All course materials, including lectures, class notes, quizzes, exams, handouts, presentations, and other materials provided to students for this course are protected intellectual property. As such, the unauthorized purchase or sale of these materials may result in disciplinary sanctions under the Campus Student Code. Intra-molecular or bonding forces are found within a molecule Force Model Physical basis Energy (kj/ mole) Example Ionic Covalent Intermolecular Forces (IMFs) are electrostatic in nature (attraction between opposite charges) Opposite charges that are permanent in nature result in stronger IMFs Opposite charges that are temporary/ transient in nature result in weaker IMFs Relative strength of intermolecular forces is directly related to: melting/boiling points (Tf/ Tb) Stronger IMFs higher Tf and Tb enthalpy of fusion ( Hfus) higher Hfus enthalpy of vaporization ( Hvap) higher Hvap Viscosity higher viscosity surface tension higher surface tension solubility of the substances like dissolves like We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 1 of 13
Boiling Points and Atomic Weight Interactions of Nonpolar Molecules London Dispersion Forces (LDF) IMFs between nonpolar molecules caused by temporary/induced dipoles in the molecules Temporary (induced) dipole Separation of charge produced in an atom or molecule by a momentary uneven distribution of electrons Factors affecting strength of dispersion: We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 2 of 13
Polarizability Relative ease with which the electron cloud in a molecule, ion, or atom can be distorted, inducing a temporary dipole Higher polarizability stronger dispersion Shape of molecules Increased surface area increased interactions between molecules Linear molecules have higher dispersion than branched molecules of similar MW We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 3 of 13
Interactions Involving Polar Molecules Ion dipole interaction Attractive force between an ion and a molecule that has a permanent dipole Sphere of hydration Cluster of water molecules surrounding an ion in aqueous solution Dipole Dipole Interactions Hydrogen bond We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 4 of 13
Boiling points are related to attractive forces H-bonding between complementary strands of DNA Responsible for forming the double helix shape We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 5 of 13
Relative Strengths of Intermolecular Forces Rank the following compounds in order of increasing boiling point: CH3OH, CH3CH2CH2CH3, and CH3CH2OCH3 We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 6 of 13
Flow chart to determine strongest type of IMFs present For each pair of substances, identify the key bonding and/or intermolecular force(s), and predict which one of the pair has the higher boiling point CH3NH2 or CH3F CH3OH or CH3CH2OH CH3CH2CH2CH2CH2CH3 (n-hexane) or 2,2-dimethylbutane [Structures given] H H H H H H H H H H C H H C C C C C C H H H H H H H H H C C C C H C H H H H H H We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 7 of 13
Properties of Water Other related examples We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 8 of 13
Factors Affecting Physical States Intermolecular forces Temperature Pressure Phase diagram A graphical representation of how the stabilities of the physical states of a substance depend on temperature and pressure Equilibrium lines represent phase changes between solid/liquid, liquid/gas, and solid/gas under specific conditions (T, P). Triple point Temperature/pressure where all three phases of a substance coexist Phase Diagram for Water We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 9 of 13
Describe the phase changes that occur when the temperature of CO2 is increased from 100 C to 200 C at a pressure of 25 atm We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 10 of 13
Inter-molecular forces are found between molecules (revisted) Kinetic view of the three states of matter Solid Attractive Forces vs. Kinetic Energy Properties Liquid Gas Liquid-gas equilibrium We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 11 of 13
The effect of temperature on the distribution of molecular speeds Factors affecting Vapor Pressure As temperature increases, the fraction of molecules with enough energy to enter the vapor phase increases, more particles are in the gas phase and the vapor pressure is higher The weaker the intermolecular forces, the more easily particles enter the vapor phase, more particles are in the gas phase and the vapor pressure is higher Diethyl ether Ethanol water Vapor pressure as a function of temperature and intermolecular forces Vapor pressure increases as temperature increases Vapor pressure decreases as the strength of the intermolecular forces increases We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 12 of 13
Vapor Pressure and Boiling Point The boiling point of a liquid is the temperature at which the vapor pressure equals the atmospheric pressure The normal boiling point of a liquid is observed at standard atmospheric pressure, exactly 1 atm As the external pressure on a liquid increases, the boiling point increases We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 13 of 13