CHEM 1405 Concept Review: Intermolecular Forces & Phase Changes

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Intermolecular Forces:  The forces that exist between molecules (or atoms) which are largely responsible for many physical properties and how molecules arrange themselves in the structure of a substance.  The three main types of intermolecular forces are Hydrogen Bonding, Dipole-Dipole Forces, and London Dispersion Forces.

 

London Dispersion Forces:  The forces between molecules which are caused by instantaneous (temporary) dipoles caused by the random motion of electrons in a molecule and/or induced dipoles due to the collision of molecules and even the electrostatic forces between molecules that don’t necessarily collide but come close to one another.  The strength of London dispersion forces increases with increased polarizability.  In general, the larger and/or longer the molecule (or atom), the more polarizable it is.

In this picture, the temporary dipoles of two atoms create an attraction between the atoms,

causing the two atoms to “stick” together.

 

 

Dipole-Dipole Forces:  The intermolecular forces due to the electrostatic attraction between the partially positive end of one polar molecule (permanent dipole) and the partially negative end of a neighboring polar molecule.  Since the dipoles in polar molecules are permanent rather than instantaneous and temporary, the intermolecular forces in these molecules tend to be significantly stronger than those in molecules of comparable size that only have London dispersion forces.

 

Hydrogen Bonding:  A powerful intermolecular force which is due to the uniquely strong attractive force that occurs between a hydrogen atom that is bonded to a highly electronegative atom (N, O, or F) and a lone pair of electrons on a highly electronegative atom from a different molecule (or at least from a different place on the same molecule).

Hydrogen Bonding in Water

Ion-Dipole Forces:  The strongest of the intermolecular forces which occurs when an ionic compound is mixed with a polar compound (particularly one with hydrogen bonding). 

 

 

Summary of Intermolecular Forces

Type of Intermolecular Force	Present in…	Strength of this Type of Intermolecular Force
London Dispersion Forces	ALL molecules and atoms	Depends on the size and polarizability of the atom or molecule.  These forces can be VERY significant for larger molecules.
Dipole-Dipole Forces	polar molecules	Depends on the magnitude of the dipole moment (i.e. how large the partial positive and negative are).  These forces are generally stronger than dispersion forces.
Hydrogen Bonding	molecules with H-N, H-O, or H-F bonds	VERY strong intermolecular force.  It is essentially an extreme form of dipole-dipole force.
Ion-Dipole Forces	solutions where an ionic compound is dissolved in a polar protic solvent	The STRONGEST intermolecular force which can be more than twice as strong as hydrogen bonding.

 

 

 

Phase Changes and the Heating Curve

Heating Curve

What it tells us

Melting is changing from a solid to a liquid. Freezing is changing from a liquid to a solid. Boiling is changing from a liquid to a gas. Condensing is changing from a gas to a liquid. The melting point and the freezing point are both at the same temperature.  Likewise, the boiling point and condensation point are also at the same temperature.   Heat can either increase temperature or change the phase but not both at the same time.

 

Boiling and Melting: To accomplish these phase changes, the intermolecular forces of the molecules must be overcome by the kinetic energy of the particles.  The stronger the intermolecular forces, the more difficult it is to overcome those forces, and therefore the higher the temperature (i.e.-average kinetic energy) that is needed to bring about the phase change.  The energy added to the flat part of the heating curve above is being used to help the molecules break free from the intermolecular forces holding them together.