Interactive Forces

Interactive Forces

Interactive Forces

Intermolecular Forces

Matter commonly exists in 3 states. The state of matter is influenced by the physical properties of a substance.

For liquids & solids, the condensed states, many of the physical properties are the result of the attraction between the molecules in the substance.

Most solids are ionic substances.What holds them together?

Most liquids are covalent molecules.

What is the primary reason for the difference between gases & the condensed states?

The state of matter of a substance depends on the relationship between

1) the KE (keeps the molecules apart) &

2) attraction between the particles (draws molecules together):Think Coulomb’s Law

The stronger of the 2 “wins”.

In gases the molecules are moving fast enough (high KE) that the attraction between molecules can’t hold the molecules together. BUT if the attractive forces are significant, it can lead to deviations from the ideal gas law at low T & high P, which can lead to condensation in these situations.

For the condensed states, the average KE is less than the attraction between molecules so they are held together.

A change in state of matter can be done in two ways

1) increase KE (T)

2) increase P which brings molecules closer together & attractive forces are greater when in closer proximity

Trends exist in the relative strength of attractive forces, but the structure and size of parties involved play a very important role in determining the overall strength of a particular inter or intra molecular attraction.

Intermolecular forces (IMF) or van der Waals forces

IMF:Attractive forces between molecules that causes the molecules to “come together”. These only work when molecules are in close proximity. As the distance increases, the force between molecules is weaker, as explained by Coulomb’s Law.

IMF are much weaker than bond energies (force of the bond between atoms)

Eg. HCl H H-Cl = 431 kJ Hvap = 75 kJ

London dispersion forces:attractive forces between ALL atoms & molecules

The movement of electrons around the atoms causes an instantaneous dipole moment. That is, e- are concentrated on one side of the atom AND, due to Coulombic forces & close proximity to another atom, it forces e neighboring atom into an instantaneous dipole. Because of repulsion, e- move to the opposite side of the neighboring molecule.

Analogy: toddlers running around

Polarizability: how easily an atom can be forced into an instantaneous dipole moment

Larger molecules have a higher degree of polarizability because of their high # of electrons. Thus they have larger LDF.

  • Explains how nonpolar substances, with seemingly no attractive forces can stick together & form liquids & solids.
  • Often the strongest force between LARGE molecules (like biological molecules)
  • Stronger in long chains of molecules with double or triple bonds (these have pi bonds) because it increases the amount of surface area.

Dipole-dipole:-only between polar molecules

  • Attractive force due to Coulombic interaction between dipoles in a polar substance.
  • Maximize attractions & minimize repulsions
  • In 2 molecules of similar size & mass, the greater the polarity of the molecule the stronger the dipole-dipole force.

H bonding Ad wes 56 JPGHydrogen bonding: a special type of dipole-dipole attraction; H bonded w/ atoms w/ high electronegativity, such as N, O, F

Explains the properties of water.

In hydrogen bonding the shared e- basically “belongs” to the more electronegative atom. So H is just a proton hanging out on the molecule. Because there are no electrons orbiting the H interfering with the dipole it is attracted to, the H can get VERY close to the dipole & since they are closer the attractive Coulombic force is stronger.

There is a very strong dipole compared to the size of the molecule.

Ion – dipole: between ionic & polar substance;

requires a salt & polar covalent liquid

Attractive force between an ion & the opposite dipole in a polar molecule

Eg. NaCl in H2O

Eg. Identify the attractive forces holding the molecules together in the list below.

Ne BaCl2CuBr in CH3OH



Order of relative strength of Intermolecular forces (all things being equal):

Melting & boiling points

Varying strengths of attraction explains differences in boiling and melting points of substances.

If a substance has a larger IMF then the attraction between molecules is ______due to stronger forces. Thus

______energy is required to break the attraction due to Coulombic forces.

Similar to magnets. Magnets that are stronger have a higher attractive force and require faster motion to keep them separated (higher T = Higher KE)

What are in the bubbles of a liquid that is boiling?

BLB table 112 amp 3 BP of substances jpg

Viscosity & surface tension

Viscosity: resistance to flow

Molasses is very viscousdue to a large IMF

Surface tension: E required to increase the surface area of a liquid (ie. To overcome the “inward” forces & spread out.)

Surface molecules experience a net inward force, so they bead up or pack close together.

Vapor Pressure

Vapor pressure: pressure a vapor exerts on its liquid at the equilibrium point between evaporation & condensation.

As liquid molecules escape (evaporate) the # of molecules that condense starts to increase because since more are evaporating (going into the gas state) there is a greater likelihood that a gas will hit the surface of the liquid & re-enter the liquid state. When the rate of condensation = evaporation (dynamic equilibrium) the pressure the gas exerts on the liquid is the equilibrium vapour pressure.

Relationship between vapor pressure & temperature :

Volatile: a substance that evaporates quickly, this is because it has low/high IMF, and thereforelow/high VP

Nonvolatile: does not evaporate quickly due to low/high IMF, which results in a low/high VP because fewer/more molecules escaped to the gas state, few/more had enough energy to overcome the IMF & escape into gas state

Larger molecules tend to be less volatile due to ______LDF.

From some BLB book BLB figure 1112 IMF flow chart jpg