two things affect inter molecular forces: › charge › distance bigger distance equals less...
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Two things affect intermolecular forces:› Charge› Distance
Bigger distance equals LESS attraction! More charge equals MORE attraction! Coulomb’s Law
› F ~ charge/distance2
› Distance has more effect since it is squared.
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Bonds are permanent attractions, IMF’s are temporary
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Covalent› Sharing of electrons in permanent tug of
war. Ionic
› Stealing electrons and then being attracted by opposite charges
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Ion Dipole Hydrogen Bonding Dipole-Dipole London Dispersion forces
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Between an ion and a dipole!› Ion = permanently charged particle› Dipole = a polar molecule
Almost exclusively seen in aqueous solutions.
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Between two dipoles!› Dipoles are polar molecules
that have no net charge, but the charge is distributed unequally.
› Positive end of one attracts negative end of the other.
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ALL covalent compounds experience this force.› It is between the electrons in one atom
and the nucleus of another atom.› More mass and/or more atoms means
more dispersion forces.
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› More mass doesn’t cause an increase in LDF!
› More mass infers that there are more P and N, which MEANS there are more electrons.
› It is an increase in the number of e- which causes a larger e- density which leads to a stronger temporary dipole.
› You can use molar mass to figure it out since a larger molar mass implies a bigger density but you can't use the words molar mass in your justification in a Free Response Question
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Occurs in molecules in which hydrogen is having FON!› The most electronegative atoms are F, O
and N. With a hydrogen, there is excessive polarization.
› This excessive polarization attracts the lone pair of electrons.
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Strongest: Ion-Dipole (very strong)
Hydrogen Bonding (strong)Dipole-Dipole
Dispersion Forces (very weak)
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More IMF’s or stronger IMF’s affect the following properties:› Boiling point› Melting Point› Viscosity (thickness)› Vapor Pressure› Surface tension
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Stronger IMF› Higher boiling point
Weaker IMF› Lower boiling point
Compare CS2 to CO2
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Both are linearBoth are nonpolar covalentBoth have LDF
CS2 has stronger LDF – larger molecule means larger e- density
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Stronger IMF› Higher melting point
Weaker IMF› Lower melting point
Think of red rover… Compare HBr to Cl2
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Both are liner HBr has LD, Dipole Cl2 has LD, nonpolar
HBr has stronger IMF – higher melting point
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Stronger IMF› Higher viscosity
Weaker IMF› Lower viscosity
More attached to each other, the hard to flow, like solids vs. liquids.
Compare CH3OCH3 to CH3CH2OH
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CH3OCH3 LD, Dipole
CH3CH2OH LD, HB
Therefore CH3CH2OH has stronger IMF and a higher viscosity
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Stronger IMF› Lower vapor pressure
Weaker IMF› Higher vapor pressure
Again…think red rover.
Compare H2S to H2O2
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H2S LD, Dipole
H2O2 LD, HB
H2O2 has stronger IMF therefore lower vapor pressure
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Resistance of a liquid to increase in its surface area
Stronger IMF› Higher surface tension
Weaker IMF› Lower surface tension
Compare H2CO to H2O
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H2CO LD, Dipole
H2O LD, HB
Hydrogen bonding is a stronger IMF than Dipole, therefore H2O has a stronger surface tension
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Stronger IMF› Higher boiling point› Higher melting point› Higher viscosity› Higher surface tension› Lower vapor pressure
Weaker IMF› Lower boiling point› Lower melting point› Lower viscosity› Lower surface tension› Higher vapor pressure