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Horvath Practice Problems 90

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

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The fi rst law for this case says 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 fi ned a new measure of energy called enthalpy ( H ) Enthalpy is de fi 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 reminds us that this is true only for constant pressure. (We will always work at constant pressure in this course.) Consider again the expanding gas example. Now if we add 10J of heat to the gas then
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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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