Enthalpy, changes

Most chemical reactions and physical changes occur at constant (usually atmospheric) pressure. 

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

An enthalpy change is sometimes loosely referred to as a heat change or a heat of reaaion. The enthalpy change is equal to the enthalpy or heat content, H, of the substances produced minus the enthalpy of the substances consumed. 

It is impossible to know the absolute enthalpy (heat content) of a system. Enthalpy is a state funaim, however, and it is the change in enthalpy in which we are interested this can be measured for many processes. In the next several sections, we focus on chemical reactions and the enthalpy changes that occur in these processes. We first discuss the experimental determination of enthalpy changes. 

We can directly measure changes in enthalpy, in a calorimeter (see Section 2.9). For example, if we place one mol of pure H2 in a calorimeter with a surplus of O2, and initiate the oxidation reaction with an infinitesimal spark, the temperature of the gas will rise, explosively. If we then cool the mixture to its starting temperature by letting it transfer heat to a large mass of water, we can measure the temperature increase of the water, from which we can compute the amount of heat released by this reaction. The measured quantity is the molar heat of combustion of H2. It is also the negative of the heat of formation (enthalpy change of formation from the elements) of H2O, which is listed in Table A.8 as 285.8 kJ/mol at 25°C. 

We can use simple calorimetry, starting from the elements for many simple compounds, but not for complex ones. For example, we cannot begin with C and CI2 and produce pure CCI4 in a calorimeter, as we did with H2O. However, if we have some reaction of the form A + B — C + D, for which we can experimentally measure the enthalpy change and for which we know the enthalpies of formation fi om the 

Physical and Chemical Equilibrium for Chemical Engineers, Second Edition. Noel de Nevers. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

Goldberg and Prosen (155) have proposed a general theory similar to this and have applied it to other types of scanning calorimetry. 

Van Dooren and Muller investigated in great detail by factorial designs the effects of apparatus, test substance, reference material, atmosphere (152), as well as heating rate and particle size (153,159,160) on results in quantitative DSC. The former set of experimental factors are called baseline-related characteristics if the curve is described using these parameters, large standard deviations should be taken into account. A baseline equation, based on heat balance considerations, is 

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The enthalpy changes due to dimerization are determined from the van t Hoff relation. For a dimerization reaction between species i and j... 

The analysis of the heat exchanger network first identifies sources of heat (termed hot streams) and sinks (termed cold streams) from the material and energy balance. Consider first a very simple problem with just one hot stream (heat source) and one cold stream (heat sink). The initial temperature (termed supply temperature), final temperature (termed target temperature), and enthalpy change of both streams are given in Table 6.1. 

ANstreams = enthalpy change between feed and product streams AI/react = reaction enthalpy (negative in the case of exothermic reactions)... 

Figure B.l shows a pair of composite curves divided into vertical enthalpy intervals. Also shown in Fig. B.l is a heat exchanger network for one of the enthalpy intervals which will satisfy all the heating and cooling requirements. The network shown in Fig. B.l for the enthalpy interval is in grid diagram form. The network arrangement in Fig. B.l has been placed such that each match experiences the ATlm of the interval. The network also uses the minimum number of matches (S - 1). Such a network can be developed for any interval, providing each match within the interval (1) satisfies completely the enthalpy change of a strearh in the interval and (2) achieves the same ratio of CP values as exists between the composite curves (by stream splitting if necessary).

Negative ions also have two unique thennodynainic quantities associated with them the electron affinity, EA, defined as the negative of the enthalpy change for addition of an electron to a molecule at 0 K [117. 121. 122]... 

The enthalpy change AH for a temperature change from to T2 can be obtained by integration of the constant pressure heat capacity... 

Batch calorimeters are instmments where there is no flow of matter in or out of the calorimeter during the time the energy change is being measured. Batch calorimeters differ in the way the reactants are mixed and in the method used to detennine the enthalpy change. Enthalpy changes can be measured by the various methods... 

This method is used to locate phase transitions via measurements of the endothennic enthalpy of phase transition. Details of the teclmique are provided elsewhere [25, 58]. Typically, the enthalpy change associated with transitions between liquid crystal phases or from a liquid crystal phase to the isotropic phase is much smaller than the melting enthalpy. Nevertheless, it is possible to locate such transitions with a commercial DSC, since typical enthalpies are... 

The enthalpy (strictly, the enthalpy change) for a reaction can readily be calculated from enthalpies of formation AHf which can often be obtained from tables of data. 

From the above discussion, we might expect that endothermic reactions for which the enthalpy change is large cannot take place. However, a further consideration of the equation... 

These values indicate a rapid fall in thermal stability of the halide from fluorine to iodine, and hydrogen iodide is an endothermic compound. If we now examine the various enthalpy changes involved. we find the following values (in kJ) ... 

Essentially the same processes occur when chlorides (for example) of non-metallic elements dissolve in water. Thus, the enthalpy changes for hydration chloride can be represented ... 

The above Born-Haber cycle represents the enthalpy changes in the formation of an alkali metal halide MX from an alkali metal (Li. Na, K, Rb. Cs) and a halogen (Fj. CI2. Br2 or I2). 

Consider first two substances which have very similar molecules. He, hydrogen fluoride and HCl. hydrogen chloride the first is a Weak acid in water, the second is a strong acid. To see the reason consider the enthalpy changes involved when each substance in water dissociates to form an acid ... 

The enthalpy changes AH involved in this equilibrium are (a) the heat of atomisation of the metal, (b) the ionisation energy of the metal and (c) the hydration enthalpy of the metal ion (Chapter 3). 

The enthalpies for the reactions of chlorine and fluorine are shown graphically in Figure 11.2 as the relevant parts of a Born-Haber cycle. Also included on the graph are the hydration energies of the two halogen ions and hence the enthalpy changes involved in the reactions... 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

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