Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: nrp88 on November 18, 2008, 08:31:59 PM
-
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
-
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.