Class 19 - IMF General Chemistry II: Atomic and Molecular Structure PDF

General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Unit 3: Structural Properties Intermolecular Forces FOCUS: Understand what the IMF are and how they relate to the ph...

General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Unit 3: Structural Properties Intermolecular Forces FOCUS: Understand what the IMF are and how they relate to the physical properties of substances Prof. German Perez  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Objectives 3. 1 Intermolecular Forces 3.1.1 Describe each of the following intermolecular forces: dispersion force, dipole-dipole force, H-bonding, Ions –dipole. 3.2 Intermolecular Forces and Physical Properties 3.2.1 Describe the effects of increasing dispersion forces on boiling and melting points. 3.2.2 Describe the effects of dipole forces on boiling and melting points. 3.2.3 Describe the effect of hydrogen bonding on the boiling point and melting point. 2  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Schematic Representations of the Three States of Matter  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Intramolecular and Intermolecular Bonding Intramolecular bonding - Occurs within molecules Condensed states of matter - Liquids and solids Forces involved Covalent bonding Ionic bonding Intermolecular bonding: Occurs between molecules Copyright © Cengage Learning. All rights reserved 4  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Intermolecular forces Mainly classified as: Dispersion forces Dipole – dipole force (aka Keesom interactions) H-bonding Ion – dipole interaction 5  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Dipole–Dipole Forces Forces that act between polar molecules Dipole–dipole attraction: Electrostatic attraction between molecules with dipole moments Molecules orient themselves in a way that the positive and negative ends are close to each other In a condensed state, dipoles find the best compromise between attraction and repulsion  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Characteristics of Dipole–Dipole Forces Approximately 1% as strong as covalent or ionic bonds Strength of the forces decreases as the distance between the dipoles increases  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Hydrogen Bonding Significantly strong dipole–dipole forces Prevalent in molecules that have a hydrogen atom bound to a highly electronegative atom Causative factors Polarity of the bond Proximity of the dipoles Influenced by the size of the hydrogen atom Influences physical properties of molecules  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Figure 10.3 - Hydrogen Bonding in Water Hydrogen bonding among water molecules The polar water Note that the small size of the hydrogen atom allows molecule for close interactions  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Figure 10.4 - Boiling Points of the Covalent Hydrides of the Elements in Groups 4A, 5A, 6A, and 7A  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 London Dispersion Forces Forces that exist among noble gas atoms and nonpolar molecules An accidental instantaneous dipole that occurs in an atom can induce a similar dipole in a neighboring atom Leads to an interatomic attraction that is weak and short-lived Can be significant for large atoms  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 London Dispersion Forces (Continued) Polarizability - Indicates the ease with which the electron cloud of an atom can be distorted to give a dipolar charge distribution As the atomic number increases, the number of electrons increases Increases the probability of the occurrence of momentary dipole interactions Used by nonpolar molecules to attract each other  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Chemical and Physical Changes at the Macroscopic Level Physical change Change in the form of the substance, but not in its chemical composition Chemical change Change of substances into other substances with different properties and different composition  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 IMF and the physical properties of substances 14  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Distinguishing Chemical and Physical Changes Aided by a molecular-level perspective Example of a physical change Phase change - Boiling of water Energy as heat is transferred to liquid water Intermolecular forces (hydrogen bonding) are overcome, and water vapor is formed as a result H 2 O ( l ) ⎯⎯ → H 2O ( g )  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Distinguishing Chemical and Physical Changes (continued) Example of a chemical change Combustion of methane (CH4) to form CO2 and H2O CH 4 + 2O 2 ⎯⎯ → CO 2 + 2H 2 O Chemical bonds between C and H in CH4 and between O atoms in O2 must be broken Chemical bonds between C and O in CO2 and H and O in H2O must be formed  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Dipole-Induced Dipole Interaction Permanent dipole on a polar molecule induces a dipole on a neighboring nonpolar molecule Strength depends on the natures of both the polar molecule and the nonpolar molecule Larger the magnitude of the dipole in a polar molecule, the better able it is to induce a dipole in a neighboring molecule  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Dipole-Induced Dipole Interaction (continued) Nonpolar molecules with a greater number of electrons have an increased polarizability Increases the ease with which a dipole is induced  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Ion – dipole interaction 19  General Chemistry II: Atomic and Molecular Structure Chemistry Department FALL 2024 Bibliography Paul Flowers, Klaus H. Theopold, Edward J. Neth, R. Langley, William R. Robinson, Chemistry 2nd Ed. (2019, Openstax), Sections 10.1. 20