5_PropertiesOfLiquidsAndSolids(S)

5_PropertiesOfLiquidsAndSolids(S) - Properties of Liquids...

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CHEM 1000 3.0 Structure of liquids and solids 1 Properties of Liquids and Solids •Petrucci, Herring Madura and Bissonnette : Chapter 12
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CHEM 1000 3.0 Structure of liquids and solids 2 Van der Waal Forces • In the section on gases we said there were forces between the molecules. Now we know more about bonding and electronic structure we can revisit the question. • What kinds of forces exist between molecules?
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CHEM 1000 3.0 Structure of liquids and solids 3 Van der Waal Forces • Van der Waal forces are those attractive forces between molecules that are responsible for: – Real gas behaviour (Van der Waal “a”) – Condensation • There are a number of such forces resulting in different strengths of interactions between molecules.
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CHEM 1000 3.0 Structure of liquids and solids 4 Van der Waal Forces London or Dispersion Force • The quantum concept of electronic structure talks about the probability of an electron being in a certain region at a certain time. • Even for an atom or a perfectly symmetrical molecule there is a finite chance that the electronic charges are not uniformly distributed. • This produces an instantaneous dipole .
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CHEM 1000 3.0 Structure of liquids and solids 5 Van der Waal Forces London or Dispersion Force • The instantaneous dipole can then influence neighbouring molecules and induce a dipole in them. (an induced dipole ) • This gives two dipoles that can attract. This is usually called a dispersion force or a London force . Molecule on average Instantaneous dipole
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CHEM 1000 3.0 Structure of liquids and solids 6 Van der Waal Forces London or Dispersion Force a. Normal b. Instantaneous dipole c. Instantaneous dipole (left) induced dipole (right)
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CHEM 1000 3.0 Structure of liquids and solids 7 Van der Waal Forces London or Dispersion Force This force is going to be strongest in a molecule with a large number of electrons or for an elongated molecule. The ability or tendency of a molecule to have charge separation occur is called polarizability . The effect of London forces can often be seen in the boiling points of similar compounds.
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CHEM 1000 3.0 Structure of liquids and solids 8 Effect of London Forces • Bpt(He) = 4K • Bpt(Rn) = 211K Rn is larger ____________________________________ n-pentane is more elongated CH 3 -CH 2 -CH 2 -CH 2 -CH 3 Bpt 36.1 o C Bpt 9.5 o C C C H 3 CH 3 CH 3 CH 3
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CHEM 1000 3.0 Structure of liquids and solids 9 Van der Waal Forces Dipole-Dipole interactions • Bonds where the elements have different electronegativity have a permanent charge separation. E.g. HCl •H Cl • If this shows up in the molecule we say the molecule has a permanent dipole.
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CHEM 1000 3.0 Structure of liquids and solids 10 Van der Waal Forces Dipole-Dipole interactions • For molecules with permanent dipoles, Dipole-Dipole interactions can occur.
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CHEM 1000 3.0 Structure of liquids and solids 11 Van der Waal Forces Dipole-Dipole interactions
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CHEM 1000 3.0 Structure of liquids and solids 12 Van der Waal Forces Dipole-Dipole interactions • Dipole-dipole interactions are in addition to the London forces.
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5_PropertiesOfLiquidsAndSolids(S) - Properties of Liquids...

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