Intermolecular Forces and States of Matter

INTERMOLECULAR FORCES States of matter • Physical properties of liquids and solids are due to intermolecular forces • Gases are highly compressible and assume the shape and volume of their container. o Gas molecules are far apart and don’t interact much with each other • Liquids are almost incompressible and assume the shape but not the volume of their container o Liquid molecules are closer to each other and interact • Solids are incompressible and have a definite shape and volume • Solids and liquids are condensed phases Intermolecular forces • Covalent bonds are intramolecular forces • Intermolecular forces are much weaker than intramolecular forces • When a substance melts or boils (changes state) intermolecular forces are broken • Boiling points reflect intermolecular force strength. A high boiling point indicates strong forces • Van der Waals forces are intermolecular forces between neutral molecules Ion dipole forces • An interaction between an ion and the partial charge on a polar molecule • It is the strongest of all intermolecular forces • NaCl in water is a good example Dipole-Dipole forces • Forces that exist between neutral polar molecules • The partially positive area attracts the partially negative area • Generally weaker than ion-dipole • Higher dipole moment= higher boiling point London dispersion forces • Weakest of intermolecular forces • Attraction of adjacent nonpolar molecules • Electron distribution creates a temporary dipole( instantaneous). This dipole induces an adjacent molecule. That then attract each other • London dispersion forces exist among all molecules • Effected by: o Proximity o Polarizability is the eases that the electron cloud can be deformed. The larger a molecule, the more polarizable o Molecule shape Hydrogen bonds • A special type of dipole-dipole interaction • H bonding requires: o H bonded to an electronegative element(F,O and N) o It also requires an unshared pair of electrons on a nearby electronegative ion or element • Electrons in the H-X bond lie closer to X • Hydrogen bonds are very important in biological systems Viscosity • Resistance of a liquid to flow • Depends on: o Intermolecular forces. Higher = more viscous o Tendency of molecules to become tangled o Temperature lower = more viscous Surface tension • Bulk molecules (those in the liquid) are equally attracted to their neighbors • Surface molecules are attracted to the bulk molecules. Therefore they are more closely packed. This causes the liquid to behave like it has a skin. • Surface tension is the amount of energy required to increase the surface area of a liquid by a unit amount • Strong intermolecular forces cause high surface tension. Water with hydrogen bonding, Hg even stronger with metallic bonds • Cohesive forces bind molecules to each other • Adhesive forces bind molecules to a surface • Meniscus is the shape of a liquid surface. If adhesive forces are greater, the liquid will be more attracted to the container and will form a U • Capillary action is the movement up narrow tubes. Liquid will climb until adhesive and cohesive forces are balanced by gravity Phase changes • Phase changes are changes of state o Sublimation: solid to gas, endothermic o Melting or fusion: solid to liquid, endothermic, heat of fusion o Vaporization: liquid to gas, endothermic, heat of vaporization o Deposition: gas to solid, exothermic o Condensation: gas to liquid, exothermic o Freezing: liquid to solid, exothermic • Critical temperature is the highest temp at which a substance can exist as a liquid • Critical pressure is the pressure required for liquefaction at the critical temp Vapor pressure • Some molecules on the surface of a liquid have enough energy to escape • These molecules move to the gas phase • As the gas phase increases, some molecules return to the liquid • When the pressure becomes constant, this is the equilibrium vapor pressure • Liquids that evaporate easily are termed volatile Phase diagrams • Plot of temperature vs pressure • Triple point is the temp and pressure at which all 3 phases are in equilibrium Bonding in solids • There are 4 types of solids o Molecular o Covalent network o Ionic o Metallic • Molecular o Atoms or molecules held together by intermolecular forces o Weak forces yield low melting points o Poor thermal and electrical conductivity o CH4 , sucrose • Covalent network o Atoms held together in large networks by covalent bonds o Higher melting points o Diamond,graphite, quartz • Ionic solids o Ions held together by ionic bonds o Hard, brittle, high melting point o Sodium chloride • Metallic solids o Consist entirely of metal atoms o Can be soft or hard o High melting points o Good conductors o Ductile o Al, Cu, Fe, Au o Metal ions float in a sea of delocalized electrons