Intermolecular Forces MCQ - Practice Questions with Answers

Intermolecular forces are the physical forces or just the interactions which act between the neighbouring molecules. 

The forces of attraction and repulsion between the interacting particles (atoms and molecules) are called intermolecular forces. These forces hold the molecules together which are covalently bonded. Intermolecular forces are not encountered in systems that employ ionic bonding.

Van der Waal (1837 - 1923), a Dutch scientist explained that the attractive forces present between the molecules lead to the deviation of real gases from the ideal gas behaviour. which we will study later in this chapter. So, to honor the scientist, intermolecular forces are also known as van der Waals forces. These forces of attraction exist between polar as well as non-polar molecules. These are the electrostatic forces of attraction that exist between an area of negative charge on one molecule and an area of positive on a second molecule. There are three types of van der Waals forces or interactions namely, induced dipole- induced dipole interactions, dipole-induced dipole interactions, and dipole-dipole interactions. There is a fourth type of interaction which is a particularly strong type of dipole-dipole interactions called hydrogen bonding.

• Dispersion Forces or London Forces: (Associated with non-polar structures)

These non-polar molecules are electrically symmetrical, so there is no dipole moment. But sometimes the temporary dipoles can be formed. The electrons of a neutral molecule keep on oscillating with respect to the nuclei of the atoms. As a result of this, at a given instant, positive charge may be concentrated in one region and the negative charge in another region of the molecule. Thus the non- polar molecule develops momentarily dipole due to unsymmetrical electronic charge distribution. Now, this polarised molecule distorts the electron density of the neighbouring molecule and dipoles are developed in the neighbouring molecule. These interactions are therefore also known as dipole-induced dipole interactions. Now, the attraction between the two oppositely charged ends of the two neighbouring molecules attracts each other and this type of force of attraction is called London Force after the name of the German Physicist Fritz London who proposed this type of force of interaction. This force is also known as dispersion force. 
London forces are attractive in nature and the interaction energy is inversely proportional to the sixth power of the distance between the two interacting particles.

where r is the internuclear distance between two interacting particles, These forces work only at short distances of approximately 500 p.m.
The magnitude of the forces depends upon the polarisability of the atom or molecule: polarisability is the ease with which the electrons and nuclei can be displaced from their average positions. Larger the polarisability, stronger are the London forces. 

• Dipole-Induced Dipole Forces : (Between a polar and a non-polar molecule) 

This type of forces operate between a polar molecule which has a permanent dipole and a non-polar molecule whose electron density is symmetrical. A polar molecule may sometimes polarise a non-polar molecule thereby deforming Its electronic cloud which lies in its vicinity. So, in this manner it induces the di-polarity in the non polar molecule.

Interaction Energy is inversely proportional to the sixth power of the distance between the two molecules.

The induced dipole moment depends upon mainly on two factors. 

(a) Dipole Moment : Present in the permanent dipole (polar molecule).

(b) Polarisability of the electrically neutral molecule (Non-polar molecule). Molecules of larger size get easily, thereby strength of attractive interactions increases. 

Dispersion forces also operate in such cases. So, the cumulative effect of dispersion forces and dipole induced dipole interactions exists. That means the total effect is greater than the sum of individual effects of dispersion forces and dipole-induced dipole interactions.

• Dipole-Dipole Forces: (Associated with polar structures) 
  Polar molecules have a partially positive side and a partially negative side or a dipole. Dipole-dipole forces operate between the molecules which are though neutral but possess permanent dipole. The separation of partial charges depends upon the electronegativity of the bonded atoms in a molecule. The partial charges are indicated by the Greek letter "δ"  In these type of forces the partial positive end of the one molecule is attracted towards the negative end of the other molecule. 
  
  
  Example: Dipole-dipole forces are present between the two HCI molecules. Chlorine being more electronegative pulls the shared pair of electrons towards itself. So, it has a partial negative charge() on it and hydrogen atom has a partial positive charge ().

• The interaction energy is dependent upon the distance between the polar molecules 
  (a) Stationary polar molecules: Dipole-dipole interaction energy is inversely proportional to the third power of the distance between stationary polar molecules (in solids).
  
  (b) Rotating polar molecules: Dipole-dipole interaction energy is inversely proportional to the sixth power of the distance between the rotating polar molecules (like water). 
  
  Dipole-dipole forces are stronger than the London forces because permanent dipoles are involved but weaker than the ion-ion interaction because partial charges present in polar molecules are always less than the unit electronic charge () present on the ions.
  The magnitude of this type of interaction depends upon the dipole moment of the molecule concerned. The greater is the dipole moment, the greater is the dipole-dipole interaction. Besides the dipole-dipole interaction, polar molecules can also interact by London forces. Thus the total intermolecular forces in polar molecules increase.

• Hydrogen Bonding: (Associated with polar structures) 
  Hydrogen bonding is a special type of dipole-dipole interaction. But the difference is that for hydrogen bonding there is a large difference in electronegativity of the covalently bonded atoms. That means hydrogen bonds are formed between the N—H , O—H and H—F bonds. thus, hydrogen bonding is the force of attraction between the hydrogen atom attached to the highly electronegative atom and the electronegative atom of the other polar molecule. Example H₂O, NH₃, HF etc.
  
  In the above case of hydrogen fluoride, hydrogen atom forms a weak bond which is hydrogen bond with the electronegative fluorine atom represented by a dotted line. As the difference in the electronegativity values of the bonded atoms produce large partial charges so, the resulting intermolecular forces are strong but weaker than the actual chemical bonds. The energy of the hydrogen bond varies between 12.6 to 41.8 kJ/mol.
  
  Repulsive Forces :
  When the molecules come very close to each other or just come in contact with each other, then repulsive forces operate between the electron clouds of the two molecules and between the nuclei of two molecules. Moreover, as the distance separating the two molecules decreases, the magnitude of repulsive force increases. 
  
  Note : As the distance separating the molecules is less in liquids and solids therefore, more repulsive forces come into play when these are tried to compress. That is why liquids and solids are hard to compress.