Intermolecular forces

Intermolecular Forces Ms L Chu Sunday 17 October 2010

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Intermolecular Forces ,[object Object],[object Object],[object Object]

Types of intermolecular forces 1) Permanent dipole – dipole forces 2) Van der Waals’ forces 3) Hydrogen bonding

Permanent dipole-dipole forces ,[object Object],[object Object],Green lines represent the dipole-dipole forces Increasing polarity of the molecules increases the strength of attraction

Van der Waals’ Forces ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Evidence : Noble gases are monoatomic, existing as single atoms in the gaseous phase at room temperature.

[object Object],[object Object],[object Object],[object Object],[object Object],How do these arise? Electrons are in constant motion. So at any instant there is more negative charge on one side of the molecule than other and so possesses an instantaneous electric dipole.

This then induced dipoles in neighbouring molecules causing weak induced dipole-induced dipole attractions between the molecules. ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],As the size of the atoms or molecules increases there are more electrons so the temporary partial charge is bigger resulting in stronger attraction

Average charges are spread evenly At any instance the electrons are concentrated on one side of the atom or molecule more than the other. This gives the atom or molecule a temporary partial negative charge - a temporary dipole moment. This dipole moment will induce a temporary dipole in a neighboring atom by attracting/repelling its electron charge cloud. A fraction of a second later the electron distribution changes causing and the temporary dipole-dipole attraction ( Van der Waals attraction) to break. These forces are weaker than hydrogen bond and permanent dipole forces.

Hydrogen bonding ,[object Object],[object Object],[object Object],[object Object]

[object Object],H O H H H H O O O H O H H H H H-bond Here the lone pair of electrons on oxygen is attracted to the positive nucleus of the hydrogen atoms forming H-bond. Each water molecule can form 2 h-bonds. How many H-bond can HF forms?

Essential requirements for an H-bond are: ,[object Object],[object Object],Water Liquid: water has a higher boiling point than most other hydrides (see graph) due the H-bonds present in the molecules. Greater energy is required to break these strong bonds.

[object Object],[object Object],[object Object],[object Object],Ice: a three-dimensional hydrogen-bonded lattice. In this lattice each oxygen is surrounded by a tetrahedron of hydrogen atoms bonded to other oxygen atoms. Ice has unusual properties in that it has a lower density than liquid water (water has a maximum density is at 4 0 C).

Structure of ice Four hydrogen bonds, two through its hydrogen atoms and two through its oxygen atoms hold each water molecule in place

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Intermolecular forces

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