First law of thermodynamics enthalpy

We introduced the thermodynamic property of enthalpy, H, in Chapter 6. There we noted that the change in enthalpy equals the heat of reaction at constant pressure. Now we want to look at this property again, but define it more precisely, in terms of the energy of the system. We begin by discussing the first law of thermodynamics. 

According to the law of conservation of energy, the total energy remains constant energy is neither created nor destroyed. 

Top Urea is used as a plant fertilizer because it slowly decomposes in the soil to provide ammonia. Boffom A molecular model of urea. 

Thermodynamics is normally described in terms of three laws. The first law of thermodynamics is essentially the law of conservation of energy applied to thermodynamic systems. To state the law, you need to understand what the internal energy of a system is and how you can change the internal energy. 

The internal energy, U, is the sum of the kinetic and potential energies of the particles making up the system. The kinetic energy includes the energy of motion of electrons, nuclei, and molecules. The potential energy results from the chemical bonding of atoms and from the attractions between molecules. 

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The first law of thermodynamics (enthalpy) expresses the equivalence and interchangeability of the different forms of energy (heat, work, etc.), so that a molar mass of polysaccharide, for example, undergoing transformation from A to B, absorbs or evolves an increment of energy (AE) expressed (Glasstone and Lewis, 1960 Knight, 1970) as... 

Coffee-cup calorimeter Bomb calorimeter Standard enthalpy change First law of thermodynamics AH versus AE... 

This chapter introduces the first law of thermodynamics and its applications in three main parts. The first part introduces the basic concepts of thermodynamics and the experimental basis of the first law. The second part introduces enthalpy as a measure of the energy transferred as heat during physical changes at constant pressure. The third part shows how the concept of enthalpy is applied to a variety of chemical changes, an important aspect of bioenergetics, the use of energy in biological systems. 

We saw in Section 6.11 that the first law of thermodynamics implies that, because enthalpy is a state function, the enthalpy change for the reverse of a process is the negative of the enthalpy change of the forward process. The same relation applies to forward and reverse chemical reactions. For the reverse of reaction A, for instance, we can write... 

The lattice enthalpy of a solid cannot be measured directly. However, we can obtain it indirectly by combining other measurements in an application of Hess s law. This approach takes advantage of the first law of thermodynamics and, in particular, the fact that enthalpy is a state function. The procedure uses a Born-Haber cycle, a closed path of steps, one of which is the formation of a solid lattice from the gaseous ions. The enthalpy change for this step is the negative of the lattice enthalpy. Table 6.6 lists some lattice enthalpies found in this way. 

What Do We Need to Know Already The discussion draws on concepts related to the first law of thermodynamics, particularly enthalpy (Section 6.8) and work (Sections 6.2 and 6.3). 

How do we determine the energy and enthalpy changes for a chemical reaction We could perform calorimetry experiments and analyze the results, but to do this for every chemical reaction would be an insurmountable task. Furthermore, it turns out to be unnecessary. Using the first law of thermodynamics and the idea of a state function, we can calculate enthalpy changes for almost any reaction using experimental values for one set of reactions, the formation reactions. 

The specific application of the first law of thermodynamics to study chemical reactions is referred to as thermochemistry. Thermochemistry is concerned with the measurement or calculation of the heat absorbed or given out in chemical reactions. Precisely therefore, thermochemistry is the part of thermodynamics dealing with enthalpy (i.e., heat content) changes accompanying chemical reactions. In this context, it will be useful to refer to some of the important terms associated with thermal effects. 

We could introduce Hess s generalization into thermodynamics as another empirical law, which is similar to the first law. However, a firm theoretical framework depends on a minimum of empirical postulates. Thermodynamics is so powerful a method precisely because it leads to so many predictions from only two or three basic assumptions. Hess s law need not be among these postulates, because it can be derived directly from the first law of thermodynamics perhaps most conveniently by using a new thermodynamic function, enthalpy. 

Also, AH values are required to calculate the temperature dependence of equilibrium constants. For aU these reasons, it is desirable to have tables of AH values available, so that the enthalpies of various transformations can be calculated readily. In many of these calculations, we make use of Hess s law, which is now firmly established on the basis of the first law of thermodynamics. We can then calculate AH for reactions for which the heat effect is difficult to measure but that can be expressed as sums of reactions with known values of AH. 

A general relationship between enthalpy and temperature can be obtained from classical thermodynamics for a pure substance. Begin with the first law of thermodynamics, stated as either of the following ... 

The first law of thermodynamics leads to a broad array of physical and chemical consequences. In the following Sections 3.6.1-3.6.8, we describe the formal theory of heat capacity and the enthalpy function, the measurements of heating effects that clarified the energy and enthalpy changes in real and ideal gases under isothermal or adiabatic conditions, and the general first-law principles that underlie the theory and practice of thermochemistry, the measurement of heat effects in chemical reactions. 

Figure 3.16 Schematic representation of enthalpy decompositions having exact (straight arrows) or approximate (wavy arrow) consistency with the first law of thermodynamics.

Figure 3.16 summarizes various enthalpy decomposition schemes that are justified by the first law of thermodynamics. The results of innumerable thermochemical measurements based on these decompositions provide eloquent testimony to the accuracy and generality of the first law. 

This relation is a direct consequence of the definition of enthalpy by Equation (I) and of the mathematical statement of the first law of thermodynamics, namely that the change in internal energy. AT. is equal to the heat adsorbed minus the work done q - PAY). It is clear that this thermodynamic relation does not define absolute values of enthalpy or internal energy. Changes in enthalpy, however, are readily measured by calorimetric techniques, and the relative enthalpy values nre sufficient for all therinochcmical calculations. 

This relation, which we can trace back to the first law of thermodynamics, is illustrated in Fig. 6.21. If we find, for example, that the enthalpy of vaporization of mercury is 59 kj-mol-1 at its boiling point, then we immediately know that the enthalpy change occurring when mercury vapor condenses at that temperature is —59 kj-mol. This value tells us that 59 kj of heat is released when 1 mol Hg(g) condenses to a liquid. 

Cooling curves such as the two in Figure 8.7 were determined for constant enthalpy (or Joule-Thomson) expansions, obtained from the First Law of Thermodynamics for a system flowing at steady-state, neglecting kinetic and potential energy changes ... 

The only two functions actually required in thermodynamics are the energy function, obtained from the first law of thermodynamics, and the entropy function, obtained from the second law of thermodynamics. However, these functions are not necessarily the most convenient functions. The enthalpy function was defined in order to make the pressure the independent variable, rather than the volume. When the first and second laws are combined, as is done in this chapter, the entropy function appears as an independent variable. It then becomes convenient to define two other functions, the Gibbs and Helmholtz energy functions, for which the temperature is the independent variable, rather than the entropy. These two functions are defined and discussed in the first part of this chapter. 

Thermochemistry is concerned with the determination of the heat absorbed by a system when some process occurs within the system. The quantity of heat absorbed may be determined experimentally by the use of calorimeters or by calculation from prior knowledge of the thermodynamic properties of the system. The equations relating the heat absorbed by a system for a given process to the change of energy or enthalpy of the system for the change of state that occurs during the process are the mathematical statements of the first law of thermodynamics. They are Equations (2.26) and (2.30), written here as... 

In accordance with the law of energy conservation (the first law of thermodynamics) Eq. 11.20 is valid for the enthalpy flow and the enthalpy change in the stationary state ... 

The first law of thermodynamics states that energy cannot be created or destroyed (i.e., the total energy of a system is always constant). This means that if the internal energy of a reaction increases then there must be a concomitant uptake of energy usually in the form of heat. Enthalpy (H) is a parameter used to describe the energy of a system as heat... 

The major source of the energy required for excitation is the reaction enthalpy, AH,. There are numerous examples, however, of chemiluminescent reactions in which the energy of the observed photon is greater than AHt (Bartlett and Landis, 1979 Horn et al., 1978-79 Adam, 1977 Wilson, 1976 Turro et al., 1974b Mumford, 1975 McCapra, 1966 Lechtken et al., 1973). In these cases where —AH, < AE, additional energy may be provided by the activation enthalpy of the reaction AH+. Thus, the first law of thermodynamics is satisfied by the requirements of (3). An objection to the inclusion of AH to satisfy the energy requirement had been made on thermodynamic grounds (Perrin, 1975) but was later shown to be incorrect (Lissi, 1976 E. B. Wilson, 1976). 

Born-Haber s cycle — Hess s law establishes that the enthalpy of a reaction is the same independently whether the reaction proceeds in one or several steps. It is a consequence of the first law of thermodynamics, which states the conservation of energy. Born and -> Haber applied Hess s law to determine the - enthalpy of formation of an ionic solid. The formation of an ionic crystal from its elements according to Born-Haber s cycle can be represented by the following diagram. 

The zeroth law of thermodynamics involves some simple definition of thermodynamic equilibrium. Thermodynamic equilibrium leads to the large-scale definition of temperature, as opposed to the small-scale definition related to the kinetic energy of the molecules. The first law of thermodynamics relates the various forms of kinetic and potential energy in a system to the work which a system can perform and to the transfer of heat. This law is sometimes taken as the definition of internal energy, and introduces an additional state variable, enthalpy. 

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