TOPIC: Intermolecular Forces
How do particle diagrams of liquids & solids compare to those
of gases?
Describe relative positions and motions of particles in each of 3 phases
SOLID LIQUID GAS
Why do some substances exist as gases, some as liquids, and some as solids at room temp?
Part of answer has to do with forces between separate molecules (called intermolecular forces)
Intermolecular forces between molecules. They are weaker. Intramolecular forces are between individual atoms (we will learn this later)
Intramolecular forces
Intermolecular forces
Intermolecular Forces-IMF
Inter means “between” or “among”Intermolecular forces = forces
between neighbouring molecules
All molecules have Dispersion forces (the regents calls these Van der Waals)
2 other types of forces (IMF): 1. Dipole-Dipole forces2. Hydrogen bonds
-if one of these are present, they are more important.
Most atoms don’t have a charge, unless they are ions, so we often refer to them as having partial charges and write them like this…
This separation of Charge is responsible
For the forcesBetween themolecules
1. Dispersion Forces (van der waals)1. Dispersion Forces (van der waals):
● weakest IMF ● occur between nonpolar molecules
●Click here for animation (slide 4 of 13)
• NonNonpolar means nono poles (+/-)
• Can’t tell one end of molecule from other end
• electrons are evenly distributed
•instantaneous and momentary•fluctuate•results from motion of electrons
if charge cloud not symmetrical will induceinduce asymmetry in neighbor’s charge cloud!
4 categories of Nonpolar Molecules(you need to memorize)
Noble Gas molecules: He, Ne, Ar, Kr, Xe, Rn
diatomics if both atoms are same: (7) H2, N2, O2, Cl2, F2, I2, Br2
Pure Hydrocarbons (CxHy): CH4, C2H6, C3H8
small symmetrical molecule CO2, CF4, CCl4
Dispersion Forces and Size
Dispersion forces ↑ with molecule size larger the electron cloud, the greater the
fluctuations in charge can be Rn > Xe > Kr > Ar > Ne > He I2 > Br2 > Cl2 > F2
C8H18 > C5H12 > C3H8 > CH4
2. 2. Dipole-dipole forcesDipole-dipole forces: • intermediate IMF • occur between polar molecules (they have a
partial charge at each pole – one is typically much larger than the other)
• Click here for animation (slide 3 of 13)
Dipole-dipole Forces & Polar Molecules
Molecule shows permanent separation of charge; has poles: one end partly (-) & one end partly (+)
Polar means molecule has poles: (+) & (-)
geometry and electron distribution are not symmetrical
Polar Molecules
3. 3. Hydrogen bondsHydrogen bonds:
• strongest IMF • occur between molecules that have: H-F H-O or H-N bonds ONLY
Hydrogen Bonding
H-O N-H
Occurs between molecules with H-F, H-O, or H-N bonds
Hydrogen Bonding
Hydrogen bonding is extreme case of dipole-dipole bondingF, O, and N are all small and electronegative
strong electrons attraction H has only 1 electron, so if being pulled away H
proton is almost “naked” H end is always positive & F, O, or N end is always negative
Strength of Hydrogen BondingFluorine most electronegative element, so
H-F bonds are most polar and exhibit strongest hydrogen bonding
H-F > H-O > H-N
(H-bonding…sound like FON to me!!!)
Hydrogen bonding: • strongest IMF• influences physical props a great deal
H-F > H-O > H-N
Strongest Intermolecular
Force
Hydrogen Bonding
Dipole-Dipole
Dispersion
Indicate type of IMF for each molecule:
NH3
ArN2
HClHFNeO2
HBrCH3NH2
• Hydrogen bonding• Dispersion forces• Dispersion forces• Dipole-dipole forces• Hydrogen bonding• Dispersion• Dispersion• Dipole-dipole• Hydrogen bonding
OH
H
OH
H
H-Bonding = strongest IMFmuch harder to “pull” molecules apart
C
Dispersion Forces= weakest IMFmuch easier to “pull” molecules apart
C H
H
HH
H
H
HH
DEMO…
GycerolWaterEthanolAcetone
IMF vs.
Physical PropertiesIf IMF then:
Boiling point Melting point Heat of Fusion Heat of Vaporization Surface tension Viscosity
while: Evaporation Rate
Change from solid to liquid w/o changing temp
Change from liquid to gas w/o changing temp
Rate at which conc. will go from liquid to gas
If IMF are strong, substance will be solid or liquid at room tempParticles want to clump together
If IMF are weak, substance will be gas at
room tempParticles free to spread apart
Why do some substances exist as gases, some as liquids, and some as solids at room temp?
#1 reason = IMF
#2 reason = temperature (avg. KE)
IMF vs. Temp – it’s a balancing act
IMF is weak = gas (YOU expect this)
IMF is strong = solid/liquid (YOU expect this)
IMF weak but Temp is low = molecules will move slower, having more opportunity to interact, may be a solid/liquid
IMF is strong but Temp is high = molecules will move faster so ___________
Determining phaseDetermining phase“Competition” between strength of IMF &
average KE determines phase
Boiling point of N2 is 77 K (-196˚C)IMF are very weak dispersion forces
REMEMBER…
Temp = average KEIf we change T we change KEIncrease KE will help “pull” molecules
apart (overcome IMF)