Intermolecular+forces

Forces of attraction exist between the molecules of a substance. There are 3 types of intermolecular forces of attraction:
 * Van der waals’ forces
 * Dipole-Dipole forces
 * Hydrogen Bonding

//**Van der waals’ forces **// Non-polar molecules usually have temporary dipoles. That is they have a dipole for a short instant every now and then. This temporary dipole is caused due to the random movements of the valence electrons of an atom such that every now and then they are positioned in a way that creates a temporary dipole on the molecule. Usually when a molecule has a dipole it tends to have an inductive effect on the neighbouring molecules. As a result the molecules attract each other. The larger the van der waals’ forces between the molecules, the higher the boiling point of the molecule as more energy is required to overcome the stronger forces of attraction between the molecules. The strength of the van der waals’ forces depends on the //molar mass// and //surface area// of the molecule. The more elongated the molecule the higher the van der waals' forces of attraction and thus the higher the boiling point of the substance. Also the larger the molar mass, the stronger the van der waals’ forces. That is the reason why the strength of the van der waals’ forces betweent the molecules increases as you go down group 7.

// **Dipole-Dipole forces ** // Polar molecules have positive and negative poles with permanent dipoles. Thus there is an electrostatic force of attraction between polar molecules. This force of attraction is called a dipole-dipole force. Dipole-dipole forces of attraction are much stronger than van der waals’ forces, and therefore between two molecules of similar mass, one polar and the other one non-polar, the polar one will have a higher boiling point (because polar molecules have dipole-dipole forces of attraction which are much stronger than van der waals’ forces between non-polar molecules). 

//**Hydrogen Bonding **//

This usually occurs in molecules where hydrogen is bonded to a highly electronegative atom such as Flourine, oxygen or nitrogen. The molecule will be polar, with the hydrogen atom having a partial positive charge. Due to its partial positive charge, the hydrogen will from a strong dipole-dipole force of attraction with the lone electron pair on a neighbouring atom. However this dipole-dipole force of attraction is given its own special name, ‘Hydrogen bonding’. This is because hydrogen bonding is much stronger than the other intermolecular forces of attraction. 

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If the intermolecular forces of attraction were to be arranged from the weakest to the strongest, then we would have: //

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Van der waals’ < dipole-dipole < Hydrogen bonding

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">As has been hinted earlier, the strength of the intermolecular forces of attraction has an effect on the boiling point of a substance. This is because when a substance boils (from liquid to gas) the molecules become further apart and thus the intermolecular forces between them need to be overcome; thus the stronger these forces are, the higher the amount of energy required to overcome them and the higher the boiling point.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">That is the reason why the hydrides of nitrogen, oxygen and fluorine have a much higher boiling point than the hydrides of the other elements which are in the same group as them.


 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">In the hydride of nitrogen (that is ammonia NH 3 ) there is hydrogen bonding between molecules, one hydrogen from one molecule, is attracted to the lone pair on the nitrogen atom), thus NH 3 has a high boiling point. However, the hydrides of other elements in group 5 (eg. PH 3 ) have a much lower boiling point than NH 3, because they are non-polar and have van der waals’ forces of attraction between them.


 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The hydride of oxygen (H <span style="font-family: Tahoma,sans-serif; font-size: 14pt; vertical-align: sub;">2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O) also has hydrogen bonding where the two hydrogen on one molecule, hydrogen bond to the two lone pair on the oxygen of a neighbouring molecule. Thus water has a higher b.p (due to hydrogen bond) than the hydrides of other atoms in group 6 (eg. H 2 S).


 * <span style="color: #0000ff; font-family: Tahoma,sans-serif; font-size: 80%;">By the end of this lesson you should be able to: **
 * **<span style="color: #0000ff; font-family: Tahoma,sans-serif; font-size: 80%; line-height: 27px;">describe the 3 type of intermolecular forces of attraction which exist **
 * **<span style="color: #0000ff; font-family: Tahoma,sans-serif; font-size: 80%; line-height: 27px;">explain how the intermolecular forces of attraction arise from the structural features of the molecule(polarity..) **
 * **<span style="color: #0000ff; font-family: Tahoma,sans-serif; font-size: 80%; line-height: 27px;">explain how the strength of intermolecular forces affects the boiling point of the substance **
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