Intermolecular Forces LACC Chem101

States of Matter Solids Liquids Gases High density Little translational motion (if any) Rotations/vibrations give temperature Intermolecular forces cause 3D structures Liquids Translational motion limited by frequent collisions High degree of intermolecular forces Gases Low density Very high speed translational motion Little intermolecular forces Often approximated as no interactions (ideal gases) LACC Chem101

Phase Transitions Melting Point Boiling Point Sublimation Point Melting: Solid  Liquid Freezing: Liquid Solid Boiling Point Condensation: Gas  Liquid Vaporization: Liquid  Gas Sublimation Point Sublimation: Solid  Gas Deposition: Gas  Solid LACC Chem101

Phase Diagrams Show equilibrium lines between phases Phases defined by both temperature and pressure Key Parts Triple point Critical point Normal boiling point LACC Chem101

Latent Heat Heat transfer of a constant temperature process Heat energy added/lost, but no associated temperature change Energy used to change structure of the material Energy transfer changes the internal energy of the substance, and is therefore an enthalpy We define enthalpies phase changes: LACC Chem101

Heating/Cooling Curve Graphical representation of the temperature/energy of a substance Example: Water moving from ice to gas LACC Chem101

Water example The first step is to design a pathway: Calculate the amount of energy required to change 15.0g of ice at -5.00C to steam at 125.0C. The first step is to design a pathway: q1 = msDT for ice from -5.0 to 0.0 oC, the specific heat of ice is 4.213 J/g oC q2 = DHfus for ice to liquid at 0.0oC q3 = msDT for liquid 0.0oC to 100.0 oC q4 = DHvap for liquid to steam at 100.0oC q5 = msDT for steam 100.0 to 125.0 oC; the specific heat of steam is 1.900 J/g oC qT = q1 + q2 + q3 + q4 + q5 The next step is to calculate each q: q1= (15.0 g) (4.213 J/g oC) (0.0 - (-5.0) oC) = 316 J q2 = (15.0 g) (335 J / g) = 5025 J q3= (15.0 g) (4.184 J/g oC) (100.0 - (0.0) oC) = 6276 J q4 = (15.0 g) (2260 J / g) = 33900 J q5= (15.0 g) (1.900 J/g oC) (110 - 100 oC) = 285 J qT = 316 J + 5025 J + 6276 J + 33900 J + 285 J = 45.8 kJ LACC Chem101

Intermolecular Forces Not to be confused with intramolecular forces These are forces between atoms within molecules Covalent/Ionic bonds Intermolecular Forces occur between molecules Types of forces depend on compound type Neutral molecules Dipole-Dipole forces London dispersion Hydrogen bonding Ionic compounds Ion-dipole forces LACC Chem101

Intermolecular Forces Intermolecular forces affect boiling and melting points Stronger force requires more energy to break the molecules apart Type of Interaction Energy Range (kJ/mol) Intermolecular Van der Waals 0.01 - 10 Hydrogen bond 10 - 40 Chemical Bond Ionic 100 - 1000 Covalent LACC Chem101

Intermolecular Forces Ion-Dipole between ions and polar molecules strength is dependent on charge of the ions or polarity of the bonds usually involved with salts & H20 Dipole-Dipole between neutral polar molecules weaker force than ion-dipole positive dipole attracted to negative dipole molecules should be relatively close together strength is dependent on polarity of bonds London dispersion all molecules and compounds involves instantaneous dipoles strength is dependent on Molar Mass (size) contributes more than dipole-dipole shape contributes to strength LACC Chem101

Hydrogen Bonding Exists between hydrogen atom in a polar bond and a lone pair of electrons on a nearby electronegative species Oxygen, Fluorine, and Nitrogen Special case of dipole-dipole interaction Stronger than dipole-dipole and London dispersion Accounts for water’s notable properties High boiling point for small size Solid expands from liquid volume Less dense than the liquid “universal” solvent High heat capacity LACC Chem101

Interacting molecules or ions Flowchart of Intermolecular Forces Interacting molecules or ions Are polar Are ions Are polar molecules involved? molecules involved? and ions both present? Are hydrogen atoms bonded to N, O, or F atoms? London forces Dipole-dipole hydrogen bonding Ion-dipole Ionic only (induced forces forces Bonding dipoles) Examples: Examples: Examples Example: Examples: Ar(l), I2(s) H2S, CH3Cl liquid and solid KBr in NaCl, H2O, NH3, HF H2O NH4NO3 NO NO YES NO YES Yes YES NO LACC Chem101 Van der Waals forces

Properties of Liquids Viscosity Surface tension Resistance of a liquid to flow Depends on attractive forces between molecules May also be caused by structural features (entanglement) Surface tension Energy required to increase the surface area of a liquid (Energy/Area) Due to interactions between molecules Also lack of interactions at an interface LACC Chem101

Vapor Pressure Pressure exerted by a vapor in equilibrium with its liquid or solid state Changes with intermolecular forces Involves an equilibrium between liquid and gas Volatile Nonvolatile LACC Chem101

Clausius-Clapeyron Equation Higher temperature causes a weakening of intermolecular forces This causes higher vapor pressure Relationship is a differential equation: LACC Chem101

Clausius-Clapeyron Example The vapor pressure of ethanol at 34.9C is 100.0mmHg. The normal boiling point of ethanol is 78.5C. Calculate the heat of vaporization. LACC Chem101

Crystalline Solid Composed of crystal lattices Unit cell: Geometric arrangement of lattice points smallest repeating unit in cell structure Edge lengths and angles describe unit cell Many types of unit cells Metals and salts are usually cubic LACC Chem101

Unit cell example Determine the number of ions in the lithium fluoride unit cell. The structure is a face centered cube. LACC Chem101

Molecular Solids Solid composed of molecules held together by van der Waals forces Require large number of atoms surrounding center for maximum attraction Close-packing arrangement variations Hexagonal close-packed Cubic close-packed Similar to face-centered cubic Coordination number Number of nearest neighbors Highest is 12 LACC Chem101

Metallic Solids Sea of delocalized electrons Usually cubic or hexagonal close-packed LACC Chem101

Covalent Network Directional covalent bonds Hybridization affects structure Structure gives physical properties Examples Tetrahedral structures: diamond, Si, Ge, Sn sp3 hybridized Face-centered cubic cells Hexagonal Sheets: graphite, carbon nanotubes sp2 hybridized Electrical properties delocalized electrons LACC Chem101

CRYSTALLINE SOLIDS Type of solid lattice site Type of force properties of examples particle type between particles solids IONIC positive & electrostatic high M.P. NaCl negative ions attraction nonvolatile Ca(NO3)2 hard & brittle poor conductor POLAR polar dipole-dipole & moderate M.P. Sucrose, MOLECULAR molecules London Dispersion moderate C12H22O11 forces volatility Ice, H2O NONPOLAR Nonpolar London Dispersion low M.P., Argon, Ar, MOLECULAR molecules & forces volatile Dry Ice, CO2 atoms MACRO- atoms covalent bonds extremely high Diamond, C MOLECULAR between atoms M.P. nonvolatile Quartz, SiO2 Covalent- Arranged in Very Hard Network Network Poor conductor METALLIC metal atoms attraction between variable M.P. Cu, Fe outer electrons low volatility Al, W and positive good conductor atomic centers LACC Chem101

TYPE OF MELTING POINT HARDNESS ELECTRICAL SOLID OF SOLID & BRITTLENESS CONDUCTIVITY Molecular Low soft & brittle Nonconducting Metallic Variable Variable hardness, conducting malleable Ionic High to very hard & brittle Nonconducting high solid (conducting liquid) Covalent Very high Very hard Usually Network nonconducting LACC Chem101

X-ray diffraction Method for determining atomic/molecular structure of a crystal Atoms reflect x-rays directionally Angles and intensities of diffracted light rays give 3D picture of electron densities Atom positions then deduced Planes of atoms act as reflecting surfaces X-rays reflect and make diffraction pattern on photographs Constructive interference give more intense waves Only at angles where X-rays are in-phase Destructive interference cancels Type of unit cell and size can be determined LACC Chem101

X-Ray Diffraction If molecular, the position of atoms can be determined using the Bragg equation LACC Chem101