Reactions and Gibbs free energy

Heat of Reaction and Gibbs Free Energy Change... 

Appendix B also includes values for 5 (298.15) and A/ Gr (298.15), from which entropies of reaction and Gibbs free energies of reactions can be calculated at 298.15 K from... 

Why Do We Need to Know This Material The second law of thermodynamics is the key to understanding why one chemical reaction has a natural tendency to occur bur another one does not. We apply the second law by using the very important concepts of entropy and Gibbs free energy. The third law of thermodynamics is the basis of the numerical values of these two quantities. The second and third laws jointly provide a way to predict the effects of changes in temperature and pressure on physical and chemical processes. They also lay the thermodynamic foundations for discussing chemical equilibrium, which the following chapters explore in detail. 

Calculate the standard reaction entropy, enthalpy, and Gibbs free energy for each of the following reactions from data found in Appendix 2A ... 

To find the connection between cell potential and Gibbs free energy, recall that ir Section 7.14 (Eq. 21) we saw that the change in Gibbs free energy is the maximum nonexpansion work that a reaction can do at constant pressure and temperature ... 

Standard reaction enthalpy (X = H) and Gibbs free energy (X = G) from standard enthalpies of formation ... 

In order to have a consistent basis for comparing different reactions and to permit the tabulation of thermochemical data for various reaction systems, it is convenient to define enthalpy and Gibbs free energy changes for standard reaction conditions. These conditions involve the use of stoichiometric amounts of the various reactants (each in its standard state at some temperature T). The reaction proceeds by some unspecified path to end up with complete conversion of reactants to the various products (each in its standard state at the same temperature T). 

The enthalpy and Gibbs free energy changes for a standard reaction are denoted by the... 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

Since reaction 14 can be considered similar to reaction 13, approximations between EA and Gibbs free-energy are possible this subject has been reviewed by Kebarle and Chowdhury40. 

Enthalpy and Gibbs free energy changes and equilibrium constants for the reaction CO(g)+2H2(g) = CH30H(g)... 

We represent A///° and A/G° as the standard enthalpy and Gibbs free energy changes for the reaction in which the chemical substance is formed from the elements in their stable form, as they occur in nature at T = 298.15 K.rr For ions in solution, the values tabulated are relative to the standard enthalpy and Gibbs free energy of formation of the H+ ion being set equal to zero.ss... 

The existence of an energy balance is not sufficient to answer all questions about a chemical reaction. Does a given reaction take place at all If so, to what extent does it proceed Questions relating to the processes and extent of chemical reactions require the introduction of some new thermodynamic functions which, like E and //, are properties of the state of the system. These new functions are entropy, S, and Gibbs free energy, G. In order to answer these and other questions, a mathematical statement of the second law of thermodynamics is required ... 

Is there a function which can represent the balance between these two opposing factors, and thus be a measure of the tendency for a reaction to take place Such a function does exist and is known as the free energy Junction or simply free energy. We now derive two different free energies, namely, Helmholtz free energy and Gibbs free energy. 

Solution The electrode reactions, standard electrode potentials, and Gibbs free energies are as follows ... 

Table 5.4 Enthalpy and Gibbs free energy of reactions in standard conditions (kj/mol).

To apply the preceding concepts of chemical thermodynamics to chemical reaction systems (and to understand how thermodynamic variables such as free energy vary with concentrations of species), we have to develop a formalism for the dependence of free energies and chemical potential on the number of particles in a system. We develop expressions for the change in Helmholtz and Gibbs free energies in chemical reactions based on the definition of A and G in terms of Q and Z. The quantities Q and Z are called the partition functions for the NVT and NPT systems, respectively. 

Thermodynamics is the basis of all chemical transformations [1], which include dissolution of chemical components in aqueous solutions, reactions between two dissolved species, and precipitation of new products formed by the reactions. The laws of thermodynamics provide conditions in which these reactions occur. One way of determining such conditions is to use thermodynamic potentials (i.e., enthalpy, entropy, and Gibbs free energy of individual components that participate in a chemical reaction) and then apply the laws of thermodynamics. In the case of CBPCs, this approach requires relating measurable parameters, such as solubility of individual components of the reaction, to the thermodynamic parameters. Thermodynamic models not only predict whether a particular reaction is likely to occur, but also provide conditions (measurable parameters such as temperature and pressure) in which ceramics are formed out of these reactions. The basic thermodynamic potentials of most constituents of the CBPC products have been measured at room temperature (and often at elevated temperatures) and recorded in standard data books. Thus, it is possible to compile these data on the starter components, relate them to their dissolution characteristics, and predict their dissolution behavior in an aqueous solution by using a thermodynamic model. The thermodynamic potentials themselves can be expressed in terms of the molecular behavior of individual components forming the ceramics, as determined by a statistical-mechanical approach. Such a detailed study is beyond the scope of this book. 

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. 

The assessment of the driving force follows from the reaction standard Gibbs free energies A Go, calculated by using the corresponding energies of formation of the reactant CX (AGr) and the products (AGp) in reactions (4.11a,b). 

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