Enthalpy of Chemical Reactions

Our next step is to see how the first law of thermodynamics can be applied to processes carried out under different conditions. Specifically, we will consider two situations most commonly encountered in the laboratory one in which the volume of the system is kept constant and one in which the pressure applied on the system is kept constant. 

If a chemical reaction is run at constant volume, then AV = 0 and no P-V work will result from this change. From Equation (6.1) it follows that 

We add the subscript v to remind us that this is a constant-volume process. This equality may seem strange at first, for we showed earlier that q is not a state function. The process is carried out under constant-volume conditions, however, so that the heat change can have only a specific value, which is equal to AE. 

Constant-volume conditions are often inconvenient and sometimes impossible to achieve. Most reactions occur under conditions of constant pressure (usually atmospheric pressure). If such a reaction results in a net increase in the number of moles of a gas, then the system does work on the surroundings (expansion). This follows from the fact that for the gas formed to enter the atmosphere, it must push the surrounding air back. Conversely, if more gas molecules are consumed than are produced, work is done on the system by the surroundings (compression). Finally, no work is done if there is no net change in the number of moles of gases from reactants to products. 

We now introduce a new thermodynamic function of a system called enthalpy (H), which is defined by the equation 

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Data exist for the enthalpy of chemical reactions, formation of substances from their constituent elements, combustion, fusion, neutralization, solution, vaporization, etc. 

In combustion calorimetry [47,48] the enthalpies of chemical reactions of elements and compounds with reactive gases like oxygen or fluorine are determined. 

The basic theory of the photoacoustic effect was described by Tam and Patel [279,280] and some of its applications were presented in a review by Braslavsky and Heibel [281], The first use of PAC to determine enthalpies of chemical reactions was reported by the groups of Peters and Braslavsky [282,283], The same groups have also played an important role in developing the methodologies to extract those thermodynamic data from the experimentally measured quantities [282-284], In the ensuing discussion, we closely follow a publication where the use of the photoacoustic calorimety technique as a thermochemical tool was examined [285],... 

Changes in enthalpy of chemical reactions are determined by calorimetry of reacting systems. The reaction heat is calculated from measured heats of formation, solution, and mixing, using the Hess law. 

The first law of thermodynamics considers the enthalpy of chemical reactions. The second law states that the universe spontaneously tends toward increasing disorder or randomness. 

In the 1920s the developing chemical technology required reliable physicochemical data. The accuracy of the thermochemical data available was hardly adequate for the calculations of enthalpies of chemical reactions and completely insufficient for the calculation of equilibrium constants. If reliable values of equilibrium constants were to be calculated, the accuracy of the thermochemical data would have to be increased by about one order of magnitude. The classical period of thermochemistry may be considered to have ended with this understanding. 

OBJECTIVE To consider the heat (enthalpy) of chemical reactions. 

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