4. Dispersion Interactions

Question 1

What are the three types of dipole?

Answer 1

• Permanent
• Induced
• Instantaneous

Question 2

Give the equation for the attractive dispersion force between two atoms/molecules

Answer 2

F(x) = force
U = potential energy
x = position
α = polarisability
q = charge
d = separation distance
ε = permittivity

Question 3

Give the equation for the attractive dispersion potential between two atoms/molecules

Answer 3

U(x) = potential
α = polarisability
q = charge
d = separation distance
ε = permittivity

Question 4

What force opposes the attractive dispersion force?

Answer 4

A repulsive empirical potential

Question 5

What causes the repulsive force between two atoms?

Answer 5

The Pauli exclusion principle.
When two atoms get too close, their electrons overlap so they must repel each other so that the Pauli exclusion principle is maintained.

Question 6

State the Pauli exclusion principle

Answer 6

“Two or more electrons may not occupy the same quantum state.”

Question 7

Give the equation for the repulsive potential between two atoms

Answer 7

U(x) = potential energy
B = constant
x = position

Question 8

What is the Lennard Jones potential?

Answer 8

A combination of the attractive and repulsive potentials, mimicking the change in potential with distance from an atom.

Question 9

Describe the shape of the Lennard Jones potential

Answer 9

Question 10

Give the equation for the Lennard Jones potential

Answer 10

U(x) = potential energy
B and C = constants
x = position

Question 11

Give the equation for the approximate range of a dispersion interaction

Answer 11

x = range
α = polarisability
q = charge
d = separation distance
ε = permittivity
k = Boltzmann’s constant
T = temperature

Question 12

What is the approximate range of a dispersion interaction?

Answer 12

~0.3 nm

Question 13

The dispersion interaction is _____ range.

Answer 13

Short

Question 14

What is the total interaction energy for a central atom/molecule surrounded by N nearest neighbours?

Answer 14

U_tot = total interaction energy
N = number of nearest neighbours
U = dispersion interaction

Question 15

How can dispersion interactions hold simple organic liquids and solids together when no other interactions occur?

Answer 15

So many dispersion interactions occur that the total interaction energy is high enough to hold the material together.