IMF Theory

Big Picture

Atoms can be considered small spheres that combine to form molecules using INTRAMOLECULAR (intra = within) bonds (Ionic or covalent).

Evaporation Rates

Click HERE for complete data set.

IMF Summary:

In the chart above, water would be classified as Hydrogen bonding. A relatively strong force acting between the Hydrogen of one molecule and the Oxygen of a neighboring molecule.

What is the first step in determining the types of intermolecular forces present?
Why is it important to identify if a molecule contains hydrogen bonded to oxygen, nitrogen, or fluorine?
What types of intermolecular forces are always present, regardless of polarity?
If a molecule is nonpolar, what types of intermolecular forces are present?
If a molecule is polar, what types of intermolecular forces are present?
How does the strength of intermolecular forces affect the physical properties of a substance?

However, if water interacts with an ION (Na+ , Cl- , K+ ) the attractive force is even stronger. As it is an Ion-Dipole force, these are more difficult to overcome.

IMF Flow Chart Practice

HS-PS1-3 Intermolecular Forces Activity

CK-12 Physical Properties and Intermolecular Forces

Intermolecular forces are the forces that hold molecules, ions, and atoms together. The intermolecular forces affect the substance's physical properties, such as its viscosity (resistance to flow), vapor pressure (pressure exerted by vapor), boiling/melting point and surface tension (tension/toughness of surface film).

Move the red point to see how the strength of intermolecular forces in a compound affect its physical properties.
Notice how the type of bonding present in a molecule relates to the amount of intermolecular forces it experiences.

Dispersion Forces - The Weakest

The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.

From the Gizmo Interactive: Polarity and IMFs

Electrons move within an atom to produce temporary dipoles.

London dispersion forces (video) | Khan AcademyLondon dispersion forces result from the coulombic interactions between instantaneous dipoles. Dispersion forces are present between all molecules (and atoms) and are typically greater for heavier, more polarizable molecules and molecules with larger surface areas.

Covalent: Dipole-Dipole - Slightly above Average

Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. These molecules are permanent dipoles.

Covalent: Hydrogen Bonding - The Strongest Type of Dipole-Dipole

Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom.

Temperature and Molecular Motion

As the temperature increases even more, the individual particles will have so much energy that the intermolecular forces are overcome.

As the temperature of substance increases the average distance between molecules increases as shown in the two gifs.

At the same time, due to the increased distance between molecule the attractive forces between molecules DECREASE.

Large Molecules and IMFs

The Diagram to the right shows how oil molecules are attracted to each other. The longer molecules the stronger the attraction between the two molecules. Likewise as the distance between the molecules increases the attractive force decreases (as shown above as well).

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