notes_Lecture_05_101110

# notes_Lecture_05_101110 - Lecture 5: Standard Enthalpies...

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Lecture 5: Standard Enthalpies • Reading: Zumdahl 9.5, 9.6 • Outline – Hess’s Law (9.5) • Examples of using Hess’s Law – Standard Enthalpies (9.6) • What is a standard enthalpy? • Defining standard states • Using standard enthalpies to determine Δ rxn • Heats of formation and Hess’s Law Additional examples of Ch 9 problems. We will not cover sections 9.7 or 9.8 this quarter.

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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 in a series of steps. This means we can use enthalpy changes for reactions that we can easily measure to determine the enthalpy changes for reactions that are not so easy to measure. Two characteristics of enthalpy will be useful in this endeavor: Enthalpy is a state function . If a reaction is reversed, the value of Δ H is also reversed. Enthalpy is an extensive property . If the coefficients in a balanced equation are multiplied by an integer, Δ H must be multiplied by the same integer.
Use the information below to determine Δ H for the reaction: 3 C(graphite) + 4 H 2 (g) b C 3 H 8 (g) C 3 H 8 (g) + 5 O 2 (g) b 3 CO 2 (g) + 4H 2 O(l) Δ H 1 = -2219.9 kJ C (graphite) + O 2 (g) b CO 2 (g) Δ H 2 = -393.5 kJ H 2 (g) + 1/2 O 2 (g) b H 2 O(l) Δ H 3 = -285.8 kJ 3 CO 2 (g) + 4H 2 O(l) b C 3 H 8 (g) + 5 O 2 (g) - Δ H 1 = +2219.9 kJ 3 C (graphite) + 3 O 2 (g) b 3 CO 2 (g) 3( Δ H 2 ) = -1180.5 kJ 4 H 2 (g) + 2 O 2 (g) b 4 H 2 O(l) 4( Δ H 3 ) = -1143.2 kJ 3 C(graphite) + 4 H 2 (g) b C 3 H 8 (g) - Δ H 1 + 3( Δ H 2 ) + 4( Δ H 3 ) Δ H rxn = 103.8 kJ + Hess’ Law Example

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The combustion of methane has a Δ H rxn of -802 kJ/mol: A. 802 kJ B. -401 kJ C. -1604 kJ D. 401 kJ CH 4 g ( ) + 2 O 2 g ( ) CO 2 g ( ) + 2 H 2 O g ( ) What is Δ H rxn for the following? 1 2 CO 2 g ( ) + H 2 O g ( ) 1 2 CH 4 g ( ) + O 2 g ( )
Hess Hints • Using Hess’s Law involves some degree of trial and error when you are manipulating the given chemical equations. • Some tips for success:

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## This note was uploaded on 09/29/2011 for the course CHM 101 taught by Professor Ma during the Spring '11 term at Beacon FL.

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notes_Lecture_05_101110 - Lecture 5: Standard Enthalpies...

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