Lattice Enthalpies

Question 1

What is the enthalpy change of formation?

Answer 1

This is the enthalpy change when one mole of a compound in its standard state is formed from its constituent elements in their standard states

Question 2

What are the 5 steps in formation of an ionic solid from their standard elements

Answer 2

1) atomisation of both elements involved
2) ionisation of the element that is the cation
3) electron gain of the element that is the anion
4) combination of the gaseous ions (-lattice enthalpy)

Question 3

What is lattice enthalpy?

Answer 3

This is when one mole of ionic solid is converted into its gaseous ions

It is the difference in potential energy between the ions in the solid and the ions in a gas

Question 4

Why is lattice enthalpy strongly endothermic

Answer 4

This is due to the electrostatic attraction between the ions in the solid

Question 5

How can you determine lattice enthalpy from a born haber cycle

Answer 5

Lattice enthalpy= -formation + atomisation + ionisation

Add together the parts that tell you how long the arrow line is

Question 6

What is the equation for lattice enthalpy?

Answer 6

AxBy(s)-> XA^y+ (g) + yB^x-(g)

^latt(AxBy)

Question 7

What is the drawback of using born haber to predict lattice enthalpy?

Answer 7

We can’t always determine all of the thermodynamic parameters

Question 8

In the ionic model, where does the energy change come from?

Answer 8

In the ionic model, the energy changes when ion (g) come together from the electrostatic interactions between ions

Question 9

How can you use the ionic model to estimate the lattice enthalpy

Answer 9

By considering all the electrostatic interactions present in an ionic solid, a theoretical lattice energy can be obtained
- assume compounds are completely ionic and ions are point charges

Question 10

What is the change in internal energy according to the ionic model ?

Answer 10

^U= -Z+Z-e^2/ 4pi x Eo x r

Question 11

How can you estimate lattice enthalpy with the ionic model?

Answer 11

A crystal contains many atoms, not just two so there are lots of interactions with pairs of ions

You need to sum all of the interactions
^lattU=(-Z+Z-e^2/ 4pi x Eo x r) x (6-12/{2} + 8/{3}…)
See booklet

Question 12

What is the Madelung constant?

Answer 12

A- accounts for all the interactions between ions

It’s magnitude depends on the structure of the crystal

Question 13

What are the first three terms of the Mandelung constant in NaCl?

Answer 13

For one Na+
It has 6cl- nearest neighbours, 12Na+ ions( 2nd neighbours)
8cl- ions (3rd neighbours)

= 6-12/{2} + 8/{3}…)
- describes the interactions and distance from central ion
These 3 terms account for all interactions in unit cell

Question 14

What is the overall internal lattice energy for ionic model and how do you work out lattice energy from this?

Answer 14

^lattU=-ANaZ+Z-e^2/ 4pi x Eo x r

Lattice energy= -^lattU

Question 15

What is the drawback from using ionic model to estimate lattice energy?

Answer 15

This would be accurate if ions were point charges but this is not true
Additional short range forces between the ions also need to be included
- ions are actually nuclei surrounded by an electron cloud which interact and repel at close distances

Question 16

What are the short range forces in an ionic solid?

Answer 16

These forces arise from repulsion between overlapping electron clouds and rise steeply with decreasing r

Question 17

How is the lattice enthalpy equation modified for born Mayer?

Answer 17

This accounts for short range interactions

^lattU=-ANaZ+Z-e^2/ 4pi x Eo x r x(1-p/r)

P is a compressibility constant related to electron polarisability

Question 18

How can you use born Mayer to estimate how ionic a compound is?

Answer 18

Born Mayer underestimated lattice enthalpy compared to born haber
This is means that if the BM value is close to the BH value, the compound is more ionic
If there are large deviations then the compound is less ionic
This works at BM assumes completely ionic

Question 19

Why are compounds never completely ionic?

Answer 19

This is because the cations positive charge can distort the spherical electron clouds of the anions which leads to a degree of covalent character

Question 20

What are polarisable atoms?

Answer 20

These are atoms that have electron density which is easily distorted
These are normally large anions as electron density is not held close to nucleus

Question 21

What are polarising atoms?

Answer 21

These are atoms that can induce distortions in anions

They are normally high charge density cations

Question 22

What is the drawback of using Born Mayer?

Answer 22

Assumes structure is completely ionic

Requires detailed knowledge of the structure

Question 23

How can you give an estimate of the Born Mayer estimate of lattice enthalpy if you only know A?

Answer 23

If you divide A/V, the number of ions in the chemical formula, you get very similar values

Values increase with coordination number but so does the interionic separation so this cancels out

Question 24

What is the kapustinskii formula for determining lattice energy?

Answer 24

^lattU= kVZ+Z-/r+ + r-

Use an average value of Born exponent, n, combining all constants to give k and assume that the interionic distance= r+ + r-

Question 25

What are the benefits of using kapustinskii’s equation?

Answer 25

Gives a very similar answer but uses much less information

Also allows ionic radii for polymatomic molecules to be estimated from the lattice energy of their compounds

Question 26

Describe the agreement between calculated and expected values when using these approximations of lattice energy

Answer 26

Get good agreement f alkali halides when with a hard anion F-
Poor agreement with soft anion I-
Awful agreement with soft cations- soft anion as they are highly covajektn

Question 27

Why are there deviations in calculated values if lattice energy when using the approximations

Answer 27

Must have a high degree of ionic character to work