Heat in Changes of State

Energy Changes Accompanying Phase Changes Solid Liquid Gas Melting Freezing Deposition CondensationVaporization Sublimation Energy of system Brown, LeMay, Bursten, Chemistry  2000, page 405

solid liquid gas Heat added Temperature ( o C) A B C D E Heating Curve for Water LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World, 1996, page 487

solid liquid gas vaporization condensation melting freezing Heat added Temperature ( o C) A B C D E Heating Curve for Water LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World, 1996, page 487

Heating Curves Phase Change –change in PE (molecular arrangement) –temp remains constant Heat of Fusion (  H fus ) –energy required to melt a specific amount of a substance at its melting point Heat of Vaporization (  H vap ) –energy required to vaporize a specific amount of a substance at its boiling point

Heating Curves Temperature ( o C) Time Melting - PE  Solid - KE  Liquid - KE  Boiling - PE  Gas - KE  Endothermic Exothermic

Heating Curves Temperature Change –change in KE (molecular motion) –depends on heat capacity Heat Capacity –energy required to raise the temp of 1 gram of a substance by 1°C –water has a very high heat capacity Courtesy Christy Johannesson The quantity of a substance, the amount of heat transferred, its heat capacity, and the temperature change are related in two ways: q = nCpΔT where n = number of moles of substance q = mCsΔT where m = mass of substance in grams

Calculating Energy Changes - Heating Curve for Water Temperature ( o C) Time  H = mol x  H fus  H = mol x  H vap Heat = mass x  t x C p, liquid Heat = mass x  t x C p, gas Heat = mass x  t x C p, solid

Phase Diagrams Show the phases of a substance at different temps and pressures. Courtesy Christy Johannesson