two things affect inter molecular forces: › charge › distance bigger distance equals less...

Two things affect intermolecular forces:› Charge› Distance

Bigger distance equals LESS attraction! More charge equals MORE attraction! Coulomb’s Law

› F ~ charge/distance2

› Distance has more effect since it is squared.

Bonds are permanent attractions, IMF’s are temporary

Covalent› Sharing of electrons in permanent tug of

war. Ionic

› Stealing electrons and then being attracted by opposite charges

Ion Dipole Hydrogen Bonding Dipole-Dipole London Dispersion forces

Between an ion and a dipole!› Ion = permanently charged particle› Dipole = a polar molecule

Almost exclusively seen in aqueous solutions.

Between two dipoles!› Dipoles are polar molecules

that have no net charge, but the charge is distributed unequally.

› Positive end of one attracts negative end of the other.

ALL covalent compounds experience this force.› It is between the electrons in one atom

and the nucleus of another atom.› More mass and/or more atoms means

more dispersion forces.

› More mass doesn’t cause an increase in LDF!

› More mass infers that there are more P and N, which MEANS there are more electrons.

› It is an increase in the number of e- which causes a larger e- density which leads to a stronger temporary dipole.

›  You can use molar mass to figure it out since a larger molar mass implies a bigger density but you can't use the words molar mass in your justification in a Free Response Question

Occurs in molecules in which hydrogen is having FON!› The most electronegative atoms are F, O

and N. With a hydrogen, there is excessive polarization.

› This excessive polarization attracts the lone pair of electrons.

Strongest: Ion-Dipole (very strong)

Hydrogen Bonding (strong)Dipole-Dipole

Dispersion Forces (very weak)

More IMF’s or stronger IMF’s affect the following properties:› Boiling point› Melting Point› Viscosity (thickness)› Vapor Pressure› Surface tension

Stronger IMF› Higher boiling point

Weaker IMF› Lower boiling point

Compare CS2 to CO2

Both are linearBoth are nonpolar covalentBoth have LDF

CS2 has stronger LDF – larger molecule means larger e- density

Stronger IMF› Higher melting point

Weaker IMF› Lower melting point

Think of red rover… Compare HBr to Cl2

Both are liner HBr has LD, Dipole Cl2 has LD, nonpolar

HBr has stronger IMF – higher melting point

Stronger IMF› Higher viscosity

Weaker IMF› Lower viscosity

More attached to each other, the hard to flow, like solids vs. liquids.

Compare CH3OCH3 to CH3CH2OH

CH3OCH3 LD, Dipole

CH3CH2OH LD, HB

Therefore CH3CH2OH has stronger IMF and a higher viscosity

Stronger IMF› Lower vapor pressure

Weaker IMF› Higher vapor pressure

Again…think red rover.

Compare H2S to H2O2

H2S LD, Dipole

H2O2 LD, HB

H2O2 has stronger IMF therefore lower vapor pressure

Resistance of a liquid to increase in its surface area

Stronger IMF› Higher surface tension

Weaker IMF› Lower surface tension

Compare H2CO to H2O

H2CO LD, Dipole

H2O LD, HB

Hydrogen bonding is a stronger IMF than Dipole, therefore H2O has a stronger surface tension

Stronger IMF› Higher boiling point› Higher melting point› Higher viscosity› Higher surface tension› Lower vapor pressure

Weaker IMF› Lower boiling point› Lower melting point› Lower viscosity› Lower surface tension› Higher vapor pressure