Introduction

Intermolecular forces are the interactions between molecules. Governed by the relationships between electronegativity, polarity, and molecular geometry, intermolecular forces result in many physical properties of compounds. These include:

• • Melting point

  • Freezing point

  • Boiling point

  • Evaporation

  • Surface tension

  • Form of matter: solid, liquid, gas

  • Viscosity (ability to flow)

Relative strength of attractive forces

The following forces are listed in order of strongest to weakest attraction between the molecules:

• • Network Covalent> Ion dipole>hydrogen bonding > dipole dipole > dispersion

Network covalent

• • Network covalent structures are a very strong intermolecular force. In these structures, attractive forces cause the molecules line up in a tight grid structure. This is seen in silicon or carbon structures because they are tetravalent (form 4 bonds). Examples include diamond (C), graphite (C), as well as Silicon Dioxide (SiO2).

Ion-dipole

• • Ion-dipole interactions occur between charged ions and polar molecules. Since polar molecules have areas that are slightly positively or negatively charged, these regions attract the surrounding ions. Ion-dipole interactions are stronger than hydrogen bonding and dipole-dipole interactions because the ion has a full charge as opposed to the weak charge of the dipole on a polar molecule, and given Coulomb’s law which relates magnitude of charge with the force of attraction, the attraction with the higher magnitude of charge will be stronger.

Hydrogen bonding

• • As a wise chemist once said, “Water is number one in hydrogen bonding.” Because of its low electronegativity, Hydrogen atoms assume a slightly positive charge in compounds that contain Fluorine, Oxygen, and Nitrogen, which all have high electronegativities. The hydrogen's slightly positive charge is attracted to the slightly negative charge on a Fluorine, Oxygen, or Nitrogen of another molecule. Remember, Hydrogen just wants to have FON.

  • Water is the perfect example of hydrogen bonding because the H atoms are attracted to the O atoms of another molecule. This results in water's high boiling point.

Dipole-dipole

• • Dipole-Dipole interactions are caused by differences in polarity between different points on a molecules, or the net dipole of the molecule. Similar to hydrogen bonding, slightly positive atoms on one molecule are attracted to slightly negative atoms on another, resulting in an attractive force between molecules. Remember that dipole-dipole attractions occur between two polar molecules.

  • Visit the bonding page to learn more about identifying polar and non-polar structures.

London dispersion force

• • London Dispersion forces (in the category of van Der Waals forces) occur between every molecule. The basis for this force is the creation of a temporary dipole within the electron clouds of the molecules. This temporary change in charge creates a weak attraction. As a result, molecules with more electrons will experience stronger LDFs as they are more polarizable (the increased electrons result in a stronger dipole).

Ideal gas law

• • The ideal gas law, pv=nRT, relates pressure, volume, number of moles of gas, R (the gas constant) and temperature (in Kelvin). This equation is useful to solve for any of the variables when the rest are present which occur in many different situations.

  • However, this equation is "ideal" meaning it does not take into account the IMFs between gas molecules. Generally, pressure and volume are lower than calculated as the attractions between molecules cause a slight reduction in the force exerted by molecules on the edge of the container. As a result, this equation is most accurate at high temperatures and low pressures when gas molecules are far enough apart and moving quickly enough to not be impacted by the IMFs from other molecules.

  • It is important to remember that temperature is always measured in Kelvin in this equation, and the pressure is the partial pressure of the gas in question, not necessarily the overall pressure.

Paper Chromatography

Paper chromatography is a process to separate or identify chemicals. The basis of the process is the use of a stationary phase, the paper, and a mobile phase, the solvent solution. The paper can be polar or nonpolar, and the solvent will usually have the opposite polarity. After placing a dot of ink or other solute on the bottom of the paper and dipping the end in the solvent, the solvent will begin to rise up the paper.

IMFs govern the chromatography process. The parts of the solute that travel the most are generally the most soluble and therefore attracted to the solvent, while the parts of the solute that travel the least are less soluble and are more attracted to the paper than the solvent

The Rf value is a qualitative measure of chromatography. To calculate the Rf value, use the equation (distance from origin to center of spot)/(distance from origin to solvent front), where the origin is where the samples start, and the solvent front is how far up the paper the solvent travels.