Thermodynamics Demystified
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require energy to form (an endothermic reaction); others have a negative h of , indicating that they give off energy when they are formed (an exothermic reaction). The enthalpy of formation is the enthalpy change when a compound is formed. The enthalpy change when a compound undergoes complete combustion at constant temperature and pressure is called the enthalpy of combustion. For example, the enthalpy of formation of H2 is zero, and the enthalpy of combustion of H2 is 285 830 kJ/kmol. Values are listed for several compounds in Table B.6. If the products contain liquid water, the enthalpy of combustion is the higher heating value (HHV); if the products contain water vapor, the enthalpy of combustion is the lower heating value. The difference between the higher heating value and the lower heating value is the heat of vaporization hfg at standard conditions. If the reactants and products consist of several components, the first law, allowing for shaft work but neglecting kinetic and potential energy changes, is
)
(
(
Q − WS = ∑ N i h fo + h − h o i − ∑ N i h fo + h − h o prod
react
)
(9.9)
i
where Ni represents the number of moles of substance i. The work is often zero, but not in, for example, a combustion turbine. If combustion occurs in a rigid chamber, like a bomb calorimeter, so that no work results, the first law is Q = U products − U reactants
(
)
(
= ∑ Ni h f° + h − h ° − Pv − ∑ Ni h f° + h − h ° − Pv prod
i
react
)
(9.10)
i
where we have used enthalpy (u = h – Pv ) since the hi values are tabulated. Since the volume of any liquid or solid is negligible compared to the volume of the gases, we write Eq. (9.10) as
(
)
(
Q = ∑ Ni h fo + h − h o − RT − ∑ Ni h fo + h − h o − RT prod
i
react
)
i
(9.11)
If Nprod = Nreact then Q for the rigid volume is equal to Q for the control volume for the isothermal process. We employ one of the following methods to find (h − h o ) : • For a solid or liquid: Use h − h o = C p ΔT . • For gases: 1. Assume an ideal gas with constant specific heat so that h − h o = C p ΔT . 2. Assume an ideal gas and use tabulated values for h.