Figure 12.2 Phase changes and their enthalpy changes.
Figure 12.1 Heats of vaporization and fusion for several common substances.
Figure 12.3 A cooling curve for the conversion of gaseous water to ice.
Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average E k as the most probable speed of the molecules changes. Quantitative Aspects of Phase Changes During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in E p, as the average distance between molecules changes. q = (amount)(molar heat capacity or specific heat )( T) q = (amount)(enthalpy of phase change or ΔH)
Problem How much energy is transferred when 125 g of water are converted from 25 o C to 70 o C? To 120 o C? ΔH vap = 2.26 kJ/g ΔH fus = 0.33 kJ/g sp.ht ice = 2.09 J/g o C sp.ht water = 4.18 J/g o C sp.ht steam = 2.10 J/g o C
Figure 12.8 Phase diagrams for CO 2 and H 2 O. CO 2 H2OH2O