1. Hesss Law Germain Henri Hess

2. Hesss Law In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.

3. Hesss Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH 4 -74.80 kJ C + O 2 CO 2 -393.50 kJ H 2 + ½ O 2 H 2 O-285.83 kJ Step #1: CH 4 must appear on the reactant side, so we reverse reaction #1 and change the sign on H. CH 4 C + 2H 2 +74.80 kJ

4. Hesss Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH 4 -74.80 kJ C + O 2 CO 2 -393.50 kJ H 2 + ½ O 2 H 2 O-285.83 kJ CH 4 C + 2H 2 +74.80 kJ Step #2: Keep reaction #2 unchanged, because CO 2 belongs on the product side C + O 2 CO 2 -393.50 kJ

5. Hesss Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH 4 -74.80 kJ C + O 2 CO 2 -393.50 kJ H 2 + ½ O 2 H 2 O-285.83 kJ CH 4 C + 2H 2 +74.80 kJ C + O 2 CO 2 -393.50 kJ Step #3: Multiply reaction #2 by 2 2H 2 + O 2 2 H 2 O -571.66 kJ

6. Hesss Law Example Problem Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O Reaction H o C + 2H 2 CH 4 -74.80 kJ C + O 2 CO 2 -393.50 kJ H 2 + ½ O 2 H 2 O-285.83 kJ CH 4 C + 2H 2 +74.80 kJ C + O 2 CO 2 -393.50 kJ 2H 2 + O 2 2 H 2 O -571.66 kJ Step #4: Sum up reaction and H CH 4 + 2O 2 CO 2 + 2H 2 O-890.36 kJ

7. Calculation of Heat of Reaction Calculate H for the combustion of methane, CH 4 : CH 4 + 2O 2 CO 2 + 2H 2 O H rxn = H f (products) - H f (reactants) Substance H f CH 4 -74.80 kJ O2O2 0 kJ CO 2 -393.50 kJ H2OH2O-285.83 kJ H rxn = [-393.50kJ + 2(-285.83kJ)] – [-74.80kJ] H rxn = -890.36 kJ