10.4 Phase Changes

10.4 Phase Changes    fusion (melting): s  l vaporization: l  g sublimation: s  g Phase Changes - note structures freezing: l  s condensation: g  l deposition: g  s Enthalpy – DH – heat energy  DH fusion and vaporization DH + is endothermic   Heat is added, required  DH – is exothermic   Heat is released, heat exits   DHfus = heat of fusion = energy to convert solid to liquid, energy needed to melt DHvap = heat of vaporization = energy to convert liquid to gas, energy needed to boil Ignore DS and DG – we do that in CHM 152 Look at Figure 10.9 Heating Curve Why does T stay constant during fusion (melting) and vaporization?   How is energy used between A and B, C and D?  Figure 11.38 Water 1 Ice 1  The temp stays constant during melting and boiling because all the energy is going into breaking the IMF that is holding the solid / liquid together. Note melting point and freezing point are the same temp. Boiling point and condensing point are also the same temp. mp = fp for water = 0oC bp = cp for water = 100oC Temp chgs of state 8 1 answers    Xe because it is bigger, more electrons, more London force than Kr, so higher bp CH3Cl because it is polar and CH4 is nonpolar NH3 because it has the stronger H bonding forces holding it together 10.5 Vapor Pressure  10.5 Evap, VP and bp Vaporizing liquid Liquid evaporates, gaseous molecules exert a pressure (vapor pressure) in a closed container that can be measured as shown below   Boiling – quick heating of a liquid to break IMF and turn liquid into gas. Temperature where this occurs is the bp. Evaporation – no heat added, very slow, surface molecules / atoms “pop” out and float away. Only the higher kinetic energy ones pop out, so this lowers the KE of the sample left behind, thus it feels cooler. This occurs at lower temp than the bp. VP     A liquid with a lower IMF will actually have a higher VP because the molecules evaporate easier, so more pressure A liquid with a higher IMF will have a lower VP because it is harder for the molecules to go to the gas phase, they prefer to stay liquid, so less gas, less pressure. So as IMF increase, VP decrease As IMF increase, bp increase Figure 10.11 10.6 Kinds of Solids    Amorphous: random arrangement (rubber) Ionic: ordered arrangement of ions (NaCl), has ionic bonds, high mp See Figure 10.14 Kinds of Solids   Molecular (left): covalent molecules in an ordered arrangement (sucrose, ice); intermolecular forces hold molecules together, low mp Covalent network (right): atoms connected by covalent bonds in all directions in 3D array (quartz, diamonds), high mp 2 Kinds of solids 10.11 Phase Diagram Atomic Phase diagrams  Metallic – has metallic bonds, mp varies, conducts well, solid gold, sodium, silver, etc  Features of a phase diagram:  Triple point: temperature and pressure at which all three phases (s, l, and g) exist and are in equilibrium  Vaporization curve: equilibrium between liquid and gas  Melting curve: equilibrium between solid and liquid  Normal melting point: melting point at 1 atm  Normal boiling point: boiling point at 1 atm Label Phase diagrams 27 10.11 Phase Diagram: Water Critical Temperature and Pressure Low Temp Boil   Phase Diagram  Critical point: liquid and gas phases are indistinguishable Critical temperature, Tc : highest temperature at which a substance can exist as a liquid (cannot be liquified) no matter how much pressure is applied Critical pressure, Tp : minimum pressure that must be applied to bring about liquefaction at the critical temperature Phase Diagram: Carbon DioxidePhase DiaWorked Ex. 10.11, Problems 10.17, 10.18, 19 Grams: H2O and CO2 30 3