1. 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?

2. Thermal Energy and Phase Changes
First: What happens?

3. Thermal Energy and Phase Changes
First: What happens?

4. Thermal Energy and Phase Changes
First: What happens?

5. 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

6. 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

7. 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)

8. 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)

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

10. Energy/Enthalpy Diagrams

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