# Hess's LawPage 1

Slide 1

## Hess’s Law

Germain Henri Hess

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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.”

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## Hess’s Law Example Problem

Step #1: CH4 must appear on the reactant side, so we reverse reaction #1 and change the sign on H.

CH4  C + 2H2 +74.80 kJ

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Hess’s Law Example Problem

CH4  C + 2H2 +74.80 kJ

Step #2: Keep reaction #2 unchanged, because CO2 belongs on the product side

C + O2  CO2 -393.50 kJ

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Hess’s Law Example Problem

CH4  C + 2H2 +74.80 kJ

C + O2  CO2 -393.50 kJ

Step #3: Multiply reaction #2 by 2

2H2 + O2  2 H2O -571.66 kJ

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Hess’s Law Example Problem

CH4  C + 2H2 +74.80 kJ

C + O2  CO2 -393.50 kJ

2H2 + O2  2 H2O -571.66 kJ

Step #4: Sum up reaction and H

CH4 + 2O2  CO2 + 2H2O

-890.36 kJ

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## Calculation of Heat of Reaction

Hrxn =  Hf(products) -   Hf(reactants)

Hrxn = [-393.50kJ + 2(-285.83kJ)] – [-74.80kJ]

Hrxn = -890.36 kJ