.
• The lattice enthalpy becomes less negative (less exothermic.).

Question 3

Explain how ionic charge affects the lattice enthalpy?

Answer 3

The compounds with the most negative lattice enthalpies are those which have small, highly charged ions.

Question 4

Describe how, and explain why, the lattice enthalpy of magnesium oxide differs from that of barium oxide.

Answer 4

The lattice enthalpy of magnesium oxide is more exothermic that that of barium oxide. Mg²⁺ ions have a smaller ionic radius than Ba²⁺ ions, meaning that the ions in MgO are closer together than the ions in BaO. Therefore there is stronger attraction between Mg²⁺ and O^2- ions in magnesium oxide than between Ba²⁺ and O^2- ions in barium oxide.

Question 5

Give one reason why magnesium oxide is a good material to use for making the lining of furnaces.

Answer 5

Magnesium oxide has a highly negative lattice enthalpy and this means that there is a strong attraction between its ions. Compounds with highly exothermic lattice enthalpies have high melting points and so will be stable inside the working furnace at high temperatures.

Question 6

State and explain which compound has the most exothermic, lattice enthalpy:

MgCl₂, MgBr₂ or MgI₂

Answer 6

Magnesium chloride has the most exothermic lattice enthalpy because the chloride ion is smaller than both bromide ions and iodide ions. This means the Mg²⁺ and Cl^- ions in the MgCl₂lattice can pack closer together and exert a greater attraction on each other than the ion in MgBr₂ or MgI₂.

Question 7

Explain how ionic size changes the value of the enthalpy of hydration.

Answer 7

As the ionic radius become smaller, the value of the enthalpy change of hydration becomes more negative. Hydration depends on the ability of an ion to attract and bond with water molecules. Small ions exert more attraction on water molecules and more energy is released.

Question 8

Explain how ionic charge varies the value of enthalpy of hydration.

Answer 8

As the charge on an ion increases it has a greater attraction for water molecules, and hence the hydration enthalpy is more negative. Across period 3, the ions Na+, Mg²⁺and Al³⁺ progressively decrease in size, and their enthalpy of hydration increases.