The document discusses intermolecular forces of attraction (IMFs), which are weak attractive forces between molecules that cause them to stick together without forming chemical bonds. It describes the main types of IMFs - London dispersion forces, dipole-dipole forces, ion-dipole forces, and hydrogen bonding. It explains that stronger IMFs cause molecules to cling more tightly together, making liquids harder to separate into gases through boiling and evaporation. Physical properties like boiling point, viscosity, and surface tension are determined by the strength of a substance's IMFs.

1. Intermolecular
Forces of
Attraction
Attractive forces that
cause atoms or
molecules to stick
together

2. Mixtures
Elements or
compounds blended
together but not
chemically combined

3. Mixtures
 Can be solid, liquid, or gaseous
mixtures
 Solid mixture examples:
Jewelry gold, most rocks, steel, alloys
 Liquid mixture examples
Beverages, sea water, solutions
 Gaseous mixture examples
Air, car exhaust

4. Mixtures
 Why do they form? What allows
the parts to stay mixed? Why do
they stay together?
 The components of a mixture are
held together by IMF’s –
intermolecular forces of attraction

5. Intermolecular
Forces
All intermolecular forces are:
• attractive forces
• between molecules
• do not make new compounds
• Makes the molecules “sticky”
• weaker than true “bonds”

6. Types of IMF’s
 London (dispersion) forces
 all molecules
 weakest interaction
 dipole-dipole forces
 polar molecules
 ion-dipole forces
 Between polar molecules and ions
 hydrogen bonding
 H atoms w/ O, N, F not covalently bonded to it
 strongest interaction

7. Polar forces
• “Dipole - Dipole” interactions
• positive ends attract negative
ends of other molecules
• about 1% as strong as a
covalent bond

8. Dipole - Dipole
interactions

9. London forces
• Momentary dipole-dipole
interaction
• “instantaneous” dipole
“induces” a dipole in a
neighboring molecule
• brief attractive force results

10. Step 1

11. Step 2

12. Result...

13. London forces
• Strength depends on:
• size of electron cloud
– bigger = stronger
• number of atoms in molecule
– more atoms = more electrons =
stronger

14. Ion – dipole
forces
• Between polar molecules and ions
• Aqueous solutions of ions
• Positive ends of polar molecules
attract negative ions
• Negative ends attract positive ions
• Only important in aqueous
solutions

16. Hydrogen Bonds
• Special case of polar attraction
• H atom is
• 1)bonded to high EN atom and
• 2)attracted to a high EN atom it
is not bonded to (N, O, F)
• 5 times stronger than regular
polar attractions

17. Hydrogen Bonds

18. Physical property effects
of I nter m olecular F orces
of A ttraction
…all come down to how sticky
the molecules are toward
each other…

19. The Key Idea…
The stronger the
IMF’s, the tighter the
molecules cling to
each other, the harder
it is to separate them.

20. The Key Idea…
In other words, the
harder it is to separate
molecules, the
stronger their IMF’s

21. What does that
mean…”separate the
molecules”?
Pull them apart:
Melting
Boiling
Physically move them apart
Peel them away. Wipe them
off, etc…

22. Boiling Point
Effects
Formula Boiling Polar? IM force
point
HCl -85oC Polar Dipole-
dipole
H 2S -60.7oC Polar Dipole-
dipole
H 2O 100oC Polar H-bonding
Ar -185.7oC Nonpolar london

23. Boiling Point
Effects
Formula Boiling Polar? IM force
Point
F2 -188.1oC Nonpolar London
Cl2 -34.6oC Nonpolar London
Br2 58.8oC Nonpolar London
I2 184.4oC Nonpolar London

24. Boiling Point
Effects
Formula Boiling point Polar? IM force
CH4 -161.5oC Nonpolar London
C 2H6 -88.6oC Nonpolar London
C 3 H8 -42.1oC Nonpolar London
C4H10 -0.5oC Nonpolar London
C5H12 36.1oC Nonpolar london

25. Intermolecular
Forces of
Attraction
Attractive forces that
cause atoms or
molecules to stick
together

26. Types of IMF’s
 London (dispersion) forces
 all molecules
 weakest interaction
 dipole-dipole forces
 polar molecules
 ion-dipole forces
 Between polar molecules and ions
 hydrogen bonding
 H atoms w/ O, N, F not covalently bonded to it
 strongest interaction

27. The Key Idea…
The stronger the
IMF’s, the tighter the
molecules cling to
each other, the harder
it is to separate them.

28. Solubility of a solid in
liquid
General rule: Like dissolves Like
• Polar solutes dissolve in polar solvents
• Sugar (polar solute) in H2O (polar solvent)
• Ionic solids dissolve in polar solvents
• NaCl (ionic solid) in H2O (polar solvent)
• Nonpolar solutes dissolve in nonpolar
solvents
• Dried paint (nonpolar solute) in paint stripper –
turpentine, etc. (nonpolar solvent)

29. Colligative properties of
Solutions
-Properties of solutions that are affected by
the number of dissolved particles, not the
identity of the particles
Example: vapor pressure lowering
The more particles (with little tendency to
vaporize) that are added to a solvent, the
lower the vapor pressure of the solution
becomes

30. Colligative properties of
Solutions
Density of a solution
The presence of dissolved particles (whose mass
is greater than the mass of the solvent particles) in a
solution causes the density of a solution to
increase
• Sugar water is more dense than pure water
• Sugary sodas are more dense than diet
sodas
• it takes more sugar molecules than
artificial sweetener molecules

31. Swimming in the Dead
Sea

32. Colligative properties of
Solutions
Boiling point elevation :
The presence of dissolved particles
tends to increase the boiling point of a
liquid
Salt added to water before boiling to
prepare food increases the
temperature the water boils at, and
thus cooks the food faster, as the

33. Colligative properties of
Solutions
Freezing point depression
The presence of dissolved particles tends
to lower the freezing point of a liquid
Salt or other substances are used to melt
ice, because as they dissolve into the
ice, they lower the freezing point of the
ice, and thus even more of the ice melts

34. Compressibility
• gases are compressible
– empty space between
molecules
– Weak IMF’s won’t keep
molecules together
• liquids, solids aren’t
– closely packed

35. Diffusion
• The intermingling of molecules
with each other
• gases = rapid  much empty
space
• liquids = slowly  more closely
packed
• solids = negligible  locked in
place

36. Cohesion/Adhesion
 Cohesion = molecules sticking
together
 Adhesion = molecules sticking
to a substrate

37. Viscosity
• The resistance of a fluid
to flow.
• The stronger the IMF’s,
the higher the viscosity

38. Surface Tension
• Intermolecular attractions at the
surface are only inward towards the
substance

39. Surface Tension ,
continued
• Molecules at the surface of a
liquid act as a “skin”
• the greater the IMF’s, the more
pronounced the effect
• liquids seek to lower their surface
tension
• water drops are spherical

40. Surface Tension ,
continued

41. Surface Wetting
• The spreading of a liquid
across a surface
• must have adhesion ≈
cohesion

42. Gasoline – a nonpolar
hydrocarbon
• Low surface tension
– only London forces
• low cohesion
– spreads easily across most
surfaces
• does not effectively “wet” glass

43. H2O – a polar molecule that H-
bonds
• High surface tension
– hydrogen bonding
• High cohesive forces
• High adhesive forces
– will H-bond with O’s in glass
• Good wetting agent for glass

44. • Water beads on greasy glass
– eliminates H-bonding to glass
– low adhesion

45.  Surfactants , used in
soaps and detergents,
drastically lower the surface
tension of water
– soapy water spreads more
easily over dirty surfaces

46. Vaporization
• The change of a liquid → gas is
called vaporization
• an example of separating molecules
• Evaporation: happens at the
exposed surface of a liquid
• Boiling: happens within the liquid
itself

47. Evaporation
• Liquids exert vapor pressure due
to the evaporated liquid
• The stronger the IMF’s, the
slower the evaporation rate, the
lower the vapor pressure for a
given temperature

48. Evaporation
• The higher the T of a substance,
the more molecules with enough
energy to overcome cohesive
forces/surface tension and
evaporate
• As liquid T ↑, vapor P ↑

49. Temperature is ~ the
average KE of a sample
coolest warmest
A
b
u
n
d
a
n
c
e
Energy Minimum E to evaporate

50. What happens...
• To the evaporation rate
– if the temperature = the boiling
point ?
• All the molecules, on average,
have enough energy to
vaporize
– the liquid boils

51. What happens...
• If the vapor pressure equals
the atmospheric pressure ?
• All the molecules, on
average, have enough
energy to vaporize
– the liquid boils

52. T vs Vapor
Pressure
V
a 1.00 atm
p
P
r
e
s
s
u
r
e
Temperature 100oC

53. How can you boil a
liquid?
1) Heat it to its boiling point
vapor P = air P
1) Reduce the air P above it
to equal the vapor P of the
liquid at that temperature
• air P = vapor P
 all done!

54. Intermolecular
Forces of
Attraction
Forces of attraction
between molecules