Intermolecular Forces - (CAPS) Grade 11

 INTRAMOLECULAR AND INTERMOLECULAR FORCES 

Intramolecular/Interatomic forces are forces that hold atoms together within a molecule, whereas Intermolecular forces are forces that exist between different molecules.

These are the 5 types of Intermolecular forces we will focus on: 

•  Dipole-dipole forces 
•  Mutually induced dipole forces (London forces) 
•  Dipole-induced dipole forces 
•  Ion-dipole forces 
•  Ion- induced-dipole forces 

1. Dipole-dipole forces 

When 2 Dipole molecules come into contact, the positive pole of one molecule will be attracted to the negative pole of the other molecule and both molecules will be held together by this attraction. These interactions are the strong intermolecular force of attraction. 

Special Case– Hydrogen Bonding 

Is a special dipole-dipole interaction that occurs SPECIFICALLY between a Hydrogen atom bonded to either an Oxygen, Nitrogen or Fluorine atom. Hydrogen bonding is the strongest of the dipole-dipole attractions and requires considerable energy to break these bonds which results in compounds containing these bonds having exceptionally high boiling and melting points

2. Mutually induced dipole forces (London forces): 

These Intermolecular forces exist between all types of molecules but are the weakest of forces. Molecules with more electrons will have the stronger London dispersion forces. Breaking London dispersion forces do NOT require much energy which explains why Nonpolar covalent compounds will freeze at very low temperatures.

3. Dipole-induced dipole forces 

This type of force occurs when a polar molecule induces a dipole in a non-polar molecule.

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 4. Ion-dipole forces 

Forces between ions and polar molecules. An Ion is a charged atom, therefore will be attracted to one of the poles of the polar molecule.

5. Ion- induced-dipole forces 

 Forces between ions and non-polar molecules, the ion induces a dipole leading to a weak force that holds the compound together.

INTERMOLECULAR FORCE AND MOLECULAR MASS 

The larger the molecule, the more electrons it contains and therefore the molecule will be more polar and thus the intermolecular forces between the molecules would be stronger. 

To summaries, as the molecular mass increases so does the intermolecular force. This can be illustrated using alkanes which is shown in the table below. The longer the alkane chain, i.e. the greater the molecular mass, the higher the boiling point of the alkane which indicates a stronger intermolecular force.

THE EFFECT OF INTERMOLECULAR FORCES ON BOILING POINT, MELTING POINT, VAPOUR PRESSURE AND SOLUBILITY

Boiling point 

Boiling point is the temperature at which the vapour pressure of a substance is equal to the atmospheric pressure.

Melting Point 

Melting point is the temperature at which the solid and liquid phases of a substance is at equilibrium.

Vapour Pressure 

Vapour pressure is the pressure exerted on a container when the vapour and liquid phases of a substance is at equilibrium.

Solubility 

Solubility is the ability of a certain substance to dissolve in another substance, for example sugar dissolving in water. Polar molecules are only soluble in polar solvents and non-polar molecules are only soluble in non-polar solvents.