Chemical reactions Gibbs free energy calculation

The enthalpy and Gibbs free energy calculated according to their chemical reaction at 2(XX)K ... 

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 decrease in Gibbs free energy as a signpost of spontaneous change and AG = 0 as a criterion of equilibrium are applicable to any kind of process, provided that it is occurring at constant temperature and pressure. Because chemical reactions are our principal interest in chemistry, we now concentrate on them and look for a way to calculate AG for a reaction. 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

What Do We Need to Know Already The concepts of chemical equilibrium are related to those of physical equilibrium (Sections 8.1-8.3). Because chemical equilibrium depends on the thermodynamics of chemical reactions, we need to know about the Gibbs free energy of reaction (Section 7.13) and standard enthalpies of formation (Section 6.18). Ghemical equilibrium calculations require a thorough knowledge of molar concentration (Section G), reaction stoichiometry (Section L), and the gas laws (Ghapter 4). 

The tools for calculating the equilibrium point of a chemical reaction arise from the definition of the chemical potential. If temperature and pressure are fixed, the equilibrium point of a reaction is the point at which the Gibbs free energy function G is at its minimum (Fig. 3.1). As with any convex-upward function, finding the minimum G is a matter of determining the point at which its derivative vanishes. 

At constant temperature and pressure, chemical reactions are spontaneous in the direction of decreasing Gibbs free energy. Some reactions are spontaneous because they give off energy in the form of heat (AH<0). Other reactions are spontaneous because they lead to an increase in the disorder of the system (AS>0). Calculations of AH and AS can be used to probe the driving force behind a particular reaction. 

K can often be calculated from the relationships of thermodynamics, if the Gibbs free energy for the chemical reaction can be obtained. For a gaseous reaction in which the equilibrium constant Kp is determined from equilibrium pressures of each component, the following expressions may be written ... 

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... 

Thermodynamic software packages may be used to find equilibrium compositions at prescribed temperatures and pressures. Such calculations require knowledge of feed components and products and their thermodynamic properties and are based on Gibbs free energy minimization techniques. Examples of thermodynamic packages may be found in Smith and Missen (Chemical Reaction Equilibrium Analysis Theory and Algorithms, Wiley, 1982) and in Walas (Phase Equilibria in Chemical Engineering, Butterworths, 1985). 

The direction of chemical reactions can always be predicted from thermodynamical data. Thus, if the Gibbs free energy change of a reaction is calculated,... 

The reaction coordinate calculations [100] confirmed the mechanistic hypothesis depicted in Scheme 1, i.e., the entire chemical reaction process consists of four individual steps (ES TSl INTI TS2 INT2 TS3 INT3 TS4 EB). The calculated energy barriers (A a) and Gibbs free energy barriers (AGa) are summarized in Table 3. 

When processes are conducted at constant T and P, the criteria for spontaneity and for equilibrium are stated more conveniently in terms of another state function called the Gibbs free energy (denoted by G), which is derived from S. Because chemical reactions are usually conducted at constant T and constant P, their thermodynamic description is based on AG rather than AS. This chapter concludes by restating the criteria for spontaneity of chemical reactions in terms of AG. Chapter 14 shows how to identify the equilibrium state of a reaction, and calculate the equilibrium constant from AG. 

Because G is a state function, chemical equations can be added together—with their AG° values combined as in Hess s law for changes in enthalpy—to calculate Gibbs free energy changes for chemical reactions under standard-state conditions. 

Calculate the change in Gibbs free energy for chemical reactions and identify temperature ranges in which a particular reaction is spontaneous (Section 13.7, Problems 35 0). 

Chemical equilibrium dictates the extent of reaction possible for a given temperature and pressure. For single simple reactions, an equilibrium constant approach can be used to determine the equilibrium concentration of gases for a given reaction. At equilibrium, the forward and the reverse reaction rates are equal. The equilibrium constant is calculated from the Gibbs free energy, as follows ... 

The change in Gibbs free energy (AG), which occurs as a system proceeds toward equilibrium, can be expressed as the sum of two terms. The first term is the standard free energy change (A G°), which is fixed for any given reaction. AG° can be calculated from the stoichiometry of the reaction (i.e., how many moles of one compound react with how many moles of another compound) and the standard free energies of the chemicals involved. The second term contains the reaction quotient (Q), which depends on the concentrations of chemicals present. The fact that AG can be expressed in terms of the concentrations of all chemicals present in a system makes it possible to determine in which direction a chemical reaction will proceed and to predict its final composition when it reaches equilibrium. 

Chemical equilibrium for a reaction is associated with the change in Gibbs free energy (AGr) calculated as follows ... 

Calculating the standard Gibbs free energy of reaction AG° for a given chemical reaction... 

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