Temperature can change Phase change can occur Reaction can take place When Heat is Transferred … Temperature can change Phase change can occur Reaction can take place

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) Total Energy:

Enthalpy Change and Chemical Reactions DH = energy needed to break bonds – energy released forming bonds Example: formation of water: DH = ?

Energy Diagrams

Chemical Reactions and Enthalpy Change C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) H = -2045 kJ What does this mean? How much energy is released by a 468-g tank of propane?

Chemical Reactions and Enthalpy Change C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) H = -2045 kJ How much energy is released by a 468-g tank of propane? How much water (starting at 20 oC) can this boil?

Return to Calorimetry Constant Pressure gives H Constant Volume give E Count the calorimeter, or not…

Bomb Calorimetry Experiment N2H4 + 3 O2  2 NO2 + 2 H2O Energy released = E absorbed by water + E absorbed by calorimeter Ewater = Ecalorimeter = Total E = H = energy/moles = 0.500 g N2H4 600 g water 420 J/oC

Hess’s Law If you can add the reactions, you can add the H’s.

Hess’s Law Enthalpy is a State Function.

Rules for manipulating Reaction Equations Reverse reactions: Change sign of H Multiple by a constant: Multiply sign of H by that constant

Hess’s Law: Your Lab Mg(s) + ½ O2(g)  MgO(s) H = ???

Standard Enthalpy of Formation Formation Reaction: Reaction that forms one mole of a compound from its elements in their natural states. K2CO3(s)?? Standard enthalpy of formation: Enthalpy change on forming one mole of a compound from its elements under standard conditions.