Topic 4.4: Intermolecular forces Flashcards
Intermolecular forces
Forces between molecules
Intramolecular forces
Forces between atoms
Van der Waals’ forces
Collective name given to the forces between molecules, including:
a) London forces
b) Dipole-Dipole interactions
c) Dipole-Induced dipole interactions
Explanation of London Dispersion Forces (3)
Temporary dipole-induced dipole interactions
a) Since e- in a molecule are in constant motion, at any time the electron density may be greater over one region of a molecule or atom than over another, forming a temporary dipole
b) This temporary dipole may influence the electron distribution in a neighboring atom or molecule, inducing an opposite dipole in a neighboring atom.
c) These dipoles will attract each other so that there is an intermolecular force between atoms
Explanation of the trends of the strength of intermolecular force and molecular size
The strength of IF increases with increasing molecular size
a) The larger atom in the molecule means that the greater number of e-, so the molecule is more polarizable, and the induced dipoles are larger.
Strength of London dispersion force
Weakest intermolecular force
Explanation of Dipole-Dipole forces (3)
Polar molecules form permanent dipole-dipole interactions
a) A polar molecule has always one end that is electron deficient with a partial positive charge, while the other end is electron rich with a partial negative charge (permanent dipole).
b) A dipole-dipole force occurs when the positive end of a polar molecular attracts the negative end of an adjacent polar molecule
Trend of the strength of dipole-dipole forces and polarity
For two substances of similar molecular mass, the more polar substance will have the higher boiling point
Trend of the strength of intermolecular forces and molecular masses
The difference is attributed solely to stronger London dispersion forces in the molecules with more e-
When do hydrogen bonds occur?
Between molecules with a very electronegative atom joined to a H atom
Explanation of the occurence of H bonds
a) The electronegative atom pulls away electron density from H
b) There is a strong interaction between H and the lone pairs of e- in the electronegative atom of a neighbouring molecule
Relative strength of intermolecular bonding
a) London dispersion forces
b) Dipole-Dipole forces
c) Hydrogen bonds
Intermolecular bonding in the following types of substances
a) Non-polar
b) Polar
c) H bonded to N, O, F
a) London
b) London, D-D
c) London, D-D, H bonds
What type of bonds are borken when covalent molecular substances are boiled?
Intermolecular forces, no covalent bonds
Tendency of strength of intermolecular forces and enthalpy energy
The stronger the intermolecular forces, the more energy must be supplied to break them and the higher the boiling point
