Download Intermolecular Forces

Intermolecular Forces
December 5, 2016
Clarification
• IntRAmolecular Forces: bonds that hold individual molecules
together
• These occur inside a molecule
• Covalent or ionic bonds
• IntERmolecular Forces of Attraction: these are forces, NOT
BONDS, that attract many molecules to each other
• These occur between molecules
What are Intermolecular Forces?
• Three types:
• Dispersion Forces (London)
• Dipole-Dipole Forces
• Hydrogen Bonding
• These are attractions BETWEEN molecules
• They are weaker than either ionic or covalent bonds
• The are responsible for determining whether a molecular compound
is a gas, liquid, or a solid at a given temperature
• Reminder: THEY ARE NOT BONDS, THEY ARE FORCES!!!
Dipole-Dipole Forces
• Occurs when POLAR molecules are attracted to one
another
• The electrical attraction occurs between the oppositely
charged DIPOLES of polar molecules
• The slightly negative region of a polar molecule is
weakly attracted to the slightly positive region of
another polar molecule
• Similar to, but much weaker than, ionic bonds
Dispersion Forces
• Also known as London Dispersion Forces
• These are the weakest of all intermolecular forces
• They occur between all molecules and all substances
• Caused by electrons moving more to one side of a
molecule which is closest to a neighboring molecule.
The neighboring molecule’s electrons are momentarily
repelled away, causing a momentary attraction
between the two molecules due to a temporary dipole
• Dispersion forces are stronger as the molecule gets
larger!
• Predict which will have greater dispersion force:
Methane (CH4) or Ethane (C2H6)?
Hydrogen Bonding
• Can be in the same molecule or a nearby
molecule (eg. DNA vs water).
• Always involves hydrogen and nitrogen, oxygen,
and fluorine (or other halogens)
(I have had e-NOF of this)
• Water is a very good example
• An attractive force between a hydrogen
(covalently bonded to a very electronegative
atom) and another electronegative element with
an unshared pair of electrons
Strength of Forces
• The strength of intermolecular forces varies
depending upon the type of force:
• London Dispersion
• Dipole-Dipole
• Hydrogen Bonding
Weakest
Strongest
Intermolecular Forces Animation
Take a look at the animations at the following site:
https://www.wisc-online.com/learn/naturalscience/chemistry/gch6804/intermolecular-forces
What do you need to do?
For each of the 20 molecules on the front of your long worksheet. Decide if the molecules have hydrogen bonding,
dipole-dipole forces, and/or dispersion forces.
Complete the pre-lab for intermolecular forces. This can be found under the files!