Horvath Practice Problems 90

Horvath Practice Problems 90 - 4 H = 10 J During a chemical...

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The f rstlawforth iscasesays 4 E = q (positive) + w (negative) (21.2) Therefore no matter what 4 E<q (21.3) So the change in total energy is less than the amount of heat energy we added to the system. If we add 10 J of heat energy to the system the total energy does not change by 10 J This is somewhat inconvenient so chemists have de f ned a new measure of energy called enthalpy ( H ) Enthalpy is de f ned such that at constant pressure it is equal to the amount of heat energy taken in by the system 4 H q P , (21.4) where the subscript P r em ind su
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Unformatted text preview: 4 H = 10 J During a chemical reaction it is often rather convenient to mea-sure the heat given o f by a reaction. In fact, you will do this in lab. If the heat is measured at constant pressure then it is simply the change in enthalpy for the reaction: 4 H = H products H reactants (21.5) If 4 H is positive that means that q P is positive and heat energy is taken in by the system. Reactions with positive 4 H are called endothermic reactions. If 4 H is negative that means that q P is negative and heat energy is given o f by the system. Reactions with negative 4 H are called exothermic reactions. REVIEW OF THE FIRST LAW, ENTHALPY, AND HESS LAW 89...
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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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