Intermolecular Forces

Question 1

What are Intermolecular forces?

Answer 1

• Intermolecular forces are weak interactions between dipoles of different molecules.
• They fall into 3 main categories: Induced dipole-dipole interactions (London forces), Permanent dipole-dipole interactions and Hydrogen bonding
• They are responsible for physical properties

Question 2

Types of bonds and their bond enthalpy

Answer 2

• London forces - 1-10 kJ/mol
• Permanent dipole -dipole interactions - 3-25 kJ/mol
• Hydrogen bonds - 10-40 kJ/mol
• Single covalent bonds are 150-500 kJ/mol therefore intermolecular forces are not referred to as ‘bonds’ as they are v weak

Question 3

What are London Forces?

Answer 3

• Weak intermolecular forces that exist between ALL molecules
• Only temporary, the induced dipole may disappear
• Origin of induced dipoles:
  1) Movement of electrons produces a changing dipole in a molecule
  2) At any instant, an instantaneous dipole will exist but the position constantly changes
  3) This instantaneous dipole induces another dipole on neighbouring molecule
  4) The induced dipole then induces more dipoles which then all attract one another

Question 4

Strengths of London forces

Answer 4

The more electrons in each molecule:

• The larger the instantaneous and induced dipoles
• The greater the induced dipole-dipole interactions
• The stronger the attractive forces between molecules
• As the strength of London forces increases, the boiling point increases and the number of electrons also increase

Question 5

Permanent dipole-dipole interactions

Answer 5

• Extra energy is needed to break these additional permanent dipole-dipole interactions between molecules
• This increases the boiling point

Question 6

What are simple molecular substances?

Answer 6

• They are made of simple molecules which are small units containing a definite number of atoms with a definite molecular formula.
• In solid state they form a regular structure called a simple molecular lattice
• In a simple molecular lattice, molecules are held in place by weak intermolecular forces and atoms in each molecule are bonded by strong covalent bonds
• They do not conduct electricity

Question 7

Solubility of non-polar simple molecular substances

Answer 7

• When a simple molecular compound is added to a NON POLAR solvent, IM forces are formed between molecules and solvent. These weaken the IM forces in the simple molecular lattice, causing it to break and dissolve
• When added to polar solvents, there is little interactions between molecules in the lattice and solvent. The permanent d-d forces are too strong so non polar simple molecular substances tend to be insoluble in polar solvents

Question 8

Solubility of polar simple molecular substances

Answer 8

• Polar covalent substances may dissolve in polar solvents as the polar solute and solvent molecules can attract each other.
• Solubility depends on the strength of the dipole. Some have both polar and non polar parts such as Ethanol and it can dissolve in both
• Some molecules have hydrophilic parts which will be polar and will contain EN atoms so it can interact with water and it will also contain hydrophobic parts which will be non polar and comprised of a carbon chain