Project on Intermolecular Forces. Members: Liu Wing Chiu (19) Siu Nga Chiu (24). Intermolecular Forces. The origin of intermolecular forces The classification of intermolecular forces Van der Waal’s force Hydrogen bonding
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Members: Liu Wing Chiu (19)
Siu Nga Chiu (24)
Molecules are made up of charged particles: nuclei and electrons. When one molecule approaches another, there is a multitude of interactions between the particles in the two molecules. Each electron in one molecule is subject to forces from all the electrons and the nuclei in the other molecule.
1. Van der Waals' force
2. Hydrogen bonding
1. Permanent dipoles
2. Instantaneous dipoles
3. Induced dipoles
These molecules have a permanent separation of positive and negative charge.
A simple example is HCl
The pair of electrons in the covalent bond between hydroge and chlorine is
unequally shared due to the difference in electronegativity between hydrogen and
chlorine. Chlorine has a greater electronegativity compared to hydrogen and hence
Chlorine tends to attract the bonded electron pair to itself. chlorine becomes slightly
negatively charged (-), hydroge atom has a partial positive charged (+) .The
unsymmetrical distributed charge on the HCl molecule produces a permanent
Van der Waals’ force
It can be divided into three categories:
1. Dipole-dipole Interactions
2. Instantaneous dipole-induced dipole Interactions
3. Dipole-induced dipole Interactions
The Classification of Intermolecular Force
1 Dipole-dipole Interactions
2 Dipole-induced dipole Interactions
3 Instantaneous dipole-induced dipole Interactions
In the illustration :
the H end of HCl is permanently slightly positive charge. The Cl end of HCl has a permanent slight negative charge, the "H" in one molecule is attracted to the "Cl" in a neighbor.Dipole-Dipole Interactions
Note: dispersion forces operate in all molecules whether they are polar or non-polar.
For all molecules, the more number of electron (or weaker attraction force between nucleus and electrons), causing the higher in polarizability. The degree of uneven distribution of electron cloud is higher and the strengths induction force and dispersion force become stronger. Thus the stronger van der Waals’ forces.
For molecules with similar relative molecular masses or size, the higher contact surface area, the stronger van der Waals’ forces.
Most ammonium, nitrate and sulphate salts are soluble in water since they form hydrogen bonds with water molecules.
The high solubility of alkanols in water is cause by the formation of hydrogen bond.
Carbohydrates have many –OH groups which can form hydrogen bond with water. Therefore carbohydrates with low relative molecular mass are soluble in water.Solubility
The Hardness of Calcium Sulfate (CaSO4)
This reaction is exothermic.