Question

How to predict which substance in each of the following pairs would have the greater intermolecular forces?
A. $C{O_2}$ or $OCS$
B. $Se{O_2}$ or $S{O_2}$
C. $C{H_3}C{H_2}C{H_2}N{H_2}$ or ${H_2}NC{H_2}C{H_2}N{H_2}$
D. $C{H_3} - C{H_3}$ or ${H_2}CO$
E. $C{H_3}OH$ or ${H_2}CO$

Answer 1

The strength of intermolecular forces between the molecules depends on the polarity of the molecule and the ability of that molecule to form hydrogen bonds. By using these two basics we can determine the pair which have greater intermolecular forces.

Complete step by step answer:
The strength of intermolecular forces between the molecules depends on the polarity of the molecule and the ability of that molecule to form hydrogen bonds. So, we have to compare all the pairs on the basis of these two properties.
So, In the case of $C{O_2}$ or $OCS$,
These two $C{O_2}$ and $OCS$ both are linear molecules but if we talk about the molecule of carbon dioxide it is nonpolar on the other hand the molecule of $OCS$ is polar. In the case of carbon dioxide, the bond dipole moments are equal in magnitude and point in opposite directions, so the net dipole moment will be zero. On the other hand, in the case of $OCS$ , the bond dipoles are not equal in magnitude because sulfur and oxygen have different electronegativity values.
Hence, $OCS$ has greater intermolecular forces than $C{O_2}$ .
Now, in the case of $Se{O_2}$ or $S{O_2}$ ,
If we compare them on the basis of electronegativity then, selenium and sulfur have almost the same electronegativity. And they both given molecules have a bent geometry and due to same electronegativity these both are polar.
But selenium has a bigger radius than sulfur, so we can say that it also has the electron cloud bigger than sulphur which makes it more polar.
And also the positive charge on the selenium atom will be slightly bigger than the charge on the sulfur atom, which makes the net dipole moment of selenium slightly greater as compared to sulfur.
Hence, $Se{O_2}$ has greater intermolecular forces than $S{O_2}$ .
Now, in the case of $C{H_3}C{H_2}C{H_2}N{H_2}$ or ${H_2}NC{H_2}C{H_2}N{H_2}$ ,
${H_2}NC{H_2}C{H_2}N{H_2}$ has greater intermolecular forces than $C{H_3}C{H_2}C{H_2}N{H_2}$ . This is where the ability to form hydrogen bonds comes into play. The difference between these two amines will be made by the additional -NH2 functional group present on ethylenediamine.
This second amine 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 amine group attached.
Now, in the case of $C{H_3} - C{H_3}$ or ${H_2}CO$ ,
${H_2}CO$ has greater intermolecular forces than $C{H_3} - C{H_3}$ . because methane is a nonpolar molecule because carbon hydrogen bonds are supposed to be nonpolar. And that’s why methane only has weak London dispersion forces. And on the basis of electronegativity the oxygen atom will create a permanent dipole moment in the case of formaldehyde molecules.
Now, in the case of $C{H_3}OH$ or ${H_2}CO$ ,
Methanol has greater intermolecular forces as compared to formaldehyde. Here, we will compare the ability of molecules to form hydrogen bonds. Methanol structure has one hydrogen atom directly attached to the oxygen atom which causes hydrogen bonding.

Note:
Hydrogen bonding comes in case when polarity is equal otherwise the strength of intermolecular forces between the molecules depends on the polarity of the molecule and the ability of that molecule to form hydrogen bonds. By using these two basics we can determine the pair which have greater intermolecular forces.