Lec05_drobny_11 - This reaction can also be carried out in...

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Lecture 5: Hess’ Law • Reading: Zumdahl 9.5 • Outline – Defnition oF Hess’ Law – Using Hess’ Law (many examples)
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Hess’ Law Defned • From lecture 3: Enthalpy is a state ±unction. As such, Δ H ±or going ±rom some initial state to some fnal state is pathway independent. • Hess’ Law: Δ H ±or a process involving the trans±ormation o± reactants into products is not dependent on pathway. There±ore, we can pick a single-step pathway to calculate Δ H ±or a reaction.
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Hess’ Law: An Example
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Using Hess’ Law • When calculating Δ H for a chemical reaction as a single step, we can use combinations of reactions as “pathways” to determine Δ H for our “single step” reaction.
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Example (cont.) • Our reaction of interest is: N 2 (g) + 2O 2 (g) 2NO 2 (g) Δ H = 68 kJ
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Unformatted text preview: This reaction can also be carried out in two steps: N 2 (g) + O 2 (g) 2NO(g) H = 180 kJ 2NO (g) + O 2 (g) 2NO 2 (g) H = -112 kJ 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) H = 180 kJ 2NO (g) + O 2 (g) 2NO 2 (g) H = -112 kJ N 2 (g) + 2O 2 (g) 2NO 2 (g) H = 68 kJ Example (cont.) Note the important things about this example, the sum of H for the two reaction steps is equal to the H for the reaction of interest. Big point: We can combine reactions of known H to determine the H for the combined reaction. Hess Law: Burning Methane Consider the 3 reactions: Reverse reaction 1; double reaction 3 : Add:...
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This note was uploaded on 01/18/2012 for the course CHEM 152A taught by Professor Chiu during the Fall '12 term at University of Washington.

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Lec05_drobny_11 - This reaction can also be carried out in...

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