Lattice Enthalpy

Question 1

Define enthalpy of lattice dissociation

Answer 1

The enthalpy change when 1 mole of an ionic lattice is converted into its gaseous ions

Question 2

Define enthalpy of lattice formation

Answer 2

The enthalpy change when 1 mole of an ionic solid is formed from its gaseous ions
e.g. Na+ (g) + Cl- (g) -> NaCl (s)

Question 3

Is enthalpy of lattice dissociation always endothermic or exothermic?

Answer 3

Endothermic - involves bonds breaking

Question 4

Is enthaply of lattice formation always endothermic or exothermic?

Answer 4

Exothermic - involves bonds forming

Question 5

What does lattice enthalpy refer to?

Answer 5

Enthalpy of lattice formation

Question 6

The two main factors which affect the strength of ionic bonding are …

Answer 6

Ionic charge
Size of ions

Question 7

The stronger the ionic bonding in an ionic compound…

Answer 7

the more exothermic the lattice enthalpy

Question 8

Why can’t we find lattice enthalpy experimentally?

Answer 8

There isn’t any equipment that can form & react gaseous ions.
Can’t work out from dissociation enthalpy because heating a salt forms gaseous ion pairs.

Question 9

How can we find lattice enthalpy?

Answer 9

Indirectly, using a Hess cycle.

Question 10

What is the first step in the Hess cycle to determine lattice enthalpy?

Answer 10

Separation - 1 mol of the ionic compound is separated into its constituent elements in their standard states
e.g. NaCl (s) -> Na(s) + 1/2 Cl2 (g)

• reverse of enthalpy of formation

Question 11

What is the second step in the Hess cycle to determine lattice enthalpy?

Answer 11

Atomisation - 1 mol of gaseous atoms is formed from an element in their standard state

e.g. 1/2 Cl2(g) -> Cl (g)

Question 12

We can sometimes relate an element’s enthalpy of atomisation to its bond dissociation enthalpy, but only when it’s…

Answer 12

• gaseous in standard state
• covalently bonded in its standard state

Question 13

What is the third step in the Hess cycle to determine lattice enthalpy?

Answer 13

Removing electrons (ionisation energy) - 1 mol of electrons removed from 1 mol of gaseous atoms to form 1 mol of gaseous + ions

e.g. Al (g) -> Al3+ (g) + 3e-
enthalpy change = 1st + 2nd + 3rd ionisation energies of aluminium

Question 14

What is the fourth step in the Hess cycle to determine lattice enthalpy?

Answer 14

Adding electrons (electron affinity) - 1 mol of electrons added to 1 mol of gaseous atoms to form 1 mol of gaseous - ions

e.g. N(g) + 3e- -> N 3- (g)
enthalpy change = 1st + 2nd + 3rd electron affinities of nitrogen

Question 15

Is lattice enthalpy endothermic or exothermic?

Answer 15

Exothermic
- bonds forming

Question 16

Is enthalpy of formation endothermic or exothermic?

Answer 16

Can be either, but all at A Level are exothermic

Question 17

Is enthalpy of atomisation endothermic or exothermic?

Answer 17

Endothermic
- bonds broken

Question 18

Is ionisation energy endothermic or exothermic?

Answer 18

Endothemic
- removing electrons requires energy

Question 19

Is electron affinity endothermic or exothermic?

Answer 19

1st electron affinity is exothermic.
2nd, 3rd etc electron affinities are endothemic.

Question 20

Why is 1st electron affinity exothermic and subsequent electron affinities are endothermic?

Answer 20

In an atom, the attraction between nucleus & electron being added outweighs the repulsion of other electrons. Adding electrons to atoms is exothermic.
However, 2nd, 3rd etc electron affinities mean an electron is being added to a - ion. The repulsion from the electrons outweighs the attraction the nucleus - requires energy to add electron.