1. Some Examples of Enthalpy Change 2 C(s) + 2 H 2 (g)  C 2 H 4 (g)  H = +52 kJ

2. Enthalpy Change and Chemical Reactions  H is usually more complicated, due to solvent and solid interactions. So, we measure  H experimentally. Calorimetry Run reaction in a way that the heat exchanged can be measured. Use a “calorimeter.”

5. Bomb Calorimetry Experiment N 2 H 4 + 3 O 2  2 NO 2 + 2 H 2 O Energy released = E absorbed by water + E absorbed by calorimeter E water = E calorimeter = Total E =  H = energy/moles = 0.500 g N2H4 600 g water 420 J/ o C

6. 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

7. General Rule: