Chemistry Project. Intermolecular Forces. Introduction. What are intermolecular force? They are forces of attraction or repulsion between molecules. . Two Types of Intermolecular forces. 1. van der Waals ’ Force a. Dipole-Dipole Interaction b. Dipole-induced Interaction
They are forces of attraction or repulsion between molecules.
a. Dipole-Dipole Interaction
b. Dipole-induced Interaction
c. London Forces (instantaneous Dipole-induced Dipole Interaction)
Before discuss intermolecular forces, we should know about what is DIPOLE. This can help us easier to understand the intermolecular forces.
1.Sizes of Electron cloud
2.Shapes (surface area) of Molecule
More electrons , and more distance over which they can move
Bigger the possible temporary dipoles and therefore the bigger the London forces.
Boiling point of elements
(Boiling points increase as down the group)1. Size of Electron Cloud
Butane --carbon atoms are arranged ina single chain,
2-methylpropane --shorter chain with a branch.
the electrostatic attraction between thelone pairof electrons on thehighly electronegativeatom and the slightly positivelyhydrogen atom.
As hydrogen bonds are much stronger then van der Waals’ force, more energy is needed to break the hydrogen bonds in HF, NH3 and H2O. As the result, the boiling points of these hydrides are relatively higher.
—Structure of graphite
of carbon atoms arranged in layers. Weak
van der Waals’ forces hold the layers together.
The weak van der Waals’ forces of attracion between the layers allow one layer of bonded atoms to slide easily over another layer.
This explains why graphite is soft and slippery
and can be used a lubicant.
Hydrogen bond hold the TWO nucleic acid chain together. They are formed between specific pairs of bases on the chain. Like the figures below.
In DNA replication, the hydrogen bonds are broken down. When a complementary chain is formed adjacent to each of the original chain by the formation of NEW hydrogen bonds.
Solubility Rule and Summary
“LIKES DISSOLVE LIKES”
with Non-polar Solvent
the magnitude of the temperature decrease during evaporation is an indication of the strength of the intermolecular forces present. A small drop in temperature i.e. slow rate of evaporation is an indication of strong intermolecular forces.
Surface tension is a result of intermolecular forces. Molecules at the surface of a liquid are attracted to the molecules beneath, leading to an inward force on the liquid and a kind of skin on thesurface.
1. Chan Wai Yee (6B, 4)
2. Cho Wing Yan (6B, 7)