Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles15Enthalpy H (from the Greek word enthalpien, which means to heat) is a property and is
defined as the sum of the internal energy U and the PV product.
Enthalpy change of an ideal gas is given as ΔhCTdTCTT=≅−∫ PP, av21() ( ).
Enthalpy departure is the difference between the enthalpy of a real gas and the enthalpy
of the gas at an ideal gas state and it represents the variation of the enthalpy of a gas with
pressure at a fixed temperature.
Enthalpy departure factor is the nondimensionalized form of the enthalpy departure.
Enthalpy of a chemical component at a specified state is the sum of the enthalpy of formation of
the component at 25°C, 1 atm, and the sensible enthalpy of the component relative to 25°C, 1 atm,
which is the difference between the sensible enthalpy at the specified state ad the sensible
enthalpy at the standard reference state of 25°C and 1 atm. This definition enables us to use
enthalpy values from tables regardless of the reference state used in their construction.
Enthalpy of combustion hC is the enthalpy of reaction during a steady-flow combustion
process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and
pressure and represents the amount of heat released.
Enthalpy of formation is the enthalpy of a substance at a specified state due to its
chemical composition. The enthalpy of formation of all stable elements (such as O 2 , N 2 ,
H 2 , and C) has a value of zero at the standard reference state of 25°C and 1 atm.
Enthalpy of reaction hR is defined as the difference between the enthalpy of the products
at a specified state and the enthalpy of the reactants at the same state for a complete
reaction.
Enthalpy of vaporization (or latent heat of vaporization) is the quantity hfg listed in the
saturation tables.
Entropy (from a classical thermodynamics point of view) is a property designated S and
is defined as dS =(δQ/T)int rev.
Entropy (from a statistical thermodynamics point of view) can be viewed as a measure
of molecular disorder, or molecular randomness. The entropy of a system is related to the
total number of possible microscopic states of that system, called thermodynamic
probability p, by the Boltzmann relation, expressed as S = k ln p where k is the
Boltzmann constant.
Entropy balance for any system (including reacting systems) undergoing any process
can be expressed as net entropy transfer entropy by heat and mass plus entropy
generation equals the change in entropy.