Lecture 23 - Intermolecular forces

Lecture 23 - Intermolecular forces - Molecular Interactions...

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Molecular Interactions vs “Thermal Energy” k B T (at room temperature) = 2.4 kJ/mole Intermolecular Interactions smaller than k B T are broken by k B T. (Gases) Intermolecular interactions larger than k B T are NOT broken by k B T (condensed phase: crystal lattice or fluid) Thermal energy (heat) defined as: Solid or fluid Gas
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Strong Intermolecular Forces (greater than k B T) 1) Ionic bond: Ion – Ion interactions (~400 kJ/mol) 12 0 4 qq V r πε =− 3) Dipole – Dipole interactions (~10 kJ/mol) () 2 1 12 3 12 0 2 1 cos cos 3 cos 4 θθ θ μ = r V Only depends on “r” NOT orientation. Na + --- Cl - 2) Ion – Dipole Interactions (~50 kJ/mol) 2 0 4 r q V = Na + --- OH 2 C = O --- C = O Permanent dipoles T k r V B 1 ) 4 ( 3 2 6 2 0 2 2 2 1 μμ = r r Q × = μ 1 μ 2 μ ~100 Å r
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Dipole – Dipole Interactions 3 0 4 2 r V B A πε μ = 3 0 4 r V B A = θ 12 () 2 1 12 3 12 0 2 1 cos cos 3 cos 4 θθ θ = r V T k r V B 1 ) 4 ( 3 2 6 2 0 2 2 2 1 μμ = O H H O H H δ− δ+ δ−
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Intermolecular Forces between uncharged molecules and atoms = van der Waals Forces London Forces Dipole–Induced Dipole •D ipo le-D le Strength Weak Intermolecular Forces (weaker than k B T @ room temp) V 1/r 6 He He O = C O = C O = C 6 2 4 3 r I V α = 6 0 2 4 2 1 r V πε αμ = T k r V B 1 ) 4 ( 3 2 6 2 0 2 2 2 1 μμ =
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= 6 12 4 r r V σσ ε Lennard-Jones 6-12 potential 12
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Lecture 23 - Intermolecular forces - Molecular Interactions...

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