Definition enthalpy

By differentiating Eq.(6) with respect to time, considering that the variations of the volume and total pressure are negligible, and using the enthalpy definition. Hi = CpiT, where Cp is the heat capacity of i-reactant (kJ/mol °C), Eq.(5) can be written as follows ... 

The enthalpy definition can also be used to manipulate the first and last terms, yielding... 

It is not convenient use (3.139) averaging the enthalpy equation as some of the terms do not fit within the generalized framework. Instead the averaged form of the enthalpy equation can be obtained by averaging (1.129) directly or from (3.170) and the enthalpy definition (1.128). 

You should always give an equation as an example for this and other enthalpy definitions. 

These conditions considerably simplify the two models of variation of Ti and Jfby redncing the number of state variables to the substance amount n and the temperature T for both of them. The resemblance between the two equations offers the possibility of combining them in a single one and the interesting operation is to determine their difference for expressing the product PV as given by the enthalpy definition in Equation K2.2 ... 

For polymers in the rubbery range the volume change dV is small and if the pressure is only the atmospheric pressure the pdV term is so small as to be negligible. The negative sign for the pV term in the enthalpy definition here is due to the fact that the work of atmospheric pressure is in opposition to the positive work of a tensile force on an uniaxial specimen. 

This is the situation exploited by the so-called isolation method to detennine the order of the reaction with respect to each species (see chapter B2.1). It should be stressed that the rate coefficient k in (A3,4,10) depends upon the definition of the in the stoichiometric equation. It is a conventionally defined quantity to within multiplication of the stoichiometric equation by an arbitrary factor (similar to reaction enthalpy). 

A/i the dissociation or bond energy of hydrogen (it is also, by definition, twice the enthalpy of atomisation two gram atoms being produced). 

For a simple A B reaction, suppose that the heat capacity Cp of A is larger than the heat capacity for B. The enthalpy of A rises more steeply with temperature increase than that of B hy the definition of heat capacity... 

The remaining question is how we got from G3MP2 (OK) = —117.672791 to G3MP2 Enthalpy = —117.667683. This is not a textbook of classical thermodynamics (see Klotz and Rosenberg, 2000) or statistical themiodynamics (see McQuarrie, 1997 or Maczek, 1998), so we shall use a few equations from these fields opportunistically, without explanation. The definition of heat capacity of an ideal gas... 

When these four (or three) contributions are summed for a molecule such as propene, we have the themial correction to the energy G3MP2 (OK). The result is G3MP2 Energy in the G3(MP2) output block. To this is added PV, which is equal to RT for an ideal gas, in accordance with the classical definition of the enthalpy... 

We define to be AH/(l/2 + AS )R. Since the numerator of this definition is an enthalpy and the denominator an entropy, must have the units of a temperature ... 

Equation 54 implies that U is a function of S and P, a choice of variables that is not always convenient. Alternative fundamental property relations may be formulated in which other pairs of variables appear. They are found systematically through Legendre transformations (1,2), which lead to the following definitions for the enthalpy, H, Hehnholt2 energy,, and Gibbs energy, G ... 

The definitions of enthalpy, H, Helmholtz free energy. A, and Gibbs free energy, G, also give equivalent forms of the fundamental relation (3) which apply to changes between equiUbrium states in any homogeneous fluid system ... 

The most satisfactory calciilational procedure for thermodynamic properties of gases and vapors requires PVT data and ideal gas heat capacities. The primary equations are based on the concept of the ideal gas state and the definitions of residual enthalpy anci residual entropy ... 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

Technical calculations dealing with humid air are reasonable to solve with dry air mass flow rates, because these remain constant in spite of changes in the amount of water vapor in the air. For that reason a definition for enthalpy,... 

If the definition of work is limited to mechanical work, an interesting simplification is possible. In this case, AE is merely the heat exchanged at constant volume. This is so because if the volume is constant, no mechanical work can be done on or by the system. Then AE = q. Thus AE is a very useful quantity in constant volume processes. However, chemical and especially biochemical processes and reactions are much more likely to be carried out at constant pressure. In constant pressure processes, AE is not necessarily equal to the heat transferred. For this reason, chemists and biochemists have defined a function that is especially suitable for constant pressure processes. It is called the enthalpy, H, and it is defined as... 

Applying this definition, the apparent molar enthalpy would be given by... 

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