# Lecture4 - Lecture 4: Hess' Law Reading: Zumdahl 9.5...

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Lecture 4: Hess’ Law Reading: Zumdahl 9.5 • Outline – Definition of Hess’ Law – Using Hess’ Law (examples) 1

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What is Hess’ Law • We have learned: Enthalpy is a state function. As such, ΔH for going from some initial state to some final state is pathway independent. • Hess’ Law: ΔH for a process involving the transformation of reactants into products is not dependent on pathway. Therefore, we can pick a single-step pathway to calculate ΔH for a reaction. 2
An Example 3

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Using Hess’ Law • When calculating ΔH for a chemical reaction as a single step, we can use combinations of reactions as “pathways” to determine ΔH for our “single step” reaction. 2NO 2 (g) N 2 (g) + 2O 2 (g) q 2NO 2 (g) N 2 (g) + 2O 2 (g) 4
Example (cont.) • Our reaction of interest is: N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH = 68 kJ This reaction can also be carried out in two steps: N 2 (g) + O 2 (g) 2NO(g) ΔH = 180 kJ 2NO (g) + O 2 (g) 2NO 2 (g) ΔH = -112 kJ 5

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• Our reaction of interest is: N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH = 68 kJ N 2 (g) + O 2 (g) 2NO(g) + O 2 (g) 2NO 2 (g) 6
Example (cont.) • If we take the previous two reactions and add them, we get the original reaction of interest: N 2 (g) + O 2 (g) 2NO(g) ΔH = 180 kJ 2NO (g) + O 2 (g) 2NO 2 (g) ΔH = -112 kJ N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH = 68 kJ 7

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Example (cont.) • Note the important things about this example, the sum of ΔH for the two reaction steps is equal to the ΔH for the reaction of interest. the point: We can combine reactions of known ΔH to determine the ΔH for the “combined” reaction. 8
Hess’ Law: Details • One can always reverse the direction of a reaction when making a combined reaction. When you do this, the sign of ΔH changes .

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## This note was uploaded on 01/18/2012 for the course CHEM 152B taught by Professor Zhang during the Winter '12 term at University of Washington.

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Lecture4 - Lecture 4: Hess' Law Reading: Zumdahl 9.5...

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