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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 P∆V ≈ 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, P∆V = 0, and ∆H = ∆E. Reactions in which the amount (mol) of gas does change. In these cases P∆V ≠ 0. But since q is usually so much larger than P∆V, ∆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 . Exothermic process releases heat,
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