Standard change in free energy

It is important to note that, for any given temperature, the [thermodynamic] equilibrium constant is directly related to the standard change in free energy. Since, at any given temperature, the free energy in the standard state for each reactant and product, G°, is independent of the pressure, it follows that the standard change in free energy for the reaction, AfG°, is independent of the pressure.g Therefore, at constant temperature, the equilibrium constant K. .. is also independent of the pressure. That is,... 

EXAMPLE 17.5 Calculating the Standard Change in Free Energy for a Reaction Using AG xn = A//"xn ... 

Finally, recall from Chapter 17 that the standard change in free energy for a chemical reaction (AG°) represents the maximum amount of work that can be done by the reaction. Therefore, = AG°. Making this substitution into Equation 18.2, we arrive at the following important result ... 

Notice that the fundamental quantity in the given relationships is the standard change in free energy for a chemical reaction (AG°xn)- From that quantity, we can calculate both and K. The relationships between these three quantities is summarized with the following diagram ... 

We can derive an exact relationship between Ectw (under nonstandard conditions) and 11 by considering the relationship between the change in free energy (AG) and the standard change in free energy (AG°) from Section 17.8 ... 

JGo, Ho, dSo are the standard changes in free energy of activation, enthalpy of activation, and entropy of activation for the reaction A+B (A-----------B)", so that... 

The Conventional Standard Free Energy of Solution. Returning now to the solution of a crystalline solid, let us consider the free energy of solution. Taking a uni-univalent substance let AF denote the change in free energy per mole when additional ions are added to a solution at temperature T where the solute has the mole fraction x and let us fix attention on the quantity... 

AG° = the molar standard state Gibbs free energy (the change in free energy of a reaction when the products and reactants are maintained at standard conditions)... 

From changes in free energy in standard reference conditions it is possible to calculate equilibrium constants for reactions involving several reactants and products. Consider, for example, the chemical reaction aA + bB = cC + dD at equilibrium in solution. For this reaction we can define a stoichiometric equilibrium constant in terms of the concentrations of the reactants and products as... 

The main significance of being able to calculate or to measure AG values is that we are then able to make predictions about reactions and in particular identify which reactions are likely to be control points in pathways. The absolute numerical value of the actual change in free energy (AG ) is dependant upon the actual concentrations of the reactant(s) and product(s) involved in the reaction. Comparisons of values for different reactions are meaningless unless they have been determined under identical and standardized experimental conditions. The term standard free energy (symbol AG°) is used to specify just such conditions. 

Using the data in the Table l4-6, the Standard Free Energies of Formation, calculate the change in free energy, (A, for this reaction) ... 

Thus, the rate constant for desorption can be expressed as a function of the standard free energy of desorption, A, and the change in free energy of the monolayer resulting from interactions within the film, , when the film is compressed. It is noteworthy that Equation 6 is similar to one developed by Davies (6). 

If the reactants and products were in their standard states of unit activity, then of course the voltage (E°) is the "standard cell potential," and the change in free energy is the change in "standard free energy" (AG°) ... 

When we follow the changes in free energy that occur during a reaction, we need to distinguish between the standard reaction free energy, AGr°, and the reaction free energy, AGr ... 

The standard free-energy change, A G°, for a reaction is the change in free energy that occurs when reactants in their standard states are converted to products in their standard states. As with AH° (Section 8.10), the value of AG° is an extensive property that refers to the number of moles indicated in the chemical equation. For example, AG° at 25°C for the reaction... 

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