One mole of reaction

One mole of reaction gives two moles of the product A 2B I. where... 

The symbol Am refers to the change in number of moles of gases only when one mole of reaction occurs. In common reactions (An)RT contributes a few kilojoules to the difference Qp — Qv (see Exercise 4.1 at the end of this chapter). 

The reaction also can be carried out reversibly if additional constraints are placed on the system, as in the cell illustrated by Figure 7.2. The H2 and CI2 electrodes are connected to a potentiometer. If the electromotive force of the cell is opposed by the eleetromotive force of the potentiometer, which is maintained at an infinitesimally lower value than that of the H2-CI2 cell, then the conversion to HCl can be carried out reversibly, although it would take an infinitely long time to obtain one mole of reaction. The change in the Gibbs function is the same for either the reversible or the explosively spontaneous path for carrying out the transformation, because the initial and final states are the same in both cases. However, the amount of useful (electrical) work is different, and, for the reversible path... 

We can define the left side of Equation 10.20 as AG, where the process described is one mole of reaction at constant chemical potential for reactants and products, that is, for a system large enough so that one mole of reaction can take place in the mixture without any significant change in composition or chemical potential, an infinite-copy model. As AG is a constant at constant temperature, the quantity in brackets is also a constant at constant temperamre, and, in particular, independent of the total pressure and the initial composition of the system. We therefore designate the quantity in brackets as Kp, which is the equilibrium constant in terms of partial pressures for a... 

The values of AG just calculated apply to a system in which all reactants and products are at standard pressure and which is sufficiently large that one mole of reaction does not alter the pressures appreciably. Alternatively, the expression In can be equated to dG/dn)Tp for a finite system, the initial rate of change of the total Gibbs function of the system per mole of reaction when all reactants and products are at standard pressure [2]. 

By definition, n/max = -AG, where AG is the free energy change associated with one mole of reaction and hence... 

The amount of heat shown in such an equation always refers to the reaction for the number of moles of reactants and products specified by the coefficients. We call this one mole of reaction. It is important to specify the physical states of aU substances, because different physical states have different energy contents. 

The numerical value of AH (or any other thermodynamic change) refers to the number of moles of substances specified by the equation. This amount of change of substances is called one mole of reaction, so we can express AH in units of energy/mol rxn. For brevity, the units of AH are sometimes written kJ/mol or even just kJ. No matter what units are used, be sure that you interpret the thermodynamic change per mole of reaction for the balanced chemical equation to which it refers. If a different amount of material is involved in the reaction, then the AH (or other change) must be scaled accordingly. 

Remember that a thermochemical equation can imply different numbers of moles of different reactants or products. In Example 15-5 one mole of reaction also corresponds to 3 moles of 02(g) and to 2 moles of Al203(s). 

In order to simplify the equations, properties of the whole system and properties per mole have not been distinguished by changes in notation. The context in which a property is defined is made clear in the text. In most cases the thermodynamic properties refer to one mole of material and changes in thermodynamic properties are for one mole of reaction. 

This equation applies not only to equilibria involving gases but also to equilibria in solution, and indeed to any equilibrium when the equilibrium constant is expressed in terms of mole fractions rather than partial pressures. In these circumstances AF is the volume change accompanying one mole of reaction. ... 

Ais the change in free energy when a mole of A at unit molality is transformed into a mole of B at the same concentration, both solutions behaving as if they were at infinite dilution. In other words the free-energy change for one mole of reaction with both reactants in their standard states is AG —RTlnKa. Ka, the equilibrium constant expressed in terms of activities, is exactly constant at any given temperature and pressure. As... 

AH 9 is the difference in enthalpy between the products and reactants in their standard states for one mole of reaction. For systems not at equilibrium the change in free energy for a mole of reaction, all concentrations being maintained constant is, as before,... 

Electrical work is the product of charge multiphed by the electromotive force (emf) potential ( ) of the cell. If the work done results from an electrochemical reaction with cell potential, E, and if the charge is defined for one mole of reaction in which moles... 

These relations come from the stoichiometry of the reaction as expressed by the stoichiometric coefficients in the reaction equation. The second relation, for example, expresses the fact that when one mole of reaction has occurred = 1 mol), the amount of Hi in the closed system has decreased by three moles. 

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