3.6 Enthalpy changes for solids and solutions

Question 1

Define enthalpy of atomisation

Answer 1

The enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state under standard conditions.

Question 2

Define lattice enthalpy of formation

Answer 2

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous constituent ions under standard conditions.

Question 3

Define lattice dissociation enthalpy

Answer 3

The energy required to break apart an ionic lattice into its constituent ions in a gaseous state under standard conditions.

Question 4

Define enthalpy of solution

Answer 4

The enthalpy change when one mole of an ionic solid is dissolved in a solvent to infinite dilution so that the ions no longer interact under standard conditions.

Question 5

Define enthalpy of hydration

Answer 5

The enthalpy change when one mole of aqueous ions is formed from one mole of gaseous ions.

Question 6

What type of enthalpy change is taking place in the following reaction?
NaCl(s) —> Na+(aq) + Cl-(aq)

Answer 6

Enthalpy change of solution

Question 7

What type of enthalpy change is taking place in the following reaction?
Na+(g) —> Na+(aq)

Answer 7

Enthalpy change of hydration

Question 8

What type of enthalpy change is taking place in the following reaction?
Mg2+(g) + 2Cl-(g) —> MgCl2(s)

Answer 8

Lattice enthalpy of formation

Question 9

What type of enthalpy change is taking place in the following reaction?
1/2Cl2(g) —> Cl(g)

Answer 9

Enthalpy of atomisation

Question 10

What affects the solubility of ionic compounds in water?

Answer 10

The solubility of ionic compounds depends on the balance between the hydration enthalpy of the ions in the compound and the lattice dissociation enthalpy of the compound.

Question 11

What factors affect enthalpy of hydration?

Answer 11

Hydration enthalpy of an ion depends on the amount of attraction between the ions and the water molecules. The bigger the charge and the smaller the ion, the larger the enthalpy of hydration.

Question 12

What is Hess’s law?

Answer 12

The enthalpy change of a reaction is independent of the route taken.

Question 13

How can the enthalpy change of a solution be calculated using an energy cycle?

Answer 13

Lattice breaking enthalpy and enthalpy change of hydration can be used in an energy cycle to calculate the enthalpy change of solution using Hess’s law.

Question 14

What is a Born-Haber cycle?

Answer 14

Lattice enthalpy cannot be calculated directly so a Born-Haber cycle can be used to calculate the lattice enthalpy by applying Hess’s law. A Born-Haber cycle calculates lattice enthalpy by comparing the standard enthalpy of formation of the ionic compound to the enthalpy required to make gaseous ions from the elements.

Question 15

Define standard enthalpy of formation

Answer 15

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Question 16

Define first ionisation energy

Answer 16

The enthalpy change when one mole of gaseous atoms forms one mole of gaseous 1+ ions.

Question 17

Define first electron affinity

Answer 17

The enthalpy change when one mole of gaseous 1- ions is formed from one mole of gaseous atoms.

Question 18

Give the equations for the first electron affinity of oxygen and the first ionisation energy of magnesium

Answer 18

First electron affinity of oxygen:
O(g) + e- —> O-(g)

First ionisation energy of magnesium:
Mg(g) —> Mg+ + e-

Question 19

What is the perfect ionic model?

Answer 19

The perfect ionic model assumes that all ions are perfectly spherical and that the bonds have no covalent character.

Question 20

Why are theoretical lattice enthalpies often different from experimental ones?

Answer 20

Theoretical lattice enthalpies use calculations which are based on a perfect ionic model of the lattice. The experimental values often differ because most ionic compounds have some covalent character. The more polarisation in the ionic bond, the more covalent character of the compound.

Question 21

How does exothermicity and endothermicity of enthalpy of formation act as a qualitative indicator of a compound’s stability?

Answer 21

For an exothermic reaction the products are more stable than the reactants.
For an endothermic reaction the reactants are more stable than the products.