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Transcript
States of Matter Gases, Liquids and Solids Kinetic Molecular Theory of Gases Describes the motion of gas particles Points of Kinetic Molecular Theory: Gas particles are point masses Explains low density and compressibility Gas particles are in constant motion They move in straight lines at 100-1000m/s They change direction only when they run into something Collisions with container walls cause pressure Explains Brownian motion and diffusion/effusion Brownian Motion Diffusion and Effusion Diffusion is the mixing of gases Effusion is the migration of a gas through a tiny orifice into an evacuated space Graham’s Law of effusion Kinetic energy of a particle E = ½mv2 For the same energy, a heavier particle moves more slowly [v = (2E/m)] Graham’s Law Comparing two particles with the same energy: ½mAvA2 = ½mBvB2 Rearrange and cancel: vA2/vB2 = mB/mA or vA/vB = (mB/mA) Graham’s Law Molar mass can be used for m, and the effusion rate is directly proportional to v Example problem: Find the molar mass of a gas that effuses at a rate twice as slow as helium. Solution: vA/vB = (mB/mA) If gas A is helium, vA/vB = 2, so (mB/4) = 2 (mB/4) = 4 and mB = 16g/mol Kinetic Theory Points of Kinetic Molecular Theory continued Gas particles are point masses Gas particles are in constant motion All collisions between gas particles are perfectly elastic No attractive forces between particles Gases at the same temperature have the same average kinetic energy Gas Pressure Pressure is force/area Units: N/m2 (Pascal) (kPa = 1000 Pascals) psi = pounds per square inch Barometers – pressure measured as height of a column of mercury in a mercury barometer Standard pressure = 760mmHg = 1 atm = 101.325kPa = 29.92inHg = 14.7psi Mercury barometer Open arm manometer Open arm manometer Closed arm manometer Dalton’s Law of Partial Pressures When different gases are mixed, every particle contributes equally to the total pressure In a gas mixture, the contribution of each component depends on the mole fraction of that component – more particles means more pressure PT = PA + PB + PC … Interparticle forces London dispersion forces (van der Waal’s forces) Present in all particles Dominant attractive force in non-polar molecular compounds Weakest of all interparticle forces Due to temporary dipoles formed by electron dislocation Interparticle forces Interparticle forces Interparticle forces Magnitude of London dispersion forces depends on Size of molecule – more surface area means stronger forces Polarizability of electron cloud – larger atoms’ electron clouds are more polarizable due to shielding Interparticle forces Dipole-dipole interactions – in polar molecules opposite poles attract. Stronger than LDF Interparticle forces Hydrogen bonding – strong interaction between hydrogens (d+) and electronegative atoms (d-) Hydrogen must be attached to an electronegative atom (usually O or N) Strongest non-bonding interaction Hydrogen bonding Hydrogen bonding in ice Hydrogen bonding in DNA Liquids More dense than gases, less than solids (except water) Incompressible Particles are in contact but able to move past each other Liquids (and gases) are fluids – able to flow Viscosity – resistance to flow Liquids Viscosity increases with molecular surface area (chain length) Viscosity decreases with increasing temperature Surface tension depends on strength of interparticle interactions Defined as the energy required to increase the surface area of a liquid by a certain amount Liquids Capillary action – ability of a liquid to rise in a narrow tube Happens when adhesive forces between tube and liquid are greater than cohesive forces in liquid Height to which the liquid will rise is a measure of the difference in adhesive and cohesive forces Solids Most dense state (except water) Particles vibrate in place Molecular solids Smallest particle is a molecule Molecules are composed of all nonmetals held together by covalent bonds Molecules are held next to each other by LDF, dipole-dipole interactions or H-bonds Solids Molecular solids Low MP/BP Insulators Usually crystalline Examples: water, sugar, caffeine Network solids Covalent network solids Covalent bond throughout Solids Highest MP/BP No smaller units Examples: C (diamond), Si, quartz Ionic solids All salts - composed of metal & nonmetal High MP/BP Simplest unit is “formula unit” Ions are held in place by attraction to oppositely charged ion Insulators unless melted or in solution Solids Metals Held together by nondirectional metallic bonds “Electron sea” of shared electrons Not usually brittle like crystalline solids Conductors of heat/electricity Ductile, malleable, luster Amorphous solids No regular arrangement No sharp melting point Examples: rubber, glass Crystals Have a regular, repeating pattern of atoms Unit cell is simplest repeating pattern Have sharp melting points Ionic and metallic crystals have high melting points Molecular crystals have low melting points Crystal types Cubic HALITE Cubic crystals FACE CENTERED CUBIC BODY CENTERED CUBIC Face centered cubic - halite Body centered cubic - CsCl Tetragonal crystals RUTILE (TiO2) Other crystal types HEXAGONAL (QUARTZ) ORTHORHOMBIC (CALCITE) Phase changes and energy Solid/liquid: melting and freezing Melting point: temperature at which vapor pressures of solid and liquid are equal Liquid/gas: vaporization and condensation Boiling point: temperature at which vapor pressure of liquid = atmospheric pressure Boiling: vaporization at BP Evaporation: vaporization at a lower temperature Boiling and evaporation Freezing point Phase changes Solid/gas: sublimation and deposition Energy and phase changes Melting is endothermic, freezing is exothermic Boiling is endothermic, condensation is exothermic Sublimation is endothermic, deposition is exothermic Phase diagrams Phase diagrams show conditions under which an element, compound or mixture will exist in a given state Variables are pressure (y) and temperature (x) Triple point: temperature and pressure where solid, liquid and gas can exist in equilibrium Critical temperature: temperature above which a substance cannot be liquified Critical pressure: pressure necessary to liquefy a substance at the critical temperature (together they make the critical point) Phase diagrams Phase diagrams – UF6 Phase diagrams – CO2 Phase diagram - water Phase diagrams – water (detailed) Heating curves Shows change in temperature as heat is added Slope of curve gives specific heat No change in temperature during phase changes Heating curves