Internal energy constant-pressure processes

Clearly, Aid is equal to the heat transferred in a constant pressure process. Often, because biochemical reactions normally occur in liquids or solids rather than in gases, volume changes are small and enthalpy and internal energy are often essentially equal. 

Any characteristic of a system is called a property. The essential feature of a property is that it has a unique value when a system is in a particular state. Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the size of a system, such as temperature T and pressure p. Extensive properties are those that are dependent on the size of a system, such as volume V, internal energy U, and entropy S. Extensive properties per unit mass are called specific properties such as specific volume v, specific internal energy u, and specific entropy. s. Properties can be either measurable such as temperature T, volume V, pressure p, specific heat at constant pressure process Cp, and specific heat at constant volume process c, or non-measurable such as internal energy U and entropy S. A relatively small number of independent properties suffice to fix all other properties and thus the state of the system. If the system is composed of a single phase, free from magnetic, electrical, chemical, and surface effects, the state is fixed when any two independent intensive properties are fixed. 

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... 

Enthalpy plays a role in constant-pressure processes similar to that of internal energy in constant-volume processes. The heat added to a system in a constant-pressure process is the enthalpy increase of the system. Because U, P, and V (and /, and Lt) are all state functions, H is also a state function. It is extensive. The molar enthalpy, Hm = H/n, is intensive. Dividing Eq. (23) by dTP gives... 

Thus for a mechanically reversible, constant-pressure, nonflow process, the heat transferred equals the enthalpy change of the system. Comparison of the last two equations with Eqs. (2.16) and (2.17) shows that the enthalpy plays a role in constant-pressure processes analogous to the internal energy in constant-volume processes. 

Because the internal energy of an ideal gas is a function of temperature only, both enthalpy and Cp also depend on temperature alone. This is evident from the definition H = U + PV, or H = U + RT for an ideal gas, and from Eq. (2.21). Therefore, just as A U = j CvdT for any process involving an ideal gas, so AH = J CP dT not only for constant-pressure processes but for all finite processes. 

The right side of Eq. (1.64) shows the energy effect due to the expansion of volume at a constant pressure process. For a mixture of ideal gases, the internal energy is a function of temperature only, and hence Eq. (1.64) and PV=RT yields... 

While absolute entropy values can now be determined absolute values of Internal Energy and Enthalpy cannot be conceived. For ease of calculation, related especially to metallurgical reactions (constant pressure processes), a suitable reference point of enthalpy is conventionally chosen and that is - for pure elements, the enthalpy is zero when in Standard State . Standard... 

The practical utility of the heat capacities is twofold. First, they allow us to calculate heat in constant-volume and constant-pressure processes. This is useful in energy balances. Second, they allow us to calculate changes in internal energy and enthalpy. This allows us to calculate these properties using equations rather than tables, or to obtain their values in states that are not found in tables. There is a limitation, however. Equation (. 17) maybe used only between two states of the same volume, and eg. f. iQ ) only between two states of the same pressure. The general calculation of properties between any two states will be discussed in Chanter r. 

Now we have two ways to define heat flow into a system, under two different sets of conditions. For a process at constant volume, the measurable heat flow is equal to AE, the change in internal energy. For a process at constant pressure, the measurable heat flow is equal to the change in enthalpy, AH. In many ways, enthalpy is the more useful term because constant pressure conditions are more common. A reaction carried out in a beaker in the chemistry laboratory, for instance, occurs under constant pressure conditions (or very nearly so). Thus, when we refer to the heat of a process, we are typically referring to a change in enthalpy, AH. As in previous definitions, AH refers to Fffinai -ffinraai-... 

The enthalpy, H, of a substance in J/kg represents the sum of the internal energy plus the pressure-volume term. For no reaction and a constant-pressure process with a change in temperature, the heat change as computed from Eq. (1.6-4) is the difference in enthalpy, AH, of the substance relative to a given temperature or base point. In other units, H = btu/lb , or cal/g. 

For constant pressure processes the specific enthalpy h is the measure of the internal energy possessed by the material. The specific heat is related to the internal energy as... 

Although the internal energy represents the total energy of a system, and the first law of thermodynamics is based on the concept of internal energy, it is not always the best variable to work with. Equation 2.15 shows that the change in the internal energy is exactly equal to q—if the volume of the system remains constant for a particular process. However, not all processes occur at constant volume. In fact, constant pressure processes, in which the system is exposed to the atmosphere, are more common. Enthalpy is given the symbol H. The fundamental definition of enthalpy is... 

Another frequently used term relating the enthalpy of combustion and internal energy of combustion is the heating value, which is equal to the negative of enthalpy of combustion for a constant pressure process and negative of internal energy... 

Consider a process at constant pressure for which the change in internal energy is AU and the change in volume is A V. It then follows from the definition of enthalpy in Eq. 9 that the change in enthalpy is... 

Chemists define the total internal energy of a substance at a constant pressure as its enthalpy, H. Chemists do not work with the absolute enthalpy of the reactants and products in a physical or chemical process. Instead, they study the enthalpy change, AH, that accompanies a process. That is, they study the relative enthalpy of the reactants and products in a system. This is like saying that the distance between your home and your school is 2 km. You do not usually talk about the absolute position of your home and school in terms of their latitude, longitude, and elevation. You talk about their relative position, in relation to each other. 

The first law of thermodynamics simply says that energy cannot be created or destroyed. With respect to a chemical system, the internal energy changes if energy flows into or out of the system as heat is applied and/or if work is done on or by the system. The work referred to in this case is the PV work defined earlier, and it simply means that the system expands or contracts. The first law of thermodynamics can be modified for processes that take place under constant pressure conditions. Because reactions are generally carried out in open systems in which the pressure is constant, these conditions are of greater interest than constant volume processes. Under constant pressure conditions Equation 3 can be rewritten as... 

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