Intermolecular Forces

Kinetic Molecular Theory  Describes the behavior of subatomic particles Liquids, solids, and gases are composed of small particles that have mass. Particles are in constant, random, rapid motion. Particles have collisions. Particles have an avg. KE directly related to temperature. The state of a substance at room temperature depends on the strength of the attractions between its particles.

Definition of IMF  Attractive forces between molecules.  Much weaker than chemical bonds within molecules.

Definition of IMF  Intramolecular forces: Covalent Bonding  Much stronger than chemical bonds between atoms.  Examples : nonpolar, polar sharing

Definition of IMF  Intermolecular Forces  Attractive forces between molecules.  Much weaker than chemical bonds within molecules.

Intermolecular Forces  Attractive forces between molecules or particles (ions, metal atoms, etc…)  Examples:  dispersion, (London /Vander Waals);  dipole-dipole,  dipole-ion,  hydrogen “bonding”,  metallic bonding,  ion-ion

Intermolecular Forces  Relative Strength:  Examples:  dispersion, (London /Vander Waals);  dipole-dipole,  dipole-ion,  hydrogen “bonding”,  metallic bonding,  ion-ion Weakest Strongest

Types of IMF

 London Dispersion Forces View animation online.animation

Types of IMF  Dipole-Dipole Forces + + - - View animation online.animation

Types of IMF  Hydrogen Bonding

Determining IMF  NCl 3 polar = dispersion, dipole-dipole  CH 4 nonpolar = dispersion  HF H-F bond = dispersion, dipole- dipole, hydrogen bonding

Physical Properties Liquids & Solids

Liquids vs. Solids LIQUIDS Stronger than in gases Y high N slower than in gases SOLIDS Very strong N high N extremely slow IMF Strength Fluid Density Compressible Diffusion

Liquid Properties  Surface Tension attractive force between particles in a liquid that minimizes surface area

Liquid Properties  Capillary Action attractive force between the surface of a liquid and the surface of a solid watermercury

Types of Solids  Crystalline - repeating geometric pattern covalent network metallic ionic covalent molecular  Amorphous - no geometric pattern decreasing m.p.

Types of Solids Ionic (NaCl) Metallic

Types of Solids Covalent Molecular (H 2 O) Covalent Network (SiO 2 - quartz) Amorphous (SiO 2 - glass)

Liquids & Solids Changes of State

Phase Changes

 Evaporation molecules at the surface gain enough energy to overcome IMF  Volatility measure of evaporation rate depends on temp & IMF

Phase Changes Kinetic Energy # of Particles p. 477 Boltzmann Distribution tempvolatilityIMFvolatility

Phase Changes  Equilibrium trapped molecules reach a balance between evaporation & condensation

Phase Changes  Vapor Pressure pressure of vapor above a liquid at equilibrium IMFv.p.tempv.p. depends on temp & IMF directly related to volatility p.478 temp v.p.

Phase Changes  Boiling Point temp at which v.p. of liquid equals external pressure IMFb.p.P atm b.p. depends on P atm & IMF Normal B.P. - b.p. at 1 atm

 Which has a higher m.p.? polar or nonpolar? covalent or ionic? Phase Changes  Melting Point equal to freezing point polar ionic IMFm.p.

Phase Changes  Sublimation solid  gas v.p. of solid equals external pressure  EX: dry ice, mothballs, solid air fresheners

Heating Curves Melting - PE  Solid - KE  Liquid - KE  Boiling - PE  Gas - KE 

Heating Curves  Temperature Change change in KE (molecular motion) depends on heat capacity  Heat Capacity energy required to raise the temp of 1 gram of a substance by 1°C “Volcano” clip - water has a very high heat capacity

Heating Curves  Phase Change change in PE (molecular arrangement) temp remains constant  Heat of Fusion (  H fus ) energy required to melt 1 gram of a substance at its m.p.

Heating Curves  Heat of Vaporization (  H vap ) energy required to boil 1 gram of a substance at its b.p. usually larger than  H fus …why?  EX: sweating, steam burns, the drinking bird

Phase Diagrams  Show the phases of a substance at different temps and pressures.