Thermodynamics

ing which chemical energy is released in the form of heat). Therefore, someheat is transferred from the combustion chamber to the surroundings duringthis process, which is 393,520 kJ/kmol CO 2 formed. (When one is dealingwith chemical reactions, it is more convenient to work with quantities perunit mole than per unit time, even for steady-flow processes.)The process described above involves no work interactions. Therefore,from the steady-flow energy balance relation, the heat transfer during thisprocess must be equal to the difference between the enthalpy of the productsand the enthalpy of the reactants. That is,Q H prod H react 393,520 kJ>kmol(15–5)Since both the reactants and the products are at the same state, the enthalpychange during this process is solely due to the changes in the chemical compositionof the system. This enthalpy change is different for different reactions,and it is very desirable to have a property to represent the changes inchemical energy during a reaction. This property is the enthalpy of reactionh R , which is defined as the difference between the enthalpy of the productsat a specified state and the enthalpy of the reactants at the same statefor a complete reaction.For combustion processes, the enthalpy of reaction is usually referred toas the enthalpy of combustion h C , which represents the amount of heatreleased during a steady-flow combustion process when 1 kmol (or 1 kg)of fuel is burned completely at a specified temperature and pressure(Fig. 15–17). It is expressed ash R h C H prod H react(15–6)which is 393,520 kJ/kmol for carbon at the standard reference state. Theenthalpy of combustion of a particular fuel is different at different temperaturesand pressures.The enthalpy of combustion is obviously a very useful property for analyzingthe combustion processes of fuels. However, there are so many differentfuels and fuel mixtures that it is not practical to list h C values for allpossible cases. Besides, the enthalpy of combustion is not of much usewhen the combustion is incomplete. Therefore a more practical approachwould be to have a more fundamental property to represent the chemicalenergy of an element or a compound at some reference state. This propertyis the enthalpy of formation h – f, which can be viewed as the enthalpy of asubstance at a specified state due to its chemical composition.To establish a starting point, we assign the enthalpy of formation of allstable elements (such as O 2 ,N 2 ,H 2 , and C) a value of zero at the standardreference state of 25°C and 1 atm. That is, h – f 0 for all stable elements.(This is no different from assigning the internal energy of saturated liquidwater a value of zero at 0.01°C.) Perhaps we should clarify what we meanby stable. The stable form of an element is simply the chemically stableform of that element at 25°C and 1 atm. Nitrogen, for example, exists indiatomic form (N 2 ) at 25°C and 1 atm. Therefore, the stable form of nitrogenat the standard reference state is diatomic nitrogen N 2 , not monatomicnitrogen N. If an element exists in more than one stable form at 25°C and1 atm, one of the forms should be specified as the stable form. For carbon,for example, the stable form is assumed to be graphite, not diamond.1 kmol C25°C, 1 atm1 kmol O 225°C, 1 atmChapter 15 | 763h C = Q = –393,520 kJ/kmol CCombustionprocess1 kmol CO 225°C, 1 atmFIGURE 15–17The enthalpy of combustion representsthe amount of energy released as afuel is burned during a steady-flowprocess at a specified state.