05_Hess Law and Heats of Formation

05_Hess Law and Heats of Formation - Hesss Law H is well...

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Hess’s Law 2200 H is well known for many reactions, and it is inconvenient to measure H for every reaction in which we are interested. However, we can estimate H using published H values and the properties of enthalpy.
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Hess’s Law Hess’s law states that “[i]f a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”
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Hess’s Law Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.
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Hess’s Law Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.
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Practice—Hess’s Law Given the following information: Cu( s ) + Cl 2 ( g ) CuCl 2 ( s ) H° = −206 kJ 2 Cu( s ) + Cl 2 ( g ) 2 CuCl( s ) H° = −36 kJ Calculate the H° for the reaction below: Cu( s ) + CuCl 2 ( s ) 2 CuCl( s ) H° = ? kJ
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Example—Hess’s Law Given the following information: 2 NO( g ) + O 2 ( g ) 2 NO 2 ( g ) H° = −116 kJ 2 N 2 ( g ) + 5 O 2 ( g ) + 2 H 2 O( l ) 4 HNO 3 ( aq ) H° = −256 kJ N 2 ( g ) + O 2 ( g ) 2 NO( g ) H° = +183 kJ Calculate the H° for the reaction below: 3 NO 2 ( g ) + H 2 O( l ) 2 HNO 3 ( aq ) + NO( g ) H° = ?
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05_Hess Law and Heats of Formation - Hesss Law H is well...

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