How to predict which substance in each of the following pairs would have the greater intermolecular forces ? a) #CO_2# or #OCS#; b) #SeO_2# or #SO_2#; c) #CH_3CH_2CH_2NH_2# or #H_2NCH_2CH_2NH_2#; d) #CH_3CH_3# or #H_2CO#; e) #CH_3OH# or #H2CO#.

!! LONG ANSWER !!

The strength of the intermolecular forces exhibited by a certain molecule goes hand in hand with its polarity and with its ability to form hydrogen bonds.

Right from the get-go, nonpolar molecules will have weaker intermolecular forces compared with polar molecules of comparable size.

So, here's a brief analysis of each pair (the molecule with the greater IMFs will be written in green)

• #CO_2# and #color(green)(OCS)#

You're dealing with two linear molecules, the only difference between the two being that #CO_2# is nonpolar, while #OCS# is polar.

In #CO_2#'s case, the bond dipole moments are equal in magnitude and point in opposite directions, so the net dipole moment will be zero. In #OCS#'s case, the bond dipoles are not equal in magnitude because sulfur and oxygen has different electronegativity values.

• #color(green)(SeO_2)# and #SO_2#

This one is a little more subtle. From an electronegativity stanpoint, selenium and sulfur are very similar; moreover, both molecules have a bent molecular geometry, which implies that both are polar.

However, selenium has a bigger radius than sulfur, which implies that it also has a bigger electron cloud. That translates into greater polarizability.

The positive charge that will arise on the selenium atom will be slightly bigger than that on the sulfur atom, which implies a slightly greater net dipole moment.

• #CH_3CH_2CH_2NH_2# or #color(green)(H_2NCH_2CH_2NH_2)#

This is where the ability to form hydrogen bonds comes into play. The difference between these two amines will be made by the additional #"-NH"_2# functional group present on ethylenediamine.

This second #"-NH"_2# group will provide ethylenediamine with the capability to form more hydrogen bonds with neighbouring molecules when compared with propylamine, the compound that only has one #"-NH"_2# group attached.

• #CH_3CH_3# or #color(green)(H_2CO)#

Methane, or #CH_3CH_3#, is a nonpolar molecule because the #"C-H"# bonds are considered to be nonpolar. As a result, methane will only exhibit weak London dispersion forces.

By comparison, the electronegative oxygen will create a permanent dipole moment on the formaldehyde molecule. This will allow the molecule to exhibit dipole-dipole interactions, in addition to the London dispersion forces that every molecule exhibits.

• #color(green)(CH_3OH)# or #H_2CO#

Once again, this comes down to the ability to form hydrogen bonds. Both molecules exhibit London dispersion forces and dipole-dipole interactions, but the fact that ethanol, #CH_3OH#, has a hydrogen atom directly attached to an oxygen atom will allow it to engage in hydrogen bonding.