Chemical reaction energy change calculations

Thermochemistry is an important part of explosive chemistry it provides information on the type of chemical reactions, energy changes, mechanisms and kinetics which occur when a material undergoes an explosion. This chapter will carry out theoretical thermochemical calculations on explosive parameters, but it must be noted that the results obtained by such calculations will not always agree with those obtained experimentally, since experimental results will vary according to the conditions employed. 

Given a listing of free energies of formation, the free energy change of a chemical reaction may he calculated in the same manner as you evaluated enthalpies of reaction and entropies of reaction. For the reaction that was discussed earlier,... 

The standard enthalpy and Gibbs free energy changes for a chemical reaction can be calculated from A./H° and A/G° data using the relationships... 

Calculation of equilibrium constant from emf of a cell The equilibrium constant of a chemical reaction can be calculated from the standard free-energy change by the equation... 

As in the miniLAB above, you can use calorimetry to measure the energy released or absorbed in a chemical reaction or change of state. However, sometimes carrying out an experiment is difficult or even impossible. In the next section, you ll see that there are ways to calculate energy changes. 

The change in heat energy due to the chemical reaction, q, is calculated as... 

By combining values of the enthalpies of formation of reactants and products, the enthalpy changes for every possible chemical reaction can be calculated. When diese thermochemical data are combined with entropy data, Gibbs energies and equilibriiun constants can be also determined. 

Defining chemical reactions as changes in which new bonds are formed, these may be considered as the simplest such processes known. Since only two electrons are involved, it has been possible to calculate relevant potential energy surfaces on an ab initio basis. " Trajectories over these have recently been calculated. " Discovery of the above reactions awaited beam techniques since in classic mass spectrometric methods, they could not be distinguished from reactions of hydrogen-molecule ions yielding similar products. 

In chemical reactions, the change in Gibbs free energy (AG) can be expressed as the sum of two terms. The first term is the standard free energy change (AG°), which is fixed for any given reaction. AG° can be calculated from the stoichiometry of the reaction (i.e., how many moles of one com-poimd react with how many moles of another compoimd) and the tabulated standard free energies of the chemicals involved. The second term contains the reaction quotient (Q), which depends on the concentrations of all reactants and products in the system. The fact that AG can be expressed in terms of these concentrations makes it possible to determine in which direction a chemical reaction will proceed, as well as to predict a system s final composition when it reaches equilibrium. 

Having calculated the standai d values AyW and S" foi the participants in a chemical reaction, the obvious next step is to calculate the standard Gibbs free energy change of reaction A G and the equilibrium constant from... 

The standard Gibbs-energy change of reaction AG° is used in the calculation of equilibrium compositions. The standard heat of reaclion AH° is used in the calculation of the heat effects of chemical reaction, and the standard heat-capacity change of reaction is used for extrapolating AH° and AG° with T. Numerical values for AH° and AG° are computed from tabulated formation data, and AC° is determined from empirical expressions for the T dependence of the C° (see, e.g., Eq. [4-142]). 

Estimation of the free-energy change associated with a reaction permits the calcula-aon of the equilibrium position for a reaction and indicates the feasibility of a given chemical process. A positive AG° imposes a limit on the extent to which a reaction can x cur. For example, as can be calculated using Eq. (4.2), a AG° of 1.0 kcal/mol limits conversion to product at equilibrium to 15%. An appreciably negative AG° indicates that e reaction is thermodynamically favorable. 

It is reasonable to expeet that models in ehemistry should be capable of giving thermodynamic quantities to chemical accuracy. In this text, the phrase thermodynamic quantities means enthalpy changes A//, internal energy changes AU, heat capacities C, and so on, for gas-phase reactions. Where necessary, the gases are assumed ideal. The calculation of equilibrium constants and transport properties is also of great interest, but I don t have the space to deal with them in this text. Also, the term chemical accuracy means that we should be able to calculate the usual thermodynamic quantities to the same accuracy that an experimentalist would measure them ( 10kJmol ). 

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