Forces Acting Between Molecules

Question 1

What are the three types of intermolecular force?

Answer 1

1. van der Waals forces
2. Dipole-Dipole forces
3. Hydrogen bonding

Question 2

How do van der Waals forces arise?

Answer 2

-van der Waals forces act between all atoms or molecules at all times
-They are in addition to any other intermolecular forces
-The dipole is caused by the constantlly changing position of the electron cloud, therefore meaning the more electrons, the larger the instataneous dipole will be.

Question 3

How do dipole-dipole forces arise?

Answer 3

-A dipole is a shift in electronegativity in an bond, causing it to become polar
-This causes attraction between bonds that have similar shifts in electronegativity.
-A permanent shift in electronegativity in a bond causes a permanent dipole to be formed.

Question 4

How do van der Waals forces affect boiling points?

Answer 4

Boiling points are increased by van der Waals forces when the number of elecrons present increases, as the size of the instantaneous dipole is increased. This increases the strength of the forces, requiring more thermal energy to overcome them.

Question 5

What is needed for hydrogen bonding to occur?

Answer 5

-An Oxygen, Nitrogen or Fluorine atom. This is because it is the only atom electronegative to form bonds with hydrogen atoms.

Question 6

Why do NH3, H2O and HF have higher boiling temperatures than expected?

Answer 6

Hydrogen bonding is present between the molecules in each of these compounds, and hydrogen bonds are stronger than the other intermolecular forces.