Intermolecular Forces of Attraction, Lecture notes of Molecular Chemistry

Download Intermolecular Forces of Attraction and more Lecture notes Molecular Chemistry in PDF only on Docsity! Intermolecular Forces of Attraction Attractive forces that cause atoms or molecules to stick together Mixtures Elements or compounds blended together but not chemically combined Intermolecular Forces All intermolecular forces are: • attractive forces • between molecules • do not make new compounds • Makes the molecules “sticky” • weaker than true “bonds” IMF’s + are not chemical bonds! { but. IMF’s are “Coulombic attractions” Polar forces • “Dipole - Dipole” interactions • positive ends attract negative ends of other molecules • about 1% as strong as a covalent bond Dipole - Dipole interactions a London forces • Momentary dipole-dipole interaction • “instantaneous” dipole “induces” a dipole in a neighboring molecule • brief attractive force results Result... Molecule A Molecule B London forces Strength depends on the size of e- cloud bigger e- cloud = stronger attraction The more e-’s, the more ”polarizable”, the stronger the London Forces How do you get “more” electrons? 1. Bigger atoms 2. More atoms in the molecule Hydrogen Bonds • Special case of polar attraction • H atom is • 1) bonded to high EN atom and • 2) attracted to a high EN atom it . is not bonded to (N, O, F) • 5 times stronger than regular polar attractions NOT “H-bonds” H-bonds H / oO teseeeH——N a NOT “H-bonds” H-bonds \ Nome H rereeeO / ¢ ‘ / N eeeees HoeeN Which atoms can H bond? Intermolecular Forces of Attraction Attractive forces that cause atoms or molecules to stick together Types of IMF’s 1. London (dispersion) forces  all molecules  weakest interaction 2. dipole-dipole forces  polar molecules 3. hydrogen bonding  H atoms w/ O, N, F not covalently bonded to it  strongest interaction  ion-dipole forces  Between polar molecules and ions  Aqueous solutions of ionic compounds Physical property effects of Intermolecular Forces of Attraction …all come down to how sticky the molecules are toward each other… What does that mean… ”separate the molecules”? Pull them apart from each other (but not break them!): Melting Boiling Physically move them apart Peel them away. Wipe them off, etc… Separating = breaking the attraction holding the molecules to each other What is happening when ice melts? The IMF’s (H-bonds) are being disrupted The covalent bonds are NOT breaking Boiling Point Effects Formula Boiling point Polar? IM force HCl -85oC Polar Dipole- dipole H2S -60.7oC Polar Dipole- dipole H2O 100oC Polar H-bonding Ar -185.7oC Nonpolar london Boiling Point Effects Formula Boiling Point Polar? IM force F2 -188.1oC Nonpolar London Cl2 -34.6oC Nonpolar London Br2 58.8oC Nonpolar London I2 184.4oC Nonpolar London Boiling Point Effects Formula Boiling point Polar? IM force CH4 -161.5oC Nonpolar London C2H6 -88.6oC Nonpolar London C3H8 -42.1oC Nonpolar London C4H10 -0.5oC Nonpolar London C5H12 36.1oC Nonpolar london Cohesion/Adhesion Cohesion = molecules sticking together Adhesion = molecules sticking to a substrate Viscosity • The resistance of a fluid to flow. • The stronger the IMF’s, the higher the viscosity Surface Wetting • The spreading of a liquid across a surface • must have adhesion  cohesion Surface Tension, continued • Molecules at the surface of a liquid act as a “skin” • the greater the IMF’s, the more pronounced the effect • liquids seek to lower their surface tension • water drops are spherical elem Op roo a (=e) Surfactants, used in soaps and detergents, drastically lower the surface tension of water –soapy water spreads more easily over dirty surfaces Vaporization • The change of a liquid → gas is called vaporization • an example of separating molecules • Evaporation: happens at the exposed surface of a liquid • Boiling: happens within the liquid itself Evaporation • Evaporated liquids are gases – and so exert pressure • vapor pressure • The stronger the IMF’s, the slower the evaporation rate, the lower the vapor pressure for a given temperature —-—— H 50(g) Open container molecules (water vapor) H,O(1) molecules Closed container What happens... • To the evaporation rate – if the temperature = the boiling point ? • All the molecules, on average, have enough energy to vaporize –the liquid boils What happens... • If the vapor pressure equals the atmospheric pressure ? • All the molecules, on average, have enough energy to vaporize –the liquid boils T vs Vapor Pressure Temperature V a p P r e s s u r e 1.00 atm 100oC The stronger the IMF… The higher the melting point The higher the boiling point The higher the viscosity The higher the surface tension The lower the vapor pressure Forces of Attraction as Intermolecular Forces of attraction between molecules