What happens to thermal (heat) energy? When objects of different temperature meet: Warmer object cools Cooler object warms Thermal energy is transferred q warmer = -q cooler specific heat x mass x  T = specific heat x mass x  T warmer object cooler object

Heat transfer between substances:

Thermal Energy and Phase Changes First: What happens?

Warming: Molecules move more rapidly Kinetic Energy increases Temperature increases Melting/Boiling: Molecules do NOT move more rapidly Temperature remains constant Intermolecular bonds are broken Chemical potential energy (enthalpy) increases But what’s really happening?

Energy and Phase Changes: Quantitative Treatment Melting: Heat of Fusion (  H fus ) for Water: 333 J/g Boiling: Heat of Vaporization (  H vap ) for Water: 2256 J/g

Total Quantitative Analysis Convert 40.0 g of ice at –30 o C to steam at 125 o C Warm ice: (Specific heat = 2.06 J/g- o C) Melt ice: Warm water (s.h. = 4.18 J/g- o C)

Total Quantitative Analysis Convert 40.0 g of ice at –30 o C to steam at 125 o C Boil water: Warm steam (s.h. = 1.92 J/g- o C)

Enthalpy Change and Bond Energies  H = energy needed to break bonds – energy released forming bonds Example: formation of water:  H = [498 + (2 x 436)] – [4 x 436] kJ = -482 kJ