Intermolecular Forces

1. Intermolecular Forces LACC Chem101

2. States of Matter LACC Chem101 • Solids • High density • Little translational motion (if any) • Rotations/vibrations give temperature • Intermolecular forces cause 3D structures • Liquids • High density • 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)

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

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

5. Latent Heat LACC Chem101 • 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:

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

7. Water example LACC Chem101 • 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/goC • 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

8. Intermolecular Forces LACC Chem101 • 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

9. Intermolecular Forces LACC Chem101 • Intermolecular forces affect boiling and melting points • Stronger force requires more energy to break the molecules apart

10. Intermolecular Forces LACC Chem101 • 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

11. Hydrogen Bonding LACC Chem101 • 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

12. Flowchart of Intermolecular Forces Interacting molecules or ions Are polar Are ions Are polar molecules involved? molecules involved? and ionsboth present? Are hydrogen atoms bonded to N, O, or F atoms? London forcesDipole-dipolehydrogen bondingIon-dipoleIonic only (induced forcesforcesBonding dipoles) Examples: Examples: Examples Example: Examples: Ar(l), I2(s) H2S, CH3Cl liquid and solidKBr in NaCl, H2O, NH3, HF H2O NH4NO3 NO NO YES NO YES Yes YES NO Van der Waals forces LACC Chem101

13. Properties of Liquids LACC Chem101 • Viscosity • 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

14. Vapor Pressure LACC Chem101 • 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

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

16. Clausius-Clapeyron Example LACC Chem101 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.

17. Crystalline Solid LACC Chem101 • 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

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

19. Molecular Solids LACC Chem101 • 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

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

21. Covalent Network LACC Chem101 • 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

22. 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

23. 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

24. X-ray diffraction LACC Chem101 • 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

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