Chemical and Biomolecular Engineering

Official blog of the Lehigh University Chemical Engineers

Chemical Engineering and Standard Heats of Reaction

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The final section in ChemE 31 is balancing systems of reactive process. For systems like this a large heat transfer be it cooling or heating. The change in enthalpy for a chemical reaction is defined as the heat of reaction (delta Hr). The heat of reaction can be found using a compounds heat of formation. Heat of formation is the change in enthalpy for the formation of a compound. For example Benzene (C6H6) has a heat of formation of 48.66 kJ/mol. The heat of formation comes from the chemical reaction to get benzene 6C(s) + 3H2(g). The heats of formation is found from the tables in the textbook. The heats of reaction can be found by subtracting the heats of formation of products by the heats of formation of the products. For some substances such as O2 and H2 the heat of formation is 0 this is because both are elementary species and are found that way naturally. One other method to finding the heat of reaction is to use the heats of combustion (delta Hc) is the heat produced of that substance when reacted with oxygen. These values are also found in the textbook. However the Hr found by using heat of combustion is that it is reactant minus products and not the other way around.

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