1. Intermolecular Forces
Irresistible attraction…

2. DO NOW: Rank the following phases of matter in order from least to greatest for… A) Energy B) Intermolecular Forces
High Energy
Medium Energy
Low Energy
Weak
Intermolecular
Forces
Medium
Intermolecular
Forces
Strong
Intermolecular
Forces
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 441

3. ATTRACTIVE FORCES
Always electrostatic in nature
Intramolecular forces
bonding forces
These forces exist within each molecule.
They influence the chemical properties of the substance.
Intermolecular forces
nonbonding forces
These forces exist between molecules.
They influence the physical properties of the substance.

4. Why Are Molecules Attracted to Each Other?
Intermolecular attractions are due to attractive forces between opposite charges
+ ion to − ion
+ end of polar molecule to − end of polar molecule
H-bonding especially strong
even nonpolar molecules will have temporary charges
generally over much larger distances

5. Why Are Molecules Attracted to Each Other?
Larger charge = stronger attraction
Longer distance = weaker attraction
However, these attractive forces are small relative to the bonding forces between atoms
generally smaller charges
generally over much larger distances

6. Polarity
Draw the dipole diagram for each of the following. Then, predict whether the compounds are polar or nonpolar. HI N2

7. Intermolecular forces (IMF)
London dispersion forces: The forces that exist among noble gas atoms and non-polar molecules
Dipole-dipole attraction: Molecules with dipole moments (polar molecules) can attract each other by lining up so that the positive and negative ends are close to each other.
Hydrogen bonding: when hydrogen is bonded to a highly electronegative -FON
Increasing Strength of IMF

8. Dispersion Force

9. Dispersion forces among nonpolar molecules.
separated Cl2 molecules
instantaneous dipoles

10. Increase in MM (molar mass) = Increase in London Dispersion Forces

11. Dipole-Dipole Bonds
Dipole-Dipole Attraction: the attractive force resulting when polar molecules line up such that the positive and negative ends are close to each other
Usually weak bonds (1% as strong as covalent or ionic bonds)
Become weaker as the distance between the dipoles increases
Strong forces occur between molecules in which hydrogen is bound to highly electronegative atoms (ex: nitrogen, oxygen, or fluorine)
Two Factors Account for Strength:
The great polarity of the bond
The small size of hydrogen allowing the dipole moments to be close together

12. Dipole–Dipole Attractions
Polar molecules have a permanent dipole
because of bond polarity and shape
dipole moment
as well as the always present induced dipole
The permanent dipole adds to the attractive forces between the molecules
raising the boiling and melting points relative to nonpolar molecules of similar size and shape

13. Polar molecules and dipole-dipole forces.
solid
liquid

14. Hydrogen Bonding
When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it
O─H, N─H, or F─H
Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes deshielded
exposing the H proton
The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules

15. H-Bonding HF

16. Drawing Hydrogen Bonds Between Molecules of a Substance
PROBLEM:
Which of the following substances exhibits H bonding? For those that do, draw two molecules of the substance with the H bonds between them.
(c)
(a)
(b)
Find molecules in which H is bonded to N, O or F. Draw H bonds in the format -B: H-A-.

17. (a) C2H6 has no H bonding sites.
SOLUTION:
(b)
(c)

18. Predicting the Type and Relative Strength of Intermolecular Forces
PROBLEM:
For each of the following pairs of substances, identify the dominant intermolecular forces in each substance, and select the substance with the higher boiling point.
Use the formula, structure
Bonding forces are stronger than nonbonding (intermolecular) forces.
Hydrogen bonding is a strong type of dipole-dipole force.
Dispersion forces are decisive when the difference is molar mass or molecular shape.

19. (a) MgCl2 or PCl3 SOLUTION:
(a) Mg2+ and Cl- are held together by ionic bonds while PCl3 is covalently bonded and the molecules are held together by dipole-dipole interactions. Ionic bonds are stronger than dipole interactions and so MgCl2 has the higher boiling point.

20. (b) CH3NH2 or CH3F SOLUTION:
(b) CH3NH2 and CH3F are both covalent compounds and have bonds which are polar. The dipole in CH3NH2 can H bond while that in CH3F cannot. Therefore CH3NH2 has the stronger interactions and the higher boiling point.

21. (c) CH3OH or CH3CH2OH SOLUTION:
(c) Both CH3OH and CH3CH2OH can H bond but CH3CH2OH has more CH for more dispersion force interaction. Therefore CH3CH2OH has the higher boiling point.

22. (d) Hexane(CH3CH2CH2CH2CH2CH3) or 2,2-dimethylbutane
SOLUTION:
(d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to hold the molecules together. Hexane has the larger surface area, thereby the greater dispersion forces and the higher boiling point.

23. staircase memory tool! handy moving up this while of IMFs increase
Intermolecular Forces (IMFs)
Forces (not bonds!)BETWEEN molecules
IMFs are much weaker than real bonds
Three important types to know
Have important effects on boiling points
What to Know:
definition of forces
which substances have which forces
how these forces effect boiling points
*** m’c = molecules ***
HYDROGEN “BONDING”
staircase memory tool!
Attraction of m’c to other m’c due to dipole moments of polar m’c that include an H bonded to either an N, O or F. The positive H side of one m’c attracted to negative N, O or F side of a different m’c & vice versa (even stronger than dipole-dipole)
Polar m’c (m’c with dipole moments) that have an H bonded to either an N, O or F have them
These m’c “stick together” even more strongly, and so are harder to boil; this leads to higher boiling points than m’c without hydrogen bonds.
**Please forgive the terrible job we chemists did naming these forces & remember that hydrogen bonds are not really bonds, they’re IMFs!
handy
DIPOLE – DIPOLE FORCES
Types of IMFs
London Dispersion
Dipole-Dipole
Hydrogen Bonding
moving up this
Attraction of m’c to other m’c due to dipole moments of polar m’c. Positive side of one m’c attracted to negative side of a different m’c & vice versa (strong)
Polar m’c (m’c with dipole moments) have them
These m’c “stick together” more, and so are harder to boil; this leads to higher boiling points than m’c without dipole-dipole IMFs
while
LONDON DISPERSION FORCES
of IMFs increase
Attraction of m’c to other m’c due to gravity and instantaneous or temporary dipoles (very weak)
All m’c have them
Higher molar mass, higher boiling point
Strength

24. Summary diagram for analyzing the intermolecular forces in a sample.
INTERACTING PARTICLES
(atoms, molecules, ions)
ions present
ions not present
ions only
IONIC BONDING
(Section 9.2)
nonpolar
molecules only
DISPERSION
FORCES only
polar molecules only
DIPOLE-DIPOLE
FORCES
H bonded to
N, O, or F
ion + polar molecule
ION-DIPOLE FORCES
polar + nonpolar
molecules
DIPOLE-
INDUCED DIPOLE
FORCES
HYDROGEN
BONDING
DISPERSION FORCES ALSO PRESENT