Will present the highest value of the normal boiling point??

For each of the following pairs of molecules, propose, in a reasoned way, the one
consider that it will present the highest value of the normal boiling point:
a) sulfurhydrogen or water;
b) ammonia or phosphine;
c) boron trifluoride or boron trichloride

1 Answer
Ernest Z.
May 3, 2018

Here's what I find.

Explanation:

In each case, the compound with the strongest intermolecular forces will have the highest boiling point, because it takes more energy to separate the molecules from each other and get them into the gas phase.

The strength of intermolecular forces decreases in the order

#"hydrogen bonding > dipole-dipole > London dispersion forces"#

a) Hydrogen sulfide or water

#"H"_2"S"# — strongest force is dipole-dipole
#"H"_2"O"# — strongest force is hydrogen bonding

∴ Water has the higher boiling point.

b) Ammonia or phosphine

#"NH"_3# — strongest force is hydrogen bonding
#"PH"_3# — strongest force is dipole-dipole

∴ Ammonia has the higher boiling point.

c) Boron trifluoride of boron trichloride

#"BF"_3# — strongest force is London dispersion forces
#"BCl"_3# — strongest force is London dispersion forces

#"Cl"# atoms are bigger and more polarizable than #"F"# atoms.

Thus, #"BCl"_3# has stronger London dispersion forces than #"BF"_3#.

Boron trichloride has the higher boiling point.

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