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Energy and Chemical Reactions Part 2 Enthalpy continued Many reactions involve little if any PV work so most or all of the energy change occurs as a transfer of heat. Reactions that do not involve gases. Gases do not appear in many reactions. Liquids and solids undergo very small volume changes so V 0 and PV 0. H E Reactions in which the amount (mol) of gas does not change. When the total amount of gaseous reactants equals the total amount of gaseous products, V = 0, PV = 0, and H = E. Reactions in which the amount (mol) of gas does change. In these cases PV 0. But since q is usually so much larger than PV, H is very close to E. Most of E occurs as heat transfer so H E. Exo- and Endothermic Processes Enthalpy is a state function. The enthalpy change of a reaction is called heat of reaction, H rxn , which is H final H initial or H products H reactants .... View Full Document

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