Thermodynamics

762 | ThermodynamicsATOMMOLECULENuclear energyChemical energyLatent energyATOMSensibleenergyMOLECULEFIGURE 15–14The microscopic form of energy of asubstance consists of sensible, latent,chemical, and nuclear energies.15–3 ENTHALPY OF FORMATIONAND ENTHALPY OF COMBUSTIONWe mentioned in Chap. 2 that the molecules of a system possess energy invarious forms such as sensible and latent energy (associated with a changeof state), chemical energy (associated with the molecular structure),and nuclear energy (associated with the atomic structure), as illustrated inFig. 15–14. In this text we do not intend to deal with nuclear energy. Wealso ignored chemical energy until now since the systems considered in previouschapters involved no changes in their chemical structure, and thus nochanges in chemical energy. Consequently, all we needed to deal with werethe sensible and latent energies.During a chemical reaction, some chemical bonds that bind the atoms intomolecules are broken, and new ones are formed. The chemical energy associatedwith these bonds, in general, is different for the reactants and theproducts. Therefore, a process that involves chemical reactions involveschanges in chemical energies, which must be accounted for in an energybalance (Fig. 15–15). Assuming the atoms of each reactant remain intact (nonuclear reactions) and disregarding any changes in kinetic and potentialenergies, the energy change of a system during a chemical reaction is due toa change in state and a change in chemical composition. That is,¢E sys ¢E state ¢E chem(15–4)1 kmol C25°C, 1 atm1 kmol O 225°C, 1 atmSensibleenergyATOM ATOMATOMFIGURE 15–16393,520 kJCombustionchamberBrokenchemical bondFIGURE 15–15When the existing chemical bonds aredestroyed and new ones are formedduring a combustion process, usually alarge amount of sensible energy isabsorbed or released.CO 225°C, 1 atmThe formation of CO 2 during a steadyflowcombustion process at 25C and1 atm.Therefore, when the products formed during a chemical reaction exit thereaction chamber at the inlet state of the reactants, we have E state 0 andthe energy change of the system in this case is due to the changes in itschemical composition only.In thermodynamics we are concerned with the changes in the energy of asystem during a process, and not the energy values at the particular states.Therefore, we can choose any state as the reference state and assign a valueof zero to the internal energy or enthalpy of a substance at that state. Whena process involves no changes in chemical composition, the reference statechosen has no effect on the results. When the process involves chemicalreactions, however, the composition of the system at the end of a process isno longer the same as that at the beginning of the process. In this case itbecomes necessary to have a common reference state for all substances. Thechosen reference state is 25°C (77°F) and 1 atm, which is known as thestandard reference state. Property values at the standard reference stateare indicated by a superscript (°) (such as h° and u°).When analyzing reacting systems, we must use property values relative to thestandard reference state. However, it is not necessary to prepare a new set ofproperty tables for this purpose. We can use the existing tables by subtractingthe property values at the standard reference state from the values at the specifiedstate. The ideal-gas enthalpy of N 2 at 500 K relative to the standard referencestate, for example, is h – 500 K h– ° 14,581 8669 5912 kJ/kmol.Consider the formation of CO 2 from its elements, carbon and oxygen,during a steady-flow combustion process (Fig. 15–16). Both the carbon andthe oxygen enter the combustion chamber at 25°C and 1 atm. The CO 2formed during this process also leaves the combustion chamber at 25°C and1 atm. The combustion of carbon is an exothermic reaction (a reaction dur-