Conceptually Easy Example with Annoying Algebra: If we mix 250 g H2O at 95 oC with 50 g H2O at 5 oC, what will the final temperature be?

Thermal Energy and Phase Changes First: What happens?

Thermal Energy and Phase Changes First: What happens?

Thermal Energy and Phase Changes First: What happens?

But what’s really happening? 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

Energy and Phase Changes: Quantitative Treatment Melting: Heat of Fusion (DHfus) for Water: 333 J/g Boiling: Heat of Vaporization (DHvap) for Water: 2256 J/g

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

Energy and Chemical Reactions Lots of different types of energy. We use Enthalpy: Heat exchanged under constant pressure.

Energy/Enthalpy Diagrams

Some Examples of Enthalpy Change 2 C(s) + 2 H2(g)  C2H4(g) H = +52 kJ