What happens to thermal (heat) energy?

When objects of different temperature meet:

• Warmer object cools• Cooler object warms• Thermal energy is transferred

•qwarmer = -qcooler

specific heat x mass x T = specific heat x mass x T

warmer object cooler object

Heat transfer between substances:

owood o

Jq = 1.8 5 g (-18 C)

g C

= -160 J

oCu o

Jq = 0.385 5 g (+82 C)

g C

= +160 J

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 (Hfus) for Water: 333 J/g

Boiling:

Heat of Vaporization (Hvap) for Water: 2256 J/g

Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC

Warm ice: (Specific heat = 2.06 J/g-oC)

Melt ice:

Warm water (s.h. = 4.18 J/g-oC)

Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC

Boil water:

Warm steam (s.h. = 1.92 J/g-oC)

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