19. Lattice Energy

Question 1

Define ‘lattice energy’.

Answer 1

Enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions.

Exothermic - crystalline lattice formed from oppositely charged ions, stable wrt its gaseous ions.

Question 2

What is the difference between lattice energy and lattice enthalpy?

Answer 2

The above is more accurately called lattice enthalpy; lattice energy is the internal energy change at 0K.

Question 3

Define ‘standard enthalpy change of atomisation’.

Answer 3

Enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.

Endothermic.

Question 4

Define ‘electron affinity’.

Answer 4

Enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions, under standard conditions.

Exothermic. Successive electron affinities are always endothermic.

Question 5

Define ‘standard enthalpies of: formation, combustion and neutralisation’.

Answer 5

Enthalpy change when 1 mole…

• (formation) of a compound is formed from its constituent elements
• (combustion) of a compound undergoes complete combustion in excess oxygen
• (neutralisation) of water is formed from an acid and an alkali

… under STD conditions.

Question 6

Define ‘standard enthalpy change of reaction’.

Answer 6

Enthalpy change when the amounts of the

reactants shown in the stoichiometric equation (molar quantities) react under standard conditions to give products.

Question 7

State Hess’s Law.

Answer 7

The total enthalpy change of a reaction is independent of the route the reaction takes, provided the initial and final conditions are the same.

Question 8

How do you structure a Hess cycle for lattice energy?

Answer 8

Ions (g) Ionic compound (s)

• First arrow = Enthalpy 1 (atomisation, ionisation, electron affinity)
• Second arrow = Formation (enthalpy 1 + lattice energy).

Question 9

How do you structure a Born-Haber cycle for lattice energy?

Answer 9

Elements in STD states -> gaseous ions (at, ea, i) -> ionic compound (lattice energy).
Upward arrows represent ENDO, downward represent EXO.

Question 10

From where does lattice energy arise?

Answer 10

The electrostatic forces of attraction between oppositely charged ions in the crystalline lattice.

Question 11

How is lattice energy affected by ion size?

Answer 11

Increase in ion size = LESS EXO.

• Same charge - larger ions have lower charge density.
• Same charge is spread over a larger volume.
• Forces of attraction are weaker.

Question 12

How is lattice energy affected by ion charge?

Answer 12

Increase in ion charge = MORE EXO.

• Same size - higher charge have higher charge density.
• Higher charge spread over same volume.
• Forces of attraction are stronger.

Question 13

How does ion polarisation work?

Answer 13

A charge on a cation in the lattice can attract the electrons in the anion toward the cation, distorting the electron cloud of the anion and its spherical shape.

Question 14

What is polarising power?

Answer 14

The ability of a cation to attract electrons and distort the shape of an anion.

Question 15

Which factors affect the degree of polarisation?

Answer 15

Cation charge density (polarising power).

• charge (+2 or +3)
• size (small).

Anion polarisability.

• charge (-2 or -3)
• size (large).

Question 16

How can some bonds have covalent character?

Answer 16

Cations eg. Fe3+ can have such high polarising power that they can polarise the anion, causing electrons to be shared in the bond rather than localised around the electron.

Question 17

How can some bonds have ionic character?

Answer 17

The atoms involved may have a very high electronegativity difference, causing a large amount of bond polarisation and thus charge separation.

Question 18

What is the trend in thermal stability for the Group 2 carbonates and nitrates?

Answer 18

Increases down the group - requires more energy to decompose.
Enthalpy change will be more positive (more stable wrt products).
Polarising power of the cations decreases down the group, so it is harder to weaken the C-O.

Question 19

What happens to the carbonate and nitrate ions undergoing thermal decomposition?

Answer 19

The ions have equal bond lengths, meaning that the negative charge is spread over the ion (delocalised), with the charge concentrating at the oxygen atoms.
When a cation moves near the oxygen, it polarises it - the oxygen becomes an oxide ion, bonding with the metal, and the carbon dioxide is liberated.

Question 20

Define ‘standard enthalpy change of solution’.

Answer 20

Enthalpy change when 1 mole of an ionic compound dissolves in sufficient water to form an infinitely dilute solution, under standard conditions.

Endothermic or exothermic. ‘aq’ represents large volume of water.

Question 21

Which ΔHsol values indicate solubility?

Answer 21

Negative or slightly positive. No metallic salt is completely insoluble as they all have ΔHsol values.
More positive = more insoluble.

Question 22

Define ‘standard enthalpy change of hydration’.

Answer 22

Enthalpy change when 1 mole of gaseous ions dissolves in sufficient water to form an infinitely dilute solution, under standard conditions.

Exothermic, more so for ions with high charge density.

Question 23

From where does the energy to overcome the lattice (ΔHsol) arise?

Answer 23

From the attraction between polar water molecules and the charged ions in the lattice, which enables the formation of ion-dipole bonds.
The energy released by this formation compensates for the energy required for lattice dissociation.

Question 24

How do you structure a Born-Haber cycle for ΔHsol?

Answer 24

Ionic compound -> gaseous ions (lattice dissociation enthalpy) -> aqueous ions (ΔHhyd).

Question 25

What is the trend in the solubility of the Group 2 sulfates?

Answer 25

Solubility decreases as metal ion radius increases.

Question 26

How do ΔHhyd and ΔHlatt change down Group 2 (sulfates)?

Answer 26

• ΔHhyd gets less exothermic down the group as charge density decreases.
• The decrease is large and depends on the increase in cation size, as the size of the sulfate ion does not change.
• ΔHlatt is greater if the ions (with the same charge)
  forming the lattice are small, so it gets less exothermic down the group.
• Sulfate is much larger than the cation and so it contributes more to the lattice energy change.
• The decrease is therefore smaller down the group (cation size effect diluted by large anion).

Question 27

Explain the trend in the solubility of the Group 2 sulfates.

Answer 27

Lattice energy decreases down the group by smaller amounts, whereas hydration decreases by larger values.
Apply Hess’s Law to find that this means ΔHsol is more endothermic down the group, so solubility decreases.

Question 28

What is the trend in the solubility of the Group 2 hydroxides?

Answer 28

Solubility increases as cation size increases.

Question 29

Explain the trend in the solubility of the Group 2 hydroxides.

Answer 29

• ΔHlatt decreases MORE down the group. The hydroxide ion is small, so the increasing cation size affects the lattice energy more.
• Because of this, ΔHhyd decreases LESS down the group.
• Therefore, ΔHsol becomes more exothermic down the group and solubility increases.

Question 30

Define ‘infinitely dilute solution’.

Answer 30

A solution wherein the excess of water is so large that increasing the volume of the water will not cause further absorption/release of energy.