Intermolecular Forces

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Polarity and boiling temperatures

Non-polar molecules have much lower boiling temperatures compared with polar bonds.

This tells us that forces between polar molecules are much bigger than non-polar, since it takes more energy to break then apart. 

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Dipole-dipole interactions

Polar molecules have a permanent dipole (a permanent seperation of charge).

Polar molecules are attracted to each other by permanent dipole-permanent dipole interactions.

The negative end of one molecule is attracted towards the positive end of another.

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London Forces (van der waals)

Force of attraction between non-polar bonds.

In a neutral atom, the electrons are constanlty moving around, so the distribution may concentrate at a certain point, at any time.

Therefore the positive and negative charges of the molecule are unbalanced.

Producing an instantaneous dipole

Any atom near this will experience an induced dipole

The forces are small, and the attraction lasts a short time. So it is on and off. 

These exist between all molecules. 

They are the only forces between noble gases. 

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Hydrogen Bonds

The molar enthalpy of vaporisation of a liquid is the enthalpy change when one mole of liquid changes into one mole of its gas at the boiling temperature. 

This is a direct measure of intermolecular forces in liquids.

Hydrogen bonds explain particulary strong interactions.

It exists between a molecule containing a hydrogen atom attached to an electronegative element and another molecule containing a lone pair of electrons on an electronegative element. 

Hydrogen has no inner shells, so it is unusually exposed. The lone pairs are therefore attracted to the positive hydrogen. 

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Strength between intermolecular forces

Strongest to weakest:

  • Hydrogen bonds
  • Permanent dipole-dipole forces
  • London Forces 
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Trends in Alkanes

The melting and boiling temperatures of the alkanes increases as chain length increases. 

The intermolecular forces in alkanes are weak, they are London Forces.

These forces become larger as the molecule size increases. 

Volatility is a measure of how easily molecules escape from a liquid. 

Alkanes become more volatile as the amount of branching increases, boiling temperature decreases. 


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Alkanes vs Alcohols

Alcohols tend to have much higher boiling temperature than alkanes, despite having the similar relative formula masses. 

This is because alkanes only have London forces, whereas alcohols can form hydrgen bonds too.

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Trends in Hydrogen Halides


Flurine and Chlorine are much more electronegative than Bromine and Iodine. Therefore dipole-dipole (London forces) are much greater.

This explains the higher boiling temperatures.

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The mass of a solute that dissolves 100g of solvent at a particular temperature. 

Water is a good solvent.

Non-aqueous solvents (hexane), will dissolve many substances water does not.


  • High polar solids such as ionic salts, dissolve in water.
  • Polar organic substances, such as sucrose, dissolve in water but not in hexane.
  • Non-polar solids do not dissolve in water but dissolve in hexane.
  • Non-polar liquids mix completley. They are miscible.
  • Polar liquids are miscible. 
  • A polar liquid and non-polar liquid are immiscible and form separate layers.
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Ionic solids and water

The positive and negative ions in Ionic solids are held together by electrostatic forces. 

Energy required to break down the ionic lattice is lattice energy.

Water molecules are polarised hydrogen has a 

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Solubility and water


They are soluble in water because they have a polar -OH group that can hydrogen bond to the -OH groups in the water molecule.


E.g. Haxane. 

Hexane floats on water because the hexane molecules are weaker than the hydrogen bonds between water molecules. 


Two non-polar liquids mix completey.

Both contain London forces, these mix and extend thoughout the mixture. 

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