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Bond strength varies with: -charge on ions. -distance (r) separating ions. -force varies with 1/r2. -bond energy (force acting over a distance r) then.
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n Volume occupied by gas decreases (IGL) n At higher pressures: get negative deviations from IGL -due to intermolecular attraction ( Van deWaals Equation ) n At a high enough pressure:
ABRUPT decrease (100x or more) in volume
Phase Transition: Gas → Liquid
- due to intermolecular attractive forces
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Electrostatic Attraction
n All based on electrostatic attraction, but not strong enough to be considered a chemical bond
n Recall: Ionic Bonds
- electrostatic attraction between two ions:
Na+^ Cl- Bond strength varies with: -charge on ions -distance (r) separating ions -force varies with 1/r^2 -bond energy (force acting over a distance r) then varies with 1/r -ionic bond energies: very large (300 - 600 kJ/mol)
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Ion-Dipole Interactions
n Ions can have electrostatic interactions with polar molecules:
Both attractive and repulsive forces are operative here
-energy drops off as 1/r^2
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London Dispersion Forces
n These are usually very weak interactions (0.05 - 2 kJ/mol) n Energy drops of as 1/r^6
n ALL atoms and molecules will experience London Dispersion Forces
n Magnitude of force will depend upon how easy it is to distort the electron cloud ( polarize):
- favors atoms and molecules that are LARGE and have the greatest surface area
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Boiling Point Trends
n The enthalpy change associated with vaporization is due to the disruption of these intermolecular forces n The magnitude of ∆Hovap is reflected in the boiling point temperature (Tb) for a compound n For polar molecules:
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More BP Trends
n For NONPOLAR species:
increased molar mass = greater polarizability
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Hydride Boiling Points
n Something strange in the B.P. of some hydrides: -unusually high B.P. for water -same effect observed with other 2nd Period hydrides: -Tb(HF) > Tb(HCl) -Tb(NH 3 ) > Tb(PH 3 )
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H-bonding in Water
n Hydrogen bonding in water is very significant
n Without H-bonding, we would expect water to have a B.P. of about -123 oC Hydrogen acts as a^ bridge^ to facilitate electron-sharing between the oxygen atoms on adjacent molecules
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Structure of Water
n Each oxygen can accommodate interactions with four hydrogen atoms ( bonding, 2 H-bonding)
-gives a hexagonal structure in solid phase
-more open space than liquid phase, so solid water is less dense than liquid water
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Overview of IM-Forces
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Some Properties of Liquids
-characterizes a liquid’s resistance to flow -varies with the degree of intermolecular attraction
-molecules on the surface of a bulk liquid experience intermolecular attraction only from molecules in the bulk solution below the surface
-so, surface molecules are more tightly packed than molecules in the bulk solution
