Phase Changes

Phase Changes solid liquid gas exothermic sublimination vaporizing melting solid liquid gas condensing freezing endothermic

Phase changes and their enthalpy changes. Figure 12.2 Phase changes and their enthalpy changes.

Heats of vaporization and fusion for several common substances. Figure 12.1 Heats of vaporization and fusion for several common substances.

A cooling curve for the conversion of gaseous water to ice. Figure 12.3 A cooling curve for the conversion of gaseous water to ice.

Heating Curve

Quantitative Aspects of Phase Changes Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average Ek as the most probable speed of the molecules changes. q = (amount)(molar heat capacity or specific heat )(T) During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in Ep, as the average distance between molecules changes. q = (amount)(enthalpy of phase change or ΔH)

Problem How much energy is transferred when 125 g of water are converted from 25oC to 70oC? To 120oC? ΔHvap = 2.26 kJ/g ΔHfus = 0.33 kJ/g sp.htice = 2.09 J/g oC sp.htwater = 4.18 J/g oC sp.htsteam = 2.10 J/g oC

Phase diagrams for CO2 and H2O. Figure 12.8 CO2 H2O