Energetics of chemical reactions

The rearrangement of atoms that occurs in a chemical reaction is virtually always accompanied by the liberation or absorption of heat. If the purpose of the reaction is to serve as a source of heat, such as in the combustion of a fuel, then these heat effects are of direct and obvious interest. We will soon see, however, that a study of the energetics of chemical reactions in general can lead us to a deeper understanding of chemical equilibrium and the basis of chemical change itself. 

In chemical thermodynamics, we define the zero of the enthalpy and internal energy as that of the elements as they exist in their stable forms at 298K and 1 atm pressure. Thus the enthalpies// of Xc(g), O2(g) and C(diamond) are all zero, as are those of H2 and Cl2 in the reaction 

The enthalpy of two moles of HC1 is smaller than that of the reactants, so the difference is released as heat. Such a reaction is said to be exothermic. The reverse of this reaction would absorb heat from the surroundings and be endothermic. 

In comparing the internal energies and enthalpies of different substances as we have been doing here, it is important to compare equal numbers of moles, because energy is an extensive property of matter. However, heats of reactions are commonly expressed on a molar basis and treated as intensive properties. 

We can characterize any chemical reaction by the change in the internal energy or enthalpy  

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Although the LD model is clearly a rough approximation, it seems to capture the main physics of polar solvents. This model overcomes the key problems associated with the macroscopic model of eq. (2.18), eliminating the dependence of the results on an ill-defined cavity radius and the need to use a dielectric constant which is not defined properly at a short distance from the solute. The LD model provides an effective estimate of solvation energies of the ionic states and allows one to explore the energetics of chemical reactions in polar solvents. 

The EVB approach described in this chapter provides a convenient way for estimating the energetics of chemical reactions in various solvents. However, the approximation involved in eq. (2.21) cannot be justified without detailed studies by more rigorous models. Such models will be described in Chapter 3. 

Boo, H. K. (1998). Students understanding of chemical bonds and the energetics of chemical reactions. Journal of Research in Science Teaching, 35 5), 569-581. 

This part includes a discussion of the main experimental methods that have been used to study the energetics of chemical reactions and the thermodynamic stability of compounds in the condensed phase (solid, liquid, and solution). The only exception is the reference to flame combustion calorimetry in section 7.3. Although this method was designed to measure the enthalpies of combustion of substances in the gaseous phase, it has very strong affinities with the other combustion calorimetric methods presented in the same chapter. 

How do chemical technologies and processes depend on the energetics of chemical reactions ... 

Ab initio methods can also be useful they have now reached the stage where readily available computer programs can produce information on the energetics of chemical reactions which is sufficiently accurate to be used as a guide, or to support arguments, where choices are made between alternative experimental findings. They may also be useful to the modeller where no experimental results are available. 

It is a curious fact that although chemistry is first and foremost the science of transformation, discussions of time in the chemical literature are quite meager compared to discussions of energy. There are innumerable works addressing the energetics of chemical reactions and structures, yet a far smaller number are devoted to examining the temporal aspects of these subjects. I do not mean that the rates of chemical reactions... 

First, it seems desirable, even prima facie, that we develop an intuition of how chemical reactions occur. For example, molecular orbital theory provides a fair amount of detailed intuition about the energetics of chemical reactions, i.e., when do we expect large activation barriers, when do we expect concerted reactions, what is the effect of an electrophilic substituent on reaction product distributions, etc. A similar intuition has not been available concerning the role of dynamics in chemical reactivity. It is reasonable, as a chemist, to ask how one could enhance energy transfer specifically into the reaction coordinate, thus to make the reaction more efficient and to produce better reaction yields with fewer byproducts. 

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