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So O2 has zero of heat of formation (or enthalpy of formation). The enthalpy of a reaction is defined like this: H = Hproducts-H reactants. The enthalpy of reaction in this case, is the same as the heat of combustion. Hproducts is obtained summing the heats of formation.
It can be shown that the eï¬ciency = 1â |Q out| Q in of a Carnot engine does not depend on the working substance. So, we use an ideal gas to calculate it. Since 4U = 0 during an isothermal expansion of an ideal gas, the heat absorbed during step 1 is Q in = W = Z 2 1 P dV = Z 2 1 nRT h V dV = nRT h ln V 2 V 1 .