Question

For each of the following pairs of molecules, identify the dominate intermolecular force, and answer any associated questions.

a. KNO2, CH3NO2

Which compound do you expect to have the highest melting point?

b. CH3CH2OH, CH3OCH3

Which compound do you expect to have the highest boiling point?

c. KF, BrF

Which compound do you expect to have the greatest vapor pressure?

List the dominant IMF for each molecule:

d. NH3, N2

e. CF4, CCl4

f. HCN, KCN

g. LiCl, CH3CH3

h. CH3OH, CH3F

Please explain each step.

Answer 1

a) KNO2 vs CH3NO2
KNO2 is an ionic compound whereas, CH3NO2 is a covalent compound. Ionic compounds do have higher electrostatic force of attration between ions. And hence KNO2 will have higher melting point than CH3NO2.

b) CH3CH2OH vs CH3OCH3
CH3CH2OH is an alcohol with hydroxyl group responsible for strong intermolecular hydrogen bonding. Whereas, in ether CH3OCH3, there is no hydrogen bonding possible and hence will have only weaker van der Waal's forces acting between the molecules. Thus the CH3CH2OH (ethanol) will have higher boiling point than CH3OCH3 (dimethyl ether).

c) KF vs BrF
KF is an ionic compound and hence it possess strong electrostatic force of attraction. Wereas BrF is expceted to be covalent compound. Hence KF will have lower vapor pressure as compared to BrF.

d) NH3 + N2:
N2 is non-polar molecule and NH3 is a polar molecule. These molecules will have London forces, diole-induced dipole interaction present in the molecules.

e) CF4 + CCl4
Both are non-polar molecules hence they will have London forces of interaction present in the molecules.

f) HCN + KCN

Both are polar molecule and KCN is ionic compound. Hence they will have London forces, dipole-dipole interactions.

g) LiCl + CH3CH3

LiCl is polar ionic compound whereas CH3CH3 is non-polar covalent compound. Hence they will have London forces, and dipole induced dipole interactions present in the molecules.

h) CH3OH + CH3F

Both are polar molecules and hence will have dipole-dipole interaction predominantly.