Which substance has the highest/lowest melting/boiling point, etc ?
If you are asked to rank molecules in order of melting point, boiling point, viscosity, surface tension or vapour pressure ... what they are actually asking is for you to rank them by strength of intermolecular forces (either increasing or decreasing).
Here is my strategy for this:
1. Look for molecules with hydrogen bonding capability will have the strongest intermolecular forces.
2. Look for molecules with dipoles. These will have the next strongest intermolecular forces.
3. Larger molecules will have stronger London dispersion forces. These are the weakest intermolecular forces but will often be the deciding factor in multiple choice questions.
Here is my strategy for this:
1. Look for molecules with hydrogen bonding capability will have the strongest intermolecular forces.
2. Look for molecules with dipoles. These will have the next strongest intermolecular forces.
3. Larger molecules will have stronger London dispersion forces. These are the weakest intermolecular forces but will often be the deciding factor in multiple choice questions.
Example 1
List the following molecules in order of increasing surface tension: C3H8, CH4, CH3COOH, C2H6
Answer: Higher surface tension corresponds to stronger intermolecular forces. Go through the list above.
1. One of these (CH3COOH) has the ability to hydrogen-bond. It will probably have the strongest intermolecular forces.
2. CH3COOH is the only one of these molecules to have a dipole, and we already decided it has the strongest intermolecular forces.
3. Of the molecules that are left, the largest one (C3H8) likely has the strongest London dispersion forces. The smallest (CH4) likely has the weakest intermolecular forces.
The answer is: CH4, C2H6, C3H8, CH3COOH
List the following molecules in order of increasing surface tension: C3H8, CH4, CH3COOH, C2H6
Answer: Higher surface tension corresponds to stronger intermolecular forces. Go through the list above.
1. One of these (CH3COOH) has the ability to hydrogen-bond. It will probably have the strongest intermolecular forces.
2. CH3COOH is the only one of these molecules to have a dipole, and we already decided it has the strongest intermolecular forces.
3. Of the molecules that are left, the largest one (C3H8) likely has the strongest London dispersion forces. The smallest (CH4) likely has the weakest intermolecular forces.
The answer is: CH4, C2H6, C3H8, CH3COOH
Example 2
List the following molecules in order of increasing boiling point: Br2, F2, I2, Cl2,
Answer: Higher boiling points will correspond to stronger intermolecular forces. Go through the list above.
1. None of these have hydrogen bonding.
2. None of these have dipoles.
3. Bigger molecules will have stronger London dispersion forces.
So I2 has the strongest forces, and F2 will have the weakest. Correspondingly, I2 will have the highest boiling point and F2 will have the lowest boiling point.
Answer: F2, Cl2, Br2, I2
List the following molecules in order of increasing boiling point: Br2, F2, I2, Cl2,
Answer: Higher boiling points will correspond to stronger intermolecular forces. Go through the list above.
1. None of these have hydrogen bonding.
2. None of these have dipoles.
3. Bigger molecules will have stronger London dispersion forces.
So I2 has the strongest forces, and F2 will have the weakest. Correspondingly, I2 will have the highest boiling point and F2 will have the lowest boiling point.
Answer: F2, Cl2, Br2, I2
Example 3
Which has the higher vapour pressure, C20H42 or C30H62?
Answer: Higher vapour pressures correspond to weaker intermolecular forces. Go through the list above.
1. None of these have hydrogen bonding.
2. None of these have dipoles.
3. Bigger molecules will have stronger London dispersion forces.
So, C30H62 has the stronger intermolecular forces and C20H42 has the weaker intermolecular forces.
Answer: C20H42 has the higher vapour pressure, since the molecules stick together less.
Which has the higher vapour pressure, C20H42 or C30H62?
Answer: Higher vapour pressures correspond to weaker intermolecular forces. Go through the list above.
1. None of these have hydrogen bonding.
2. None of these have dipoles.
3. Bigger molecules will have stronger London dispersion forces.
So, C30H62 has the stronger intermolecular forces and C20H42 has the weaker intermolecular forces.
Answer: C20H42 has the higher vapour pressure, since the molecules stick together less.