Hesss Law Germain Henri Hess Hesss Law In

Hess’s Law Germain Henri Hess

Hess’s 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. ”

Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4: CH 4 + 2 O 2 CO 2 + 2 H 2 O Reaction C + 2 H 2 CH 4 C + O 2 CO 2 H 2 + ½ O 2 H 2 O CH 4 C + 2 H 2 Ho -74. 80 k. J -393. 50 k. J -285. 83 k. J +74. 80 k. J Step #1: CH 4 must appear on the reactant side, so we reverse reaction #1 and change the sign on H.

Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4: CH 4 + 2 O 2 CO 2 + 2 H 2 O Reaction C + 2 H 2 CH 4 C + O 2 CO 2 H 2 + ½ O 2 H 2 O CH 4 C + 2 H 2 C + O 2 CO 2 Ho -74. 80 k. J -393. 50 k. J -285. 83 k. J +74. 80 k. J -393. 50 k. J Step #2: Keep reaction #2 unchanged, because CO 2 belongs on the product side

Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4: CH 4 + 2 O 2 CO 2 + 2 H 2 O Reaction C + 2 H 2 CH 4 C + O 2 CO 2 H 2 + ½ O 2 H 2 O CH 4 C + 2 H 2 C + O 2 CO 2 2 H 2 + O 2 2 H 2 O Ho -74. 80 k. J -393. 50 k. J -285. 83 k. J +74. 80 k. J -393. 50 k. J -571. 66 k. J Step #3: Multiply reaction #2 by 2

Hess’s Law Example Problem Calculate H for the combustion of methane, CH 4: CH 4 + 2 O 2 CO 2 + 2 H 2 O Reaction C + 2 H 2 CH 4 C + O 2 CO 2 H 2 + ½ O 2 H 2 O CH 4 C + O 2 2 H 2 + O 2 CH 4 + 2 O 2 C + 2 H 2 CO 2 2 H 2 O CO 2 + 2 H 2 O Ho -74. 80 k. J -393. 50 k. J -285. 83 k. J +74. 80 k. J -393. 50 k. J -571. 66 k. J -890. 36 k. J Step #4: Sum up reaction and H

Calculation of Heat of Reaction Calculate H for the combustion of methane, CH 4: CH 4 + 2 O 2 CO 2 + 2 H 2 O Hrxn = Hf(products) - Hf(reactants) Substance CH 4 O 2 CO 2 H 2 O Hf -74. 80 0 -393. 50 -285. 83 k. J Hrxn = [-393. 50 k. J + 2(-285. 83 k. J)] – [-74. 80 k. J] Hrxn = -890. 36 k. J