Question

Intermolecular force between a H2S molecule and a NI3 molecule is:

a.ion-dipole force

b.hydrogen bond

c.dipole-dipole force

d.dispersion (London) force

e.ionic bond

Answer: C

Please solve this step by step. Let me know if I am suppose to memorize anything. Why arnt the others it. Please provide an example of the other forces/ bonds.

Answer 1

To understand the intermolecular forces between the molecules, first, we should be fimiliar with electronegativity of various atoms. Flourine is most electronegative.

Intermolecular forces are the forces of attraction and repulsion between interacting particles(atoms, molecules, ions).

1) Dispersion(London) forces:

London forces are the weakest type of intermolecular forces. They exist between all atoms and molecules,whether they are polar or nonpolar.In non polar molecules(no electronegativity difference between two atoms) london forces are the only forces present. Examples: Molecular elements (O₂,N₂ etc) and monatomic elements (He,Ne, etc) .

London forces are caused by an uneven distribution of electrons within an atom.

Because of the constant motion of the electrons, an atom or molecule can develop a temporary (instantaneous) dipole when its electrons are distributed unsymmetrically about the nucleus.

This instantaneous dipole distort the electron density of the other atom, which is close to it and as a result a dipole is induced in second atom.

2) ionic bond: Ionic bond or ionic forces exist between ions of opposite charges.Opposite charges attract each other. These are the strongest intermolecular forces Example : NaCl, CaCl₂ etc.

3) Dipole-Dipole Forces: it act between the molecules possessing permanent dipole. "dipole", means that the molecule has two "poles". One pole(end) of the molecule has a partial positive charge while the other end has a partial negative charge. this interaction is stronger than the London forces but is weaker than the ionic forces because only partial charges are involved.

4) ion-dipole forces: This is the attraction between an ion and a polar molecule. ex NaCl dissolved in water.

5)Hydrogen bond( Special case of dipole-dipole interaction): This is found in the molecules in which highly polar N-H, O-H or H-F bonds are present.

A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine.

Now, in H2S and NI3 , there is no ionic charges on H,S,N or I , so ionic forces and ion-dipole forces are absent( because both need presence of a positive or negative ion). There are no O-H, N-H or H-F bonds in both the molecules ,so no hydrogen bond is present. London forces are present in all molecules but they are weak forces. So, the major intermolecular forces between H2S and NI3 molecules are dipole-dipole forces because both of them have permanent dipoles. S is more electronegative than H and N is more electroegative than I.