[nrp88]

A. List the types of intermolecular forces that exist between the molecules:

1: PCl5 : Dispersion
2. CS2: Dispersion
3: CH3CH2OH: Dispersion, Dipole-Dipole, Hydrogen
4 SF4: Dispersion, Dipole-Dipole

B. Which has the strongest intermolecular forces of attraction?

1. CH3CH2OH or CH3CH2Br
I chose:CH3CH2OH

2. SI4 or CI4
I chose Sulfur Tetraiodide

C. Which has the lowest boiling point?

1. CH3CH2Cl or CH3CH2OH
I chose: CH3CH2Cl

2. NH3 or PH3
I chose: NH3

D. Which has the highest vapor pressure?

1. CH3Br or CH3Cl
I chose: CH3Cl

2. F2 or HCl
I chose: F2

[shelanachium]

All fine except that PH3 boils lower than NH3. This is because N as a small, highly electronegative atom, attracts the partial positive charges on the small H atoms in adjacent molecules ('hydrogen bonding'). P is much larger and less electronegative than N so that PH3 molecules attract each other much less, so PH3 boils lower than NH3 despite its higher molecular weight. The same is found with H2O/H2S and HF/HCl where in each case the higher-molecular-weight substance is lower-boiling.