intramolecular forces vs. intermolecular forces intramolecular forces chemical bonds ...
TRANSCRIPT
Intermolecular Forces
Intramolecular Forces vs. Intermolecular Forces
Intramolecular Forces Chemical bonds
Intermolecular Forces Attractive forces between molecules
What are intermolecular forces?
NOT chemical bonds, less strength
Attractive forces between molecules involved in covalent bonding
Molecular level, not individual atoms
Develop solid and liquid physical properties
Types London Dispersion Dipole-Dipole Ion-Dipole Hydrogen Bonding
(London) Dispersion Forces
Attractive force between dipoles
TEMPORARY charge separation in an atom (instantaneous dipole) Sudden dipole in one atom causes “domino”
effect—influences electron distribution within adjacent atoms (induced dipoles
At SOME point in time, electron density is greater around one atom than the other Induces temporary dipoles in adjacent molecules
Found in ALL atoms/molecules
Intermolecular Force Strength
Influenced by:
1) Polarizability
2) Molecular Shape
1. Polarizability
How easily can a dipole be induced in a molecule or atom How easily can the electron density of an
atom/molecule be altered
polarizability, strength of intermolecular forces (WHY?)
**Atomic Size and Mass have an influence on polarizability
Example 1:
Which molecule is more likely to a gas at room temperature: F2 or Br2?
2. Molecular Shape
More contact with adjacent molecules creates more dispersion forces Bigger/elongated molecules have this advantage Compact molecules do not have as much surface area
Example 2:
Which compound has the strongest intermolecular forces?
Dipole-Dipole Forces
PERMANENT charge separation (dipole)
All polar molecules More intermolecular forces (dipole-dipole +
dispersion)
Molecules arranged so the POSITIVE end of one molecule is oriented toward the NEGATIVE end of another molecule Molecular ends with like charges repel each other
Example 3: Propane vs. acetaldehyde
Which has stronger intermolecular forces?
Example 4
Based on intermolecular forces, arrange the following in order from decreasing to increasing boiling point: CBr4
CH3CH2CH2CH3
F2
CH3CHO
Example 5
Examine the following 2 chemical compounds: BrCl and IBr. Which compound is a gas at room temperature? A solid?
Ion-Dipole
Attractive forces between ions and polar molecules Results from soluble ionic compounds in polar solvent (ex. Water) Ions split up in solvent and are surrounded by water molecules
Negative portion of water molecule surrounds POSITIVE ion
Positive portion of water molecule surrounds NEGATIVE ion
Hydrogen Bonding
Type of intermolecular force, strong dipole-dipole force
H atom is bonded to an electronegative (nonmetal) atom and is attracted to the electronegative (nonmetal) atom in a neighboring molecule
Majority of H-bonding occurs among small, very electronegative, nonmetal atoms (N, O, F)
Dotted lines represent hydrogen bonding
Hydrogen Bonding (cont.)
Considered a FORCE, not a type of bond
Force holding water molecules together
Results from a negative charge on an atom and a positive charge on hydrogen
H-X format
Example: H2O, HF
Unique Properties of Water due to Hydrogen Bonding
Freezes and expands as solid
Solid is less dense than liquid (ex. Ice floats)
High melting point
High boiling point H-bonding strength greater than other
intermolecular forces
Highest surface tension (Hg only exception)
Other compounds can easily dissolve in it
High specific heat
Biological Applications to H-Bonding
Protein Structure
Enzyme Activity
DNA structure
ALL DUE TO HYDROGEN BONDING !
Intermolecular Force Strength
All chemical compounds have dispersion forces
Other intermolecular forces are dependent on molecular structure
Generally, increasing molecular weight indicates an increase in intermolecular forces Analyze molecular structure and types of
intermolecular forces for compounds of similar molecule weight.
Similar forces, look at molecular weight
Strength of Intermolecular Forces
Example 6:
The following compounds have similar molecular weights. Arrange in order from DECREASING to INCREASING boiling point. Formaldehyde (H2C=O)
Methanol (CH3—OH)
Ethan (CH3CH3)
Example 7:
Arrange in order from DECREASING to INCREASING boiling point. Carbon tetrachloride (CCl4)
Acetone (C3H6O)
Tetrabromobutane (C4H6Br4)
Example 8:
Arrange in order from INCREASING to DECREASING boiling point. Octanoic acid—CH3(CH2)6COOH
Decane—CH3(CH2)8CH3
Nonanal—CH3(CH2)7CHO