Ch 11: Intermolecular Forces and Types of Solids. Brown, LeMay AP Chemistry Monta Vista High School. Inter- molecular Forces. Have studied INTRA molecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules — INTER molecular forces.
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Monta Vista High School
Have studied INTRAmolecular forces—the forces holding atoms together to form molecules.
Now turn to forces between molecules —INTERmolecular forces.
Forces between molecules, between ions, or between molecules and ions.
Ion size and LE
Ex: Compare IMF in Cl- (aq) and S2- (aq).
NaCl dissolving in Water
Water is highly polar and can interact with positive ions to give hydratedions in water.
Attraction between ions and dipole depends on ion charge and ion-dipole distance.
Measured by ∆H for Mn+ + H2O f [M(H2O)x]n+
Ex: What IMF exist in NaCl (aq)?
Influence of dipole-dipole forces is seen in the boiling points of simple molecules.
Compd Mol. Wt. Boil Point
N2 28 -196 oC
CO 28 -192 oC
Br2 160 59 oC
ICl 162 97 oC
A special form of dipole-dipole attraction, which enhances dipole-dipole attractions.
H-bonding is strongest when X and Y are N, O, or F
Boiling Points (ºC)
H-bonding is especially strong in water because
Accounts for many of water’s unique properties.
Animation of Ice
Ice has open lattice-like structure.
Ice density is < liquid.
And so solid floats on water.
Snow flake: www.snowcrystals.com
Ice has open lattice-like structure.
Ice density is < liquid and so solid floats on water.
H bonding in Water
One of the VERY few substances where solid is LESS DENSE than the liquid.
H bonds leads to abnormally high boiling point of water.
See Screen 13.7
See Active Figure 12.8
H-bonding is especially strong in biological systems — such as DNA.
DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O.
Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases.
—adenine with thymine
—guanine with cytosine
Portion of a DNA chain
Hydrogen bonding and base pairing in DNA.
With your elbow partner, draw the following on the same sheet of paper taking turns:
Nucleotide pairs form H-bonds
DNA double helix
How can non-polar molecules such as O2 and I2 dissolve in water?
The water dipole INDUCES a dipole in the O2 electric cloud.
Dipole-Dipole and LDFs
Solubility increases with mass the gas
Formation of a dipole in two nonpolar I2 molecules.
Induced dipole-induced dipole
The induced forces between I2 molecules are very weak, so solid I2sublimes (goes from a solid to gaseous molecules).
See Figure 12.12
In a liquid
• molecules are in constant motion
• there are appreciable intermolec. forces
• molecules close together
• Liquids are almost incompressible
• Liquids do not fill the container
To evaporate, molecules must have sufficient energy to break IM forces.
Breaking IM forces requires energy. The process of evaporation is endothermic.
HEAT OF VAPORIZATION is the heat req’d (at constant P) to vaporize the liquid.
LIQ + heat f VAP
Compd. ∆vapH (kJ/mol) IM Force
H2O 40.7 (100 oC) H-bonds
SO2 26.8 (-47 oC) dipole
Xe 12.6 (-107 oC) induced dipole
SURFACE TENSION also leads to spherical liquid droplets.
Movement of water up a piece of paper depends on H-bonds between H2O and the OH groups of the cellulose in the paper.
4. Adhesion:attraction of molecules for a surface
Effect of MW: Effect of # atoms:
pentane 36ºCNe –246°C
hexane 69ºCCH4 –162°C
C5H12 isomers: 2,2-dimethylpropane 10°C
Van der Waals forces
Ex: Identify all IMF present in a pure sample of each substance, then explain the boiling points.
I2 (s) and (g)
Pressure cooker ≈ 2 atm
Normal BP = 1 atm
10,000’ elev ≈ 0.7 atm
29,029’ elev (Mt. Everest)≈ 0.3 atm
Get in groups of four.
Two people will need to draw and two will need to explain
First drawer draws a phase diagram of water.
Second drawer draws a phase diagram of CO2
Third member explains the water’s phase diagram
Fourth member explains CO2’s phase diagram.
Ex: rubber, glass