5.2.1 Lattice Enthalpy

Question 1

Define lattice Enthalpy

Answer 1

The Enthalpy change when 1 mol of ionic lattice is formed from gaseous ions. Always exothermic

Question 2

Write the equation for the lattice Enthalpy of potassium chloride

Answer 2

K+(g) + Cl-(g) -> KCl

Question 3

What does a Born Haber cycle consist of?

Answer 3

• Enthalpy of formation (exo)
• Enthalpy of atomisation (endo)
• Ionisation energy (endo)
• 1st electron affinity (exo)
• 2nd electron affinity maybe (endo)

Question 4

Calculate the lattice Enthalpy of NaCl given this data:
First ionisation energy of sodium is +496
Enthalpy change of atomisation of Cl is +121
Enthalpy change of atomisation of Na is +108
Enthalpy of formation is -411
1st electron affinity of Cl is -346

Answer 4

346 -496 - 121 - 108 - 411 = -790 kJmol-1

Question 5

Calculate lattice Enthalpy of CaO given this data:
Enthalpy of formation is -635
Atomisation of calcium +193
Atomisation of oxygen is +248
1st ionisation energy of calcium is +590
2nd ionisation energy of calcium is +1150
1st electron affinity of O is -142
2nd electron affinity of +844

Answer 5

-844 +142 - 1150 -590 -428 -193 -635 = -3518 kJmol-1

Question 6

Define the Enthalpy change of solution

Answer 6

Enthalpy change that occurs when 1 mole of a solute dissolves in water

Question 7

Write the equation for the Enthalpy of solution of sodium chloride

Answer 7

Na+Cl- (s) -> Na+ (aq) + Cl- (aq)

Question 8

What two things occur when something dissolves in water?

Answer 8

1. Electrostatic attractions in lattice are broken (endothermic). Same value as lattice enthalpy but positive.
2. Ions stabilized by polar water molecules (exothermic)

Question 9

Define Enthalpy change of hydration

Answer 9

Enthalpy change that accompanies the dissolving of gaseous ions in water to form 1 mol of aqueous ions

Question 10

Is enthalpy of solution endothermic or exothermic? What must we do first?

Answer 10

• can be either, depending on relative sizes of lattice enthalpy and enthalpy of hydration
• we must run our calculations first using - lattice enthalpy + enthalpy of hydration

Question 11

Calculate the Enthalpy of solution given the following data:
Enthalpy of hydration of Ca2+ is -1579
Enthalpy of hydration of Br- is -348
Lattice enthalpy of CaBr2 is -2176

Answer 11

2176 - 1579 -(348 x 2) = -99kJmol -1

Question 12

What factors affect mp prediction and solubility prediction?

Answer 12

MP to do with lattice enthalpy -> ionic size (smaller = less negative LE = decreases MP), ionic charge (increases =more negative LE = increases MP)

Solubility to do with hydration -> same factors but without MP bit. If hydration enthalpies are greater than lattice enthalpy, the compound SHOULD dissolve, but not always the case.

Question 13

Explain why it is difficult to predict whether the enthalpy change of solution becomes more exothermic or less exothermic down the group from MgF2 to MgI2.

Answer 13

Halide ions gets larger down the group

Lattice enthalpy AND hydration enthalpy is less exothermic down the group

Difficult to predict whether the lattice enthalpy or hydration enthalpy has bigger effect