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Intermolecular Forces Essay

4326 words - 18 pages

Chang: Chemistry 7th Edition - Chapter 11
CH116 General Chemistry II
INTERMOLECULAR FORCES

Inter-molecular Forces
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.

Table 13.1 Summary of Intermolecular Forces
Ions
Dipoles
Induced Dipoles

(Overhead & book p 585)

Covalent bond energies 100-400 kJ/mol
Attractive forces between ions 700-100 kJ/mol
Intermolecular attractions less than 15% of bond energies

Intermolecular Forces
Ion-Ion Forces
Na+ — Cl- in salt.
These are the strongest forces.
Lead ...view middle of the document...

Dipole-Dipole Forces
Such forces bind molecules having permanent dipoles to one another.
Dipole-Dipole Forces
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

Dipole-Dipole Forces
Influence of dipole-dipole forces is seen in the boiling points of simple molecules.

Hydrogen Bonding
A special form of dipole-dipole attraction, which enhances dipole-dipole attractions.

Hydrogen Bonding
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

Query: How do you recognize when hydrogen bonding will occur between molecules?

H-Bonding Between Methanol and Water
H-Bonding Between Two Methanol Molecules
H-Bonding Between Ammonia and Water

Strength of Dipole depends on Factors:
electronegativity difference - the greater the difference, the more polarized the dipole, the stronger the attractions, the higher the melting point and boiling point, the lower the vapor pressure
atomic size - small atomic size concentrates charge on ions/dipole increasing the strength of the dipole, increasing mp and bp, lowering vapor pressure
Query: How can relative BP be predicted for an analogous series of compounds?

Hydrogen Bonding
Boiling Points
Explain the observed trends in boiling points for representative hydrogen compounds

Why are boiling points important predictors?

Overhead: Temperature vs. Period

Query: Why is the observed boiling point of dihydrogen selenide higher than that of dihydrogen sulfide but lower than that of dihydrogen oxide?
Indicators of Intermolecular Force
Enthalpy of vaporization
Boiling point - review table 13.2

(overhead - types of IA - BP/MW/#e-)

Solublility “like dissolves like”

Hydrogen Bonding in H2O
H-bonding is especially strong in water because
the O—H bond is very polar
there are 2 lone pairs on the O atom
Accounts for many of water’s unique properties.
Hydrogen Bonding in H2O
H-bonding in H2O ----> open lattice like structure of ice.
Ice density is less than that of liquid, and solid floats on water.
Hydrogen Bonding in H2O
H-bonding in H2O ----> open lattice like structure of ice.
Ice density is less than that of liquid, and solid floats on water.

Ice: 13m07an2
Hydrogen Bonding in H2O
H bonds ---> abnormally high specific heat capacity of water (4.184 g/K•mol).
This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.
Hydrogen Bonding
H bonds ---> abnormally high boiling point of water.

FORCES INVOLVING INDUCED DIPOLES
How can non-polar molecules such as Br2, I2, and N2 condense to form liquids and solids?
Consider I2 dissolving in alcohol, CH3CH2OH.
FORCES INVOLVING INDUCED DIPOLES
How can non-polar molecules such as Br2, I2, and N2...

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