Intermolecular Forces

Intermolecular forces vs Intramolecular Forces

intramolecular = within molecules = covalent / ionic bonds

intermolecular (IMF’s) = between molecules

Intermolecular forces are far stronger than Intramolecular Forces

IMF’s are up to 5% as strong

Electronegativity

top right is most electronegative (F)

bottom left is least electronegative

Lewis Dot Structure Steps

1. count valence electrons

2. Determine the central atom (least electronegative)

3. Draw single bonds from the center atom to each outer atom

4. Complete outer atom octets by adding pairs of electrons to outer atoms

5. place any leftover electrons on the central atom

6. form double or triple bonds if needed to satisfy all octets.

Ionic

contains a metal

Covalent

does not contain a metal

Polar Molecules

asymmetrical

Non-polar Molecules

symmetrical

Determining polarity questions

1. Is the molecule ionic? - if yes, polarity doesn’t matter

2. Is the molecule aqueous and does it start with H? - if yes, then its an ion

3. Is the molecule made up solely of carbon and hydrogen? - if yes, then it is non-polar

4. Are there 2 or more different elements around the central atom? - if yes, then it is polar

5. Are there any lone pairs on the central atom? - if yes, then it is polar

dipole

polar

induced dipole

non-polar

Type’s of IMF’s

dipole-dipole

hydrogen “bonding”

ion-dipole

ion-induced dipole

dipole - induced dipole

London dispersion forces

Dipole-dipole

attractive forces between polar molecules

Hydrogen “bonding”

special case of dipole-dipole

A hydrogen atom covalently bonded to N, O, or F is attached to N, O, or F on another molecule

Ion-dipole force

attractive forces between ions and polar molecules

Ion-induced dipole

attraction between an ion and a nonpolar molecule

Dipole-induced dipole

attraction between a polar molecule and a nonpolar molecule

London dispersion forces

attraction between nonpolar molecules

1. position of the electrons causes temporary polarity

2. induces polarity in the next molecule

3. Higher LDF with

   1. larger molecules

   2. more electrons

   3. more polarizable

IMF forces strength ranking

Ion-dipole forces

Hydrogen “bonding”

Dipole-dipole

Ion-induced dipole

Dipole-induced dipole

London dispersion forces

Boiling point

stronger forces = higher boiling point

Melting / freezing point

stronger forces = higher melting point

Viscosity

viscosity = resistance to flow, higher viscosity = less flowing

viscosity decreases at higher temperatures

stronger IMFs = higher viscosity

Surface Tension

molecules at the surface of a liquid pull tighter because they have fewer neighbors

stronger IMFs = higher surface tension

Vapor Pressure

The pressure above a liquid in a closed container when the rate of evaporation = the rate of condensation (equilibrium)

at higher temperatures, you get more vapor, so higher vapor pressure

higher IMFs = lower vapor pressure

Phase Diagrams

Heat Diagrams

Physical Properties of Solutions

Solution

Homogeneous mixture of 2 or more substances

Solute

substance(s) in smaller amounts

Solvent

substance present in smaller amounts

Saturated Solution

contains the maximum amount of solute that will dissolve in a given solvent at a given t

Unsaturated Solution

contains less than the maximum amount of solute at a given T

Supersaturated Solution

contains more solute than a saturated solution at a given T

∆H_soln

amount of heat required to make a solution

amount of heat generated when a solute dissolves

Lattice Energy

Must always be positive

∆H_hydr

heat of hyrdation

∆H_soln equation

∆H_soln = lattice energy + ∆H_hydr

Like Dissolves Like