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What are intermolecular forces? NOT chemical bonds, less strength Attractive forces between molecules involved in covalent bonding Molecular level, not individual atoms Develop solid and liquid physical properties Types Dipole-Dipole London Dispersion Hydrogen Bonding

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1. (London) Dispersion Forces TEMPORARILY uneven charge distribution Instantaneous movement of electrons from one location to another creating charge separation for a moment Influences electron distribution of neighboring atoms--- induced dipoles At SOME point in time, electron density is greater around one atom than the other Induces temporary dipoles in adjacent molecules Found between ALL atoms/molecules at varying degrees, some occur more frequently than others

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1. (London) Dispersion Forces (cont.) Mobile electron density attractive force between instantaneous dipole and induced dipoles (p. 450) Molecular shape influences the strength of intermolecular forces within a molecule Larger, elongated molecules have more surface area, create more dispersion forces Melting and boiling points indicative of intermolecular force strength

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1. (London) Dispersion Forces cont. Polarizability Ability of an electric field to alter a molecule/atoms electron charge density How easily can a dipole be induced in a molecule or atom polarizability, strength of intermolecular forces (WHY?) **Atomic/molecular sizes influence polarizability and strength of intermolecular forcesWHY???

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1. (London) Dispersion Forces cont. Ex. Halogens Nonpolar Size increases, force is strong and can form liquids and solids

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2. Dipole-Dipole Forces2. Dipole-Dipole Forces Found in polar molecules More intermolecular forces (dipole in addition to dispersion) Molecules arranged so the POSITIVE end of one molecule is oriented toward the NEGATIVE end of another molecule Dipole equal and opposite charge separation over a short distance in a molecule Permanent dipoles between molecules

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2. Dipole-Dipole Forces (cont.) Acts on adjacent molecules, opposite charges on neighboring molecules attract ( dipole-induced dipole forces ) Permanent dipole in one molecule induces dipole in neighboring molecule molecule polarity, influence of these forces on physical properties. Polar molecules have net attractive forces from all intermolecular forces existing in the molecules.

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Ex. HClEx. HCl

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How can we generally predict physical properties of molecular compounds? (MP, BP, etc.) dispersion force strength, molecular mass and size Polar compounds have dipole/dipole and dipole/induced forces Dispersion forces are found with ALL molecular compounds

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Example 1Example 1 Based on intermolecular forces, arrange the following in order from decreasing to increasing boiling point: CBr 4 CH 3 CH 2 CH 2 CH 3 F 2 CH 3 CHO

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Example 2Example 2 Examine the following 2 chemical compounds: BrCl and IBr. Which compound is a gas at room temperature? A solid?

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3. Hydrogen Bonding3. Hydrogen Bonding Type of intermolecular force, strong dipole-dipole force H atom is bonded to an electronegative (nonmetal) atom and is attracted to the electronegative (nonmetal) atom in a neighboring molecule Majority of H-bonding occurs among small, very electronegative, nonmetal atoms (N, O, F) Dotted lines represent hydrogen bonding

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Hydrogen Bonding (cont.)Hydrogen Bonding (cont.) Considered a FORCE, not a type of bond Force holding water molecules together Results from a negative charge on an atom and a positive charge on hydrogen H-X format

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Example: H 2 O, HFExample: H 2 O, HF

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Unique Properties of Water due to Hydrogen Bonding Freezes and expands as solid Solid is less dense than liquid (ex. Ice floats) High melting point High boiling point H-bonding strength greater than other intermolecular forces Highest surface tension (Hg only exception) Other compounds can easily dissolve in it High specific heat

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Biological Applications to H-Bonding Protein Structure Enzyme Activity DNA structure

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ALL DUE TO HYDROGEN BONDING !

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Homework p. 478 Problems #49, 51, 53, 55, 57