Horvath Practice Problems 91

Horvath Practice Problems 91 - 4 H s for the component...

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Hess’ Law Both total energy, E, and enthalpy, H , are what are called state functions A state function is a function that depends only on the partic- ular state that the system is in. It is completely independent of how the system arrived at that state. For example consider heating in water from 300 K to 310 K. You would calculate the same 4 H or 4 E regardless as to whether you heated directly from 300 K to 310 K or if you f rstcoo ledto290Kthenheatedto360Kandthencoo ledto 310 K. An important consequence of enthalpy being a state function is Hess’ law which states that if a reaction is the sum of two or more reactions, 4
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Unformatted text preview: 4 H s for the component reactions. That is 4 H is solely determined by 4 H = H products H reactants (21.6) regardless of how the reactants became the products. For example, 4 H for the reaction 2 C 6 H 6 + 15 O 2 12 CO 2 + 6 H 2 O ( 2 1 . 7 ) is the same as the sum of the 4 H s for the following reactions 2 C 6 H 6 12 C + 6 H 2 (21.8) 12 C + 6 H 2 + 15 O 2 12 CO 2 + 6 H 2 O ( 2 1 . 9 ) Hesss law is very handy because it would be impossible to measure enthalpy changes for every imaginable reaction were it not for Hess law. 90 REVIEW OF THE FIRST LAW, ENTHALPY, AND HESS LAW...
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This document was uploaded on 11/01/2011 for the course CHM 2046 at University of Florida.

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