My textbook derived the equation ΔH = ΔE + ΔnRT (H is enthalpy, E is internal energy (heat exchanged + work )) from the fact that ΔH = ΔE + PΔV at constant pressure. This derivation, however, requires that temperature be constant; otherwise, the resulting equation would be ΔH = ΔE + ΔnRΔT (ΔT instead of T). A paragraph or two later they give an example of an exothermic combustion reaction where q = -3264 kJ/mol (where q is the heat absorbed by the reaction, since it is negative heat is given off). Then they go on to use the equation ΔH = ΔE + ΔnRT. But this equation assumes that temperature is constant. How can the temperature be constant when the heat of reaction is -3264 kJ/mol? Isn't heat given off thereby lowering the temperature of the combusted substance? Thanks.