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S117Lecture132011

S117Lecture132011 - Intermolecular Interactions Covalent...

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Intermolecular Interactions Covalent bonds in molecules are strong. The breaking and making of covalent bonds involves large amounts of energy (e.g. chemical explosives). In addition to strong chemical bonds that define the connectivity between atoms within molecules , there are weak non-covalent interactions that define chemistry When these non-covalent interactions are between molecules, they are called inter- molecular interactions. For large molecules there are also weak intra- molecular forces between different parts of the same molecule. Although these interactions are weak they are very important: They determine whether a substance in a gas, liquid, or solid. They determine the shapes of large complex molecules like proteins.
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Bond Polarity: The first step towards understanding weak interactions Use electronegativity trends to classify bonds Predict how molecules behave H-bonding, melting, dissolution, etc… Large EN (difference in EN between atoms that form a bond) MORE POLAR! Explains high BP of H 2 O polar O-H causes Hydrogen-bonding
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EN= 0 EN=0.7 EN=1.9 EN= 3.0
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Bond Character EN Character Example >1.7 Highly Ionic NaCl 0.4-1.7 Polar Covalent HCl <0.4 Mostly Covalent HI 0 Nonpolar covalent H 2
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Dipole moments Molecules interaction with each other because of dipole moment. Which of the following orientations is more stable?
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O=C=O δ - δ - δ + The two dipoles cancel out. CO 2 does not have a permanent dipole moment – it is not polar. H H O Because H 2 O is bent (not symmetric) there is a net permanent dipole moment and water is polar. Due to the charge separation, water molecules interact through hydrogen bonding and this explains the BP of water δ + δ + δ -
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There is an hierarchy of intermolecular forces that can occur between any two molecules: Ion-dipole Hydrogen bonds Dipole-dipole Ion-induced dipole Dipole-induced dipole Dispersion (London) forces Increasing strength (typically but not always) Let’s get calibrated: What is a covalent bond worth? 150-1100 kJ/mol We can convert these energies to temperatures (divide by 8.314 J K -1 mol -1 ): 18,000-132,000 K The strongest known bond is in carbon monoxide (1073 kJ/mol).
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1. Ion-Dipole Energy ~ 40-600 kJ/mol Can be very strong! Stronger than a weak covalent bond. Example: Na + Cl + H 2 O O H H O H H O H H O H H Na + O H H O H H Cl + in a lattice solvation shell compensates for lattice energy. Without solvation would not dissolve.
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  • Fall '11
  • StephenJacobson
  • Atom, dipole moment, Intermolecular Interactions, permanent dipole moment, London Forces London Dispersion Forces Van der Waals Interactions

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