Intermolecular Forces

Topic 1.5

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  • Created by: Mary
  • Created on: 05-03-13 11:00

Types of Intermolecular Forces

There are three types of intermolecular forces:

1. van der Waals forces: also known as induced dipole attractions. It occurs between molecules and the dipoles that are induced between the molecules are temporary. 

2. Permanent dipole attractions: these are similar to van der Waals in that they involve a dipole, but this time the dipole is permanent. This type of bonding is stronger than vdW forces. 

3. Hydrogen bonding: occurs between a highly electronegative element (F, O or N) and hydrogen atoms. These bonds are very strong. 

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Physical Properties

1. van der Waals: compounds with vdW forces between molecules tend to have low melting and boiling points. This is because the vdW forces are relatively weak and require little energy to break the bonds. Soluble in non-polar solvents. 

2. Permanent dipole attractions: have higher melting and boiling points than compounds with vdW forces as more energy is required to break their bonds because they are stronger. As they are polar they are soluble in polar solvents. 

3. Hydrogen bonding: have high melting and boiling points owing to their strong intermolecular forces. As they are polar, they are soluble in polar solvents, such as water. 

Like dissolves like, so polar compounds are soluble in polar solvents and vice versa. 

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Hydrogen Bonding in Ice

Ice has a larger density than water. This is due to the hydrogen bonding in ice. There are strong hydrogen bonds between the oxygen atoms and the hydrogen atoms, and this leads to a structure which is an almost tetrahedral shape. In this structure there is a lot of empty space, hence ice has a higher density than liquid water. 

When ice melts to form water, this empty space becomes filled with water molecules, so it becomes less dense than the ice. 

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