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

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.”

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

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

General Rule: