6.3 - Intermolecular Forces

Question 1

Define intermolecular forces.

Answer 1

Weak interactions of attraction between dipoles of different molecules.

Question 2

What are the 3 types of intermolecular forces?

Answer 2

• Permanent dipole - permanent dipole
• London forces (instantaneous induced dipole dipole interactions) (weakest)
• Hydrogen bonding (strongest)

Question 3

What are London Forces?

Answer 3

• Weak intermolecular forces that exist between all molecules, polar or non-polar.

1. Movement of electrons generates an instantaneous dipole
2. Instantaneous dipole induces another dipole in a neighbouring molecule
3. The attraction between the temporary induced dipoles is the dipole-dipole interaction

Question 4

What does the strength of a London Dispersion Force depend on?

Answer 4

N^o of electrons

More electrons

• Larger instantaneous and induced dipoles, so greater dipole-dipole interactions and a stronger force
• Higher boiling and melting point

Question 5

What are permanent dipole-dipole interactions?

Answer 5

Interactions that act between permanent dipoles of different polar molecules.

Question 6

What is a simple molecular lattice?

Answer 6

• A simple molecular substance is made of simple molecules - small units containing a definite molecules formula, e.g. O₂
• In the solid state, simple molecules form a simple molecular lattice with regular structure, that has properties:
  
  • Molecules held together by intermolecular forces
  • Atoms within each molecule held by covalent bonds

Question 7

What are the properties of simple molecular lattices?

Answer 7

• Low MP and BP as intermolecular forces are weak and broken at low temperatures
  
  • (same as simple molecular substances)
• NB:* only the intermolecular forces are broken not the covalent ones
• Poor electrical conductors because there are no mobile charged particles

Question 8

Explain the solubility of non-polar simple molecular substances in non-polar solvents

Answer 8

non-polar simple molecular substances ARE soluble in non-polar solvents

• Intermolecular forces form between the compound and the solvent
• The interactions weaken the intermolecular forces in the simple molecular lattice so the compound dissolves

Question 9

Explain the solubility of simple molecular substances in polar solvents

Answer 9

simple molecular substances AREN’T soluble in polar solvents​

• There is little interaction between the simple molecular substance and the polar solvent
• The intermolecular bonding in the polar solvent is too strong to be broken

Question 10

Explain the solubility of polar simple molecular substances

Answer 10

• Polar simple molecular substances are soluble in polar solvents as their polarity means they can attract one another
• Solublity depends on strength of the dipole