7.4.1           

7.4.2          Intermolecular Forces

 

Covalent bonds within molecules are very strong.  Forces also exist between molecules called intermolecular forces.  These are of a number of different types

1.       Dipole-dipole attractions

2.     Van der Waals’ forces

3.     Hydrogen bonding

 

 

Dipole – dipole attractions

This type of attraction only occurs between polar molecules which have a permanent dipole and is often called permanent dipole – permanent dipole attraction. The molecules line up in such a way that the partial positive charges are adjacent to partial negative charges giving a net attractive force.

d+   d-                   d+   d-

                   H—Cl           H—Cl

 

Dipole moments of some molecules

Molecule

Dipole moment

 

argon

0

            Ar non-polar

water

 

1.84

          d-

d+      O       d+

H                H

 

 

tetrachloromethane

 

 

 

0

            Cl d-

                          

Cl d-      C d+    Cl d-

                     

                        

            Cl d-

 

hydrogen chloride

 

1.05

          d+       d-

          H       Cl      

 

 

ammonia

 

 

1.48

            d-

            N

 

d+ H              H d+

             Hd+

Van der Waals’ forces (viewed as attractions between induced dipoles), permanent dipole attractions and hydrogen bonding (in molecules containing either or both O-H and N-H bonds).

 

Van der Waals Forces

Non-polar substances such as the noble gases, methane, iodine etc. can be liquefied and solidified.  This suggests that there are forces of attraction between the atoms and molecules of non-polar elements and compounds.  Energy is needed to melt a solid and boil a liquid in order to overcome these forces of attraction.

These are explained in the following manner.  Consider two non-polar molecules that are close together.  The arrangement of the electrons is symmetrical on average.  Yet at a given instant the electron arrangement in one molecule may be unsymmetrical causing a temporary dipole which can attract the electron cloud of another molecule.

Electron cloud                                              -             +

is momentarily to                                        -        B     +

the left                                            -                 +

                           -                      +               direction of induced dipole

                         -        A     +  

                            -           +

                                            direction of temporary dipole

 

This means that both molecules have dipoles and the direction of the dipoles will be such that they attract each other.  Since the electrons are moving about at high speed the attraction has only a fleeting existence.  In the next instant the dipole at A will have changed and may be in the opposite direction.  Once again the dipole induced in B will result in attraction.  The dipoles are temporary but the attraction they produce is permanent.  Therefore in non-polar molecules induced dipole/induced dipole attractions exist.  These are called van der Waals’ forces.

 

Hydrogen bonding

Exercise 1

Plot a graph of the boiling points of Group IV, V, VI and VII hydrides against the relative molecular mass.

 

Group

Compound

 

Mr

Boiling point oC

IV

CH4

SiH4

GeH4

SnH4

16

32

77

123

-161

-112

-90

-52

V

NH3

PH3

AsH3

SbH3

17

34

78

125

-33

-90

-55

-17

IV

H2O

H2S

SeH2

TeH2

18

34

81

130

+100

-60

-41

-2

VII

HF

HCl

HBr

HI

20

36.5

81

128

+20

-85

-67

-35

 

Plot each graph on the same axis

From the results note

(i)                            Group IV

There is a general increase down the Group.  Going down the Group the size of the molecules increases and therefore the strength of the van der Waals’ forces also increases.

 

(ii)     H2O, HF and NH3 have higher than expected boiling points.  Therefore the forces between the molecules are much stronger than for other hydrides.  These strong molecular forces are called hydrogen bonds. 

Nitrogen, oxygen and fluorine are the three most electronegative elements. Therefore in the hydrides the shared electron pair is drawn towards a more electronegative atom. Since hydrogen has no inner electrons to set up forces of repulsion with the non-bonding electrons of the other atom, it can interpose itself between two electronegative atoms exerting an attractive forced on them. Hydrogen bonds therefore extra strong intermolecular permanent dipole/permanent dipole attractions. Hydrogen bonds are typically 1/10 - 1/20 of the strength of a covalent bond.

 For hydrogen bonding to occur;

1.       The hydrogen atom must be attached to an atom with high electronegativity ( O, N, F).

2.       There must be an unshared (lone) pair of electrons on the electronegative atom.   

Relationship between these attractive forces and the physical properties such as melting point, boiling point and solubility, of simple molecular substances.

 

Influence of attractive forces on physical properties

 

Van der Waals’ forces

As the size of the molecule increases, the number of electrons also increases making the molecule more polarisable and producing greater induced dipole attractions.

(i)                            The boiling points of the halogens increase with molecular size.

Cl2     -35 oC

Br2     59 oC

I2       165 oC

(ii)                         Between molecules with long chains of atoms, giving many points of contact, van der Waals’ forces are stronger.  This is why in an homologous series melting and boiling points rise e.g. the alkanes.

C2H6 is a gas

C6H14 is a liquid

C18H38  is a solid

(iii)                       Branched chain isomers have lower boiling points as the van der Waals’ forces are weaker than for unbranched chains.

(iv)                        Van der Waals’ forces are also responsible for holding together the layers in a layer structured solid such as graphite.

 

Hydrogen bonding

(i)                              Liquid water is a hydrogen bonded association of water molecules.  A substance such as ethanol, C2H5OH, will dissolve in water as the molecules of ethanol can displace molecules of water in the association.  New hydrogen bonds form between the molecules of ethanol and water.

 

     H    H               H                                         H  H

   │  │         │                     │ │

H —C— C—O—H……….O—H………….O—H…………….O—C—C—H

      │  │                  │         │  │ │

      H   H                                   H                 H    H  H

 

Chloroethane, C2H5Cl does not form H-bonds with water and is only slightly soluble in water.

(ii)                           In the liquid state,  the molecules of alcohol or associated H -  bonding consequently the boiling points of alcohol ads are higher than those of a non-associated liquids of comparable Molecular mass  eg alkene.

(iii)          Experimental determination of the relatives molecular masses of carboxylic acids shows that they exist as dimers - two molecules bonded together.

 

Hydrogen bonding in ice; open structure leading to low density (3D diagram not required).

 

 Structure of  ice

The bond and water are inclined at a tetrahedral angle 109o. The loan pair's occupied the other corners of the  tetrahedron.

Liquid water contains a associations of water molecules. Entice the arrangement of water molecules is similar, but the regularity extends throughout the whole structure. The structure spaces the molecules further apart than they are in the liquid. This is wide when water freezes its expanse and price is less dense than water resembles that of diamond.