Hesss Law Germain Henri Hess. Hesss Law In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same.

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Presentation transcript:

Hesss Law Germain Henri Hess

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.

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 kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O 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 kJ

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 kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ CH 4 C + 2H kJ Step #2: Keep reaction #2 unchanged, because CO 2 belongs on the product side C + O 2 CO kJ

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 kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ CH 4 C + 2H kJ C + O 2 CO kJ Step #3: Multiply reaction #2 by 2 2H 2 + O 2 2 H 2 O kJ

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 kJ C + O 2 CO kJ H 2 + ½ O 2 H 2 O kJ CH 4 C + 2H kJ C + O 2 CO kJ 2H 2 + O 2 2 H 2 O kJ Step #4: Sum up reaction and H CH 4 + 2O 2 CO 2 + 2H 2 O kJ

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 kJ O2O2 0 kJ CO kJ H2OH2O kJ H rxn = [ kJ + 2( kJ)] – [-74.80kJ] H rxn = kJ