Intermolecular Forces in Liquids

Intermolecular Forces in Liquids

– Intermolecular forces in liquids are collectively called van der Waals forces.

– These forces are essentially electrical in nature and result from the attraction of charges of opposite sign. 

– The principal kinds of intermolecular attractions are:

(1) Dipole-dipole attractions

(2) London forces

(3) Hydrogen bonding. 

Intermolecular Forces in Liquids

The relative size of these interactions is important so the relative effects are understood.

Intermolecular Forces in Liquids

– Clearly, normal covalent bonds are almost 40 times the strength of hydrogen bonds. 

– Covalent bonds are almost 200 times the strength of dipole-dipole forces and more than 400 times the size of London dispersion forces.

(1) Dipole–Dipole Attractions

– Dipole-dipole attractions exist between polar molecules. 

– This requires the presence of polar bonds and an unsymmetrical molecule. 

– These molecules have a permanent separation of positive and negative charge. 

– In the illustration, the H and of HCl is permanently slightly positive charge.

– The Cl end of HCl has a permanent slight negative charge.

– The H atom in one molecule is attracted to the Cl in a neighbour.

– The intermolecular force is weak compared to a covalent bond, but this dipole-dipole interaction is one of the stronger intermolecular attractions.

Intermolecular Forces in Liquids

(2) London Dispersion Forces

– London dispersion forces exist in nonpolar molecules. 

– These forces result from temporary charge imbalances. 

– The temporary charges exist because the electrons in a molecule or ion move randomly in the structure.

– The nucleus of one atom attracts electrons from the neighbouring atom. 

– At the same time, the electrons in one particle repel the electrons in the neighbour and create a short-lived charge imbalance.

– These temporary charges in one molecule or atom attract opposite charges in nearby molecules or atoms. 

– A local slight positive charge (δ+) in one molecule will be attracted to a temporary slight negative charge (δ–) in a neighbouring molecule. 

London Forces in Hydrocarbons and Organic Molecules 

– The temporary separations of charge that lead to the London force attractions are what attract one nonpolar organic molecule to its neighbours. 

– The possibilities for these interactions go up within creasing molecular size and surface.

– Also The larger surface increases the chances for the (induced)charge separation. 

– If the molecules are linear they have more surface area than if they are folded into a sphere. 

– The linear molecules have higher melting and boiling points because of the increased attractions. 

Intermolecular Forces in Liquids

(3) Hydrogen Bonding

– Hydrogen bonding is a unique type of intermolecular attraction.

– There are two requirements:

(1) Covalent bond between an H atom and either F, O, or N. These are the three most electronegative elements.

(2) Interaction of the H atom in this kind of polar bond with a lone pair of electrons on a nearby atom like F, O, or N. 

– The normal boiling point for water is 100ºC.

– The observed boiling point is high compared to the expected value.

– The predicted boiling point from the trend of boiling points for H2Te, H2Se, H2S and H2O is very low.

– If the trend continued the predicted boiling point would be below –62ºC. The (anomalous) boiling point for water is the result of hydrogen bonding between water molecules. 

– Hydrogen bonding is responsible for the expansion of water when it freezes.

– The water molecules in the solid state have a tetrahedral arrangement for the two lone pairs and two single bonds radiating out from the oxygen.

– The lone pairs on the (O) atom can be attracted to nearby water molecules through hydrogen bonds. A cage-like structure results. 

Intermolecular Forces in Liquids

Summary of Types of Intermolecular Forces

Intermolecular Forces in Liquids

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