Constant-pressure processes defined

Enthalpy (il) - A thermodynamic function, especially useful when dealing with constant-pressure processes, defined by Ef = + PV, where E is energy, P pressure, and Vvolume. [1]... 

In analogy to the constant-pressure process, constant temperature is defined as meaning that the temperature T of the surroundings remains constant and equal to that of the system in its initial and final (equilibrium) states. First to be considered are constant-temperature constant-volume processes (again Aw = 0). For a reversible process... 

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... 

Students often ask, What is enthalpy The answer is simple. Enthalpy is a mathematical function defined in terms of fundamental thermodynamic properties as H = U+pV. This combination occurs frequently in thermodynamic equations and it is convenient to write it as a single symbol. We will show later that it does have the useful property that in a constant pressure process in which only pressure-volume work is involved, the change in enthalpy AH is equal to the heat q that flows in or out of a system during a thermodynamic process. This equality is convenient since it provides a way to calculate q. Heat flow is not a state function and is often not easy to calculate. In the next chapter, we will make calculations that demonstrate this path dependence. On the other hand, since H is a function of extensive state variables it must also be an extensive state variable, and dH = 0. As a result, AH is the same regardless of the path or series of steps followed in getting from the initial to final state and... 

The adiabatic flame temperature is defined as the maximum possible temperature achieved by the reaction in a constant pressure process. It is usually based on the reactants initially at the standard state of 25 °C and 1 atm. From Equation (2.20), the adiabatic temperature (7 i[Pg.30]

Heat Content or Enthalpy. A thermodynamic property closely related to energy. It is defined by H = E + PV where E is the internal energy of the system, P is the pressure on the system and V is the volume of the system. Often it is used in differential form as in. AH = AE + PAV for a constant pressure process... 

Most of the chemical reactions run in laboratory courses are to be performed in open systems. This means that there won t be a build-up of pressure and some work will be done by the reacting system on the surroundings or, possibly, by the surroundings on the system. In such cases, the principle of conservation of energy requires that the amount of heat shifted must adjust itself to provide for the small, but significant, amount of this work. A new function, the enthalpy, H, can be defined which is related simply to the heat flow in an open or constant-pressure vessel by the definition, H = E + PV. The amount of heat absorbed (or released) in a constant-pressure process is exactly equal to AH, the increase (or decrease) in H. 

Replacement of Pext by P, however, does not require complete equilibrium mechanical equilibrium with the surroundings is sufficient. In many slow processes, the system pressure closely tracks the external pressure and can be substituted for it in Eq. (5). The most commonly encountered of these is the constant-pressure process. Because we define our constraints in the surroundings, a constant-pressure process has constant Pext. If the system has a moveable boundary and the system is initially in mechanical equilibrium with the surroundings (P, = Pcxtl), then P will remain equal to Pcxl for the following two processes ... 

For study of constant pressure processes, it is convenient to define a term called Enthalpy or Heat Content , if, equal to E + PV. Like i , it is impossible to know the absolute value of P[ for a system. But, for convenience, i/for pure elements at atmospheric pressure and 25 °C (298 °K) is taken to be zero. 

We have defined heat capacities for constant volume and constant pressure processes as follows... 

We have described above a constant pressure process, and we use the alternate term Gibbs Free Energy since, for constant pressure processes, Gibbs Free Energy is applicable. Gibbs Free Energy, F is defined mathematically as... 

We can now define the free energy for a constant pressure process (i.e., the Gibbs free energy) to be as in Equation 10-21. 

In Chapter 6 we encountered the first of three laws of thermodynamics, which says that energy can be converted from one form to another, but it cannot be created or destroyed. One measure of these changes is the amount of heat given off or absorbed by a system during a constant-pressure process, which chemists define as a change in enthalpy (A//). 

The quantity of heat transferred into or out of a system as it undergoes a chemical or physical change at constant pressure, is defined as the enthalpy change, AH, of the process. 

Thus it happens that in constant pressure processes, the enthalpy change is exactly equal to q, the total heat flow. Or putting it the other way around, q admits a potential H in constant pressure processes. Please note that because H is a state variable, AH is perfectly well-defined between any two equilibrium states. But when the two states are at the same pressure, it becomes equal to the total heat flow during the process from one to the other, and in fact AH is in practice rarely used except in these cases (another kind of use, isenthalpic expansions, is discussed in Chapter 8). 

Comparing Equation (6.8) with Equation (6.5), we see that for a constant-pressure process, qp = AH. Again, although q is not a state function, the heat change at constant pressure is equal to AH because the path is defined and therefore it can have only a specific value. 

All we have done here is notice that, because work becomes a fixed quantity in constant pressure processes, then heat does too, by the first law. And because constant pressure processes are so common (including all reactions carried out at atmospheric pressure, such as most biochemical reactions), it is convenient to have a state variable defined to equal this heat term. Defining enthalpy as in (3.15) accomplishes this, and we now have a heat of reaction term, which will be useful in all constant pressure processes. 

Because most laboratory reactions are constant-pressure processes, the heat exchanged between the system and surroundings is equal to the change in enthalpy for the process. For any reaction, we define the change in enthalpy, called the enthalpy of reaction (AH), as the difference between the enthalpies of the products and the enthalpies of the reactants ... 

The heat capacity is defined as the heat required to raise the temperature of a unit mass of substance by a unit temperature. For a constant pressure process, the heat capacity Cp is given by... 

Many liquid systems in the laboratory are contained in vessels that are open to the atmosphere and are thus maintained at a nearly constant pressure. For convenient analysis of constant-pressure processes we define a new variable, denoted by H and called the enthalpy ... 

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