Molar reaction enthalpy and heat

During a process in a closed system at constant pressure with expansion work only, the enthalpy change equals the energy transferred across the boundary in the form of heat dH = dq (Eq. 5.3.7). Thus for the molar reaction enthalpy AfH = (dH/d )r,p, which refers to a process not just at constant pressure but also at constant temperature, we can write 

Note that when there is nonexpansion work, such as electrical work, the enthalpy change is not equal to the heat. For example, if we compare a reaction taking place in a galvanic cell with the same reaction in a reaction vessel, the heats at constant T and p for a given change of are different, and may even have opposite signs. The value of AtH is the same in both systems, but the ratio of heat to advancement, dq/d, is different. 

An exothermic reaction is one for which ArH is negative, and an endothermic reaction is one for which ArH is positive. Thus in a reaction at constant temperature and pressure 

T/)ermodYnam anofG/ em/sfry, second edition, version 3 2011 by Howard De A e. Lat t version ww.chem.umd.edu/thermobook 

These comments apply not just to chemical reactions, hut to the other chemical processes at constant temperature and pressure discussed in this chapter. 

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