Phase Changes

solidliquidgas melting freezing vaporizing condensing sublimination endothermic exothermic

Phase changes and their enthalpy changes.

Heating Curve

Heating Curve of Water

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 = (specific heat )(amount)(ΔT) q = (amount)(enthalpy of phase change or ΔH)

Heats of Fusion and Vaporization

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

Phase Diagrams Pressure Temperature vaporization condensation critical point triple point Solid Liquid Gas 1 atm normal melting pt. normal boiling pt. Fusion Curve Vapor Pressure Curve Sublimation Curve melting freezing sublimation deposition

Phase diagrams for CO 2 and H 2 O. CO 2 H2OH2O