Intermolecular Forces

Forces that hold solids and liquids together may be ionic or covalent bonding or they may involve a weaker interaction called intermolecular forces. All of these forces are van der Waals forces

Intermolecular Forces Generally, the strengths of intermolecular forces are much weaker than ionic or covalent bonds The stronger the attractive force, the higher the boiling or melting points.

Types of Intermolecular Attractive Forces Ion – Dipole Forces Dipole – Dipole Forces Hydrogen Bonding London Dispersion Forces

Ion-Dipole Forces Ion-dipole forces – exist between an ion and the partial charge on the end of a polar molecule

Dipole – Dipole Forces Dipole-dipole forces – exist between neutral polar molecules, when dipoles are close together these are weaker than ion-dipole forces The molecules orient themselves to maximize the positive/negative interactions and to minimize the + + and - - interactions. These forces are typically only about 1% as strong as covalent or ionic bonds. These forces rapidly become weaker as the distance between the dipoles increases.

London Dispersion Forces London Dispersion forces – exist primarily between non-polar atoms or molecules, (including noble gases) Sometimes called induced dipole-induced dipole attraction. These forces exist between all molecules to some degree.

London Dispersion Forces The constant motion of an electron in an atom or molecule can create an instantaneous dipole moment by affecting the electron distribution of a neighboring atom This inter-atomic attraction is relatively weak and short lived. This is the weakest intermolecular force. The strength of these forces increases with increasing molecular mass

London Dispersion Forces London forces are the attractive forces that cause non-polar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently. Dispersion forces are present between any two molecules (even polar molecules) when they are almost touching (this means they are found in all substances).

London Dispersion Forces Dispersion forces are present between all molecules, whether they are polar or nonpolar. Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones (outer electrons are shielded from nucleus positive charge allowing more interactions). In a larger atom or molecule, the valence electrons are, on average, farther from the nuclei than in a smaller atom or molecule. They are less tightly held and can more easily form temporary dipoles. The ease with which the electron distribution around an atom or molecule can be distorted is called the polarizability.

London Dispersion Forces London dispersion forces tend to be: stronger between molecules that are easily polarized. weaker between molecules that are not easily polarized.

Hydrogen Bonding Hydrogen bonding – is a special type of intermolecular attraction that exists between the hydrogen atom in a polar bond (particularly an H-F, H-O or H-N bond) and an unshared electron pair on a nearby small electronegative ion or atom (usually an F, O, or N atom on another molecule). This is a specific type of dipole-dipole force

Hydrogen Bonding Two factors account for the strengths of these interactions: 1.large polarity of the bond 2.close approach of the dipoles (allowed by the very small size of the hydrogen atom)

Hydrogen Bonding Each attraction is electrostatic in nature, (involving attractions between positive and negative species) See Brown and LeMay page 403 for a flow diagram for intermolecular forces.

Polarizability Polarizability – the ease with which the charge distribution in a molecule can be distorted by an external electric field. (see B&L pg. 397) More polarizable molecules have stronger London Dispersion forces Strength increases with increasing size occurs between all polar and non-polar molecules

Properties of Liquids viscosity – the resistance of a liquid to flow The greater a liquid’s viscosity, the more slowly it flows. Viscosity decreases with increasing temperature. At higher temperatures, the greater average kinetic energy of the molecules more easily overcomes the attractive forces between molecules.

Surface Tension Surface tension – the energy required to increase the surface area of a liquid by a unit amount. Surface tension is due to an increase in the attractive forces between molecules at the surface of a liquid compared to the forces between molecules in the center, or bulk, of the liquid. This property causes fluids to minimize their surface areas. see Brown and LeMay page 404

Surface Tension When a liquid is poured onto a solid surface, it tends to bead as droplets, which is a phenomenon that depends on the intermolecular forces. Although molecules in the interior of the liquid are completely surrounded by other molecules, those at the surface are subject to attractions only from the side and from below. The effect of this uneven pull on the surface molecules tends to draw them into the body of the liquid and causes a droplet of liquid to assume the shape that has a minimum surface area (a sphere).

Phase Changes The melting process for a solid can be referred to as fusion. A heating curve is a plot of the temperature versus the amount of heat added. A cooling curve is a plot of the temperature versus the amount of heat removed. Critical temperature is the highest temperature at which a substance can exist as a liquid. The critical pressure is the pressure required to bring about liquefaction at this critical temperature.

Ch 11 Problems 5, 7-11, 13, 19, 25, 27, 33, 34, 37, 40, 47, 48, 52-54, 56, 57, 62, 65