Lattice Energy

Question 1

What is lattice energy?

Answer 1

The energy change when one mole of an ionic solid is formed from its gaseous ions
M^+ (g) + X^- (g) -> MX (g)

Question 2

What is enthalpy change of atomisation?

Answer 2

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state. It is always positive
M (s) -> M (g)
1/2 Cl2 (g) -> Cl (g)

Question 3

What is electron affinity?

Answer 3

The energy released when one mole of gaseous atoms each acquire an electron to form one mole of gaseous 1- ions
X (g) + e- -> X^- (g)

Question 4

What is a Born-Haber cycle?

Answer 4

A method for calculating lattice energy values. Uses data from experiments and Hess’ law

Question 5

What is experimental lattice energy?

Answer 5

Lattice energy value calculated from carefully measured experimental data. The ‘real life’ value.

Question 6

What is theoretical lattice energy?

Answer 6

Lattice energy value calculated from a mathematical model that assumes the ions involved are perfectly spherical. The ‘rough guess’ value.

Question 7

What is polarisation?

Answer 7

Anions are polarised if they are ‘stretched’ out of shape by a strong force of attraction.

Question 8

What factors affect polarising power?

Answer 8

Different cations have different polarising powers (ability to polarise anions) depending on their charge and size. Small, highly charged cations have the greatest polarising power

Question 9

What is polarisability?

Answer 9

How easily anions are polarised, again, depending on their size. Large anions are more polarisable

Question 10

Why do ionic substances have covalent character?

Answer 10

Most ionic bonds are not completely ionic because they are slightly covalent due to the polarisation of anions.

Question 11

What is enthalpy change of solution?

Answer 11

The enthalpy change when one mole of an ionic substance dissolves in water to give a solution of infinite dilution
MX (s) + aq -> M^+ (aq) + X^- (aq)

Question 12

What is enthalpy change of hydration?

Answer 12

The enthalpy change when one mole of gaseous ions dissolve in sufficient water to give an infinitely dilute solution. Hydration enthalpies are always negative
X^- + aq -> X^- (aq)

Question 13

What are the steps to draw a Born-Haber cycle?

Answer 13

1) Start with the reactants in their standard states
2) Draw in the enthalpy of formation (downwards)
3) Atomise the metal (upwards)
4) Ionise the metal (upwards)
5) Atomise the non-metal (upwards)
6) Add an electron to the non-metal (downwards)
7) Use the gap to calculate lattice energy

Question 14

How and why does ionic radius affect lattice energy?

Answer 14

A bigger ionic radius means there is a greater distance between the ions, so a weaker force of attraction and a less exothermic lattice energy

Question 15

How and why does charge affect lattice energy?

Answer 15

Higher charges lead to a greater electrostatic attraction therefore a more exothermic lattice energy

Question 16

For which compounds are the theoretical values for lattice energy less accurate and why?

Answer 16

It is least accurate for compounds with a large amount of covalent character. Higher charge density cations distort the anion more, and larger anions can be distorted more. The mathematical model used to calculate the theoretical value does not account for covalent character, and the more distorted the anion the further the compounds differ from the perfect spheres and so the more inaccurate the values.

Question 17

How does charge affect enthalpy of hydration?

Answer 17

A higher charge means there is a greater force of attraction between water molecules and the ion, so more bonds can form between water molecules and the ion so it is more exothermic

Question 18

How does ionic radius affect enthalpy of hydration?

Answer 18

Larger ions mean the charge density is lower so the force of attraction is weaker so less water molecules are attracted to the ion, so enthalpy of hydration is less exothermic

Question 19

How does enthalpy of solution depend on lattice energy and enthalpy of hydration?

Answer 19

The lattice must be broken before the compound dissolves, and energy is released when ions from the lattice are hydrated
- Breaking a lattice requires the same energy as making it
> if less energy is given out when hydrating the ions then enthalpy of solution is endothermic
> if hydrating ions releases more energy than it takes to break the bonds then energy is released then enthalpy of solution is exothermic