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Thermodynamics filled in class notes_Part_78

# Thermodynamics filled in class notes_Part_78 - 163 5.3...

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5.3. FIRST LAW ANALYSIS OF REACTING SYSTEMS 163 5.3 First law analysis of reacting systems One can easily use the first law to learn much about chemically reacting systems. 5.3.1 Enthalpy of formation The enthalpy of formation is the enthalpy that is required to form a molecule from combining its constituents at P = 0 . 1 MPa and T = 298 K . Consider the reaction (taken here to be irreversible) C + O 2 CO 2 . (5.123) In order to maintain the process at constant temperature, it is found that heat transfer to the volume is necessary. For the steady constant pressure process, one has U 2 U 1 = 1 Q 2 1 W 2 , (5.124) = 1 Q 2 integraldisplay 2 1 PdV, (5.125) = 1 Q 2 P ( V 2 V 1 ) , (5.126) 1 Q 2 = U 2 U 1 + P ( V 2 V 1 ) , (5.127) = H 2 H 1 , (5.128) 1 Q 2 = H products H reactants . (5.129) So 1 Q 2 = summationdisplay products n i h i summationdisplay reactants n i h i . (5.130) In this reaction, one measures that 1 Q 2 = 393522 kJ for the reaction of 1 kmole of C and O 2 . That is the reaction liberates such energy to the environment. So measuring the heat transfer can give a measure of the enthalpy difference between reactants and products.

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