Specific enthalpy definition

Fortunately, we never need to know the absolute values of H or // at specified states we only need to know AH and AH for specified changes of state, and we can determine these quantities experimentally. We may therefore arbitrarily choose a reference state tor a species and determine AH = 0 - Href for the transition from the reference state to a series of other states. If we set Href equal to zero, then H(= AH) for a specified state is the specific internal energy at that state relative to the reference state. The specific enthalpies at each state can then be calculated from the definition, H = U + PV, provided that the specific volume (V) of the species at the given temperature and pressure is known. 

The specific enthalpy change associated with the transition of a substance from one phase to another at constant temperature and pressure is known as the latent heat of the phase change (as distinguished from sensible heat, which is associated with temperature changes for a singlephase system). For example, the specific enthalpy change AH for the transition of liquid water to steam at I00°C and 1 atm, which equals 40.6 kJ/mol, is by definition the latent heat of vaporization (or simply the heat of vaporization) of water at this temperature and pressure. 

This formulation follows directly from the definition of the partial specific enthalpies. The partial specific enthalpies have to be derived from a differentiation of the specific enthalpies with respect to the mole number. 

For the case when the pressure is kept constant during the heating process, we begin by noting the definition of specific enthalpy, namely ... 

Standard thermodynamic formalism for the total differential of specific enthalpy in terms of its natural variables (i.e., via Legendre transformation, see equations 29-20 and 29-24b) allows one to calculate the pressure coefQdent of specific enthalpy via a Maxwell relation and the definition of the coefficient of thermal expansion, a. 

Mixing enthalpy is zero (i.e. no enthalpy changes upon mixing). Indeed, by definition, the specific mixing enthalpy is the difference h — X =i (cf. (4.435), (4.436)) where h is the specific enthalpy of the mixture and h is the specific... 

Equation (2.86) comes from H=U + PV the definition of specific enthalpy in terms of specific internal energy, U, pressure and volume, P and V, respectively. 

The term differential is sometimes added to enthalpy changes where infinitesimal (i.e., very small) amounts were added to a very large amount of either solution or pure component. These enthalpy changes are usually called partial (molar or specific) enthalpies of solution/mixing. The mathematical definition of partial molar enthalpies of solution/mixing is given for the copolymer B by ... 

The temperature profile of a planetary atmosphere depends both on the composition and some simple thermodynamics. The temperature decreases with altitude at a rate called the lapse rate. As a parcel of air rises, the pressure falls as we have seen, which means that the volume will increase as a result of an adiabatic expansion. The change in enthalpy H coupled with the definition of the specific heat capacity... 

We must be careful to distinguish between bond enthalpy and the dissociation enthalpy of a given bond. The latter is a definite quantity that refers to the enthalpy absorbed when a given bond of some specific compound is broken. However, bond enthalpy is an average value of the dissociation enthalpies of a given bond in a series of different dissociating species. 

The temperature gap (AT) between the two flows is chosen as the controlling parameter it determines both the enthalpy feed and the Carnot efficiency of the thermoelectric element. The value of AT is related to the heat exchanger efficiency Tiexc or e-NTU (normal thermal unit), the ratio of the heat exchanged to the total exchangeable heat. This relationship comes from the definition of e-NTU for the exchanger efficiency and, in this specific case, it has the following form [16] ... 

The key to doing process analysis is the identification of the equations that may be used to achieve zero degrees of freedom. These equations will come from a number of sources, including the balance equations themselves (Equations (1) and (19)), process specifications (such as the purity of output streams and the reflux ratio), physical relations (such as the definition of enthalpy for liquid and vapour streams) and other constraints imposed by the problem. Once a full set of equations has been developed, the equations can be solved, usually with little difficulty, and the desired results obtained. 

A second — and undoubtedly more specific — difficulty proceeds from the fact that aromaticity has at least two meanings which are fundamentally different according to whether the word is being used by a pure chemist or a physicist to the first, a compound is aromatic if its chemistry is like that of benzene, while, to the other, according to the more modern definition, a compound is aromatic if it has a low ground-state enthalpy. )... 

Suppose, for example, that a substance is to be vaporized isothermally at 130 C, but the only available value of the heat of vaporization is at 80 C. A process path from the liquid at 130 C to the vapor at the same temperature must then be chosen that includes an isothermal vaporization step at 80 C specifically cool the liquid from 130 C to 80 C, vaporize the liquid at 80 C, and then heat the vapor back to 130°C. Summing the changes in enthalpy for each of these steps yields the change in enthalpy for the given process. (By definition, the calculated value is the latent heat of vaporization at 130 C.)... 

Note that the standard temperature (77°F or T0 = 298.15 K) is used in this definition. cp i is the specific heat and Ah p is the enthalpy of formation at the standard state, both for species i. The heat flux, q, includes contributions from conduction, radiation, differential diffusion among component species, and concentration gradient-driven Dufour effect. For combustion applications, the most important contributions come from conduction and radiation. As discussed in Section 4.3, conduction heat flux follows Fourier s law (Equation 4.27) and radiation heat flux is related to the local intensity as... 

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