Standard free energy change calculation

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

Using AG values from Appendix 1, calculate the standard free energy change at 25°C for the reaction referred to in Example 17.3. 

All of the free energy calculations to this point have involved the standard free energy change, AG°. It is possible, however, to write a general relation for the free energy change, AG, valid under any conditions. This relation is a relatively simple one, but we will not attempt to derive it. It tells us that... 

One of the most important characteristics of a cell is its voltage, which is a measure of reaction spontaneity. Cell voltages depend on the nature of the half-reactions occurring at the electrodes (Section 18.2) and on the concentrations of species involved (Section 18.4). From the voltage measured at standard concentrations, it is possible to calculate the standard free energy change and the equilibrium constant (Section 18.3) of the reaction involved. 

The standard free energy change can be calculated from the equilibrium constant... 

As Equation describes, the standard free energy change for a reaction can also be calculated from AH ° and As ° for the reaction by making use of Equation under standard conditions AG° = A H ° - T AS ° Either of these equations can be used to find standard free energy changes. Which equation to use depends on the available data. Example illustrates both types of calculations. 

Equation expresses an important link between two standard quantities. The equation lets us calculate standard electrical potentials from tabulated values for standard free energies. Equally important, accurate potential measurements on galvanic cells yield experimental values for standard potentials that can be used to calculate standard free energy changes for reactions. 

Equation can be used to calculate the standard free energy change for this net reaction, applying the appropriate... 

C20-0065. Calculate the standard free energy change at 25 °C for reduction of ZnO to Zn using carbon and using carbon monoxide. 

The Van t Hoff isotherm establishes the relationship between the standard free energy change and the equilibrium constant. It is of interest to know how the equilibrium constant of a reaction varies with temperature. The Varft Hoff isochore allows one to calculate the effect of temperature on the equilibrium constant. It can be readily obtained by combining the Gibbs-Helmholtz equation with the Varft Hoffisotherm. The relationship that is obtained is... 

In order to determine in which direction the reaction is spontaneous, we need to calculate the non-standard free energy change for the reaction. To accomplish this, we will employ the equation AG = AG° + RTlnQc, where... 

Thus a useful parameter (AG ) can be calculated for any reaction (not only those operating under standard conditions) providing the relative concentrations of r and p, the temperature and the standard free energy change AG° are known. 

Given that the standard free energy change for the hydrolysis of ATP is —30.5 kj / mol, (ii) calculate the total amount (in grams) of ATP required to furnish the energy to sustain the competitor during the race. 

Calculate the approximate standard free energy change for the ionization of hydrofluoric acid, HF (K,= lx 10 ), at25°C. 

In the Marcus version of the theory, AG°, the standard free energy change of the reaction considered is taken as zero, (f) When will this apply (h) What error in the calculation of the energy of activation would neglect of AG° cause in the calculated energy of activation for Fe +e0— Fe Give a list of papers and books you consulted in researching your answer. 

The equilibrium constant is fixed and characteristic for any given chemical reaction at a specified temperature. It defines the composition of the final equilibrium mixture, regardless of the starting amounts of reactants and products. Conversely, we can calculate the equilibrium constant for a given reaction at a given temperature if the equilibrium concentrations of all its reactants and products are known. As we will show in Chapter 13, the standard free-energy change (A(3°) is directly related to Ke[Pg.61]

As an example, let s make a simple calculation of the standard free-energy change of the reaction catalyzed by the enzyme phosphoglucomutase ... 

The standard free energy of hydrolysis of ATP is -30.5 kJ/mol. In the cell, however, the concentrations of ATP, ADP, and Pi are not only unequal but much lower than the standard 1 m concentrations (see Table 13-5). Moreover, the cellular pH may differ somewhat from the standard pH of 7.0. Thus the actual free energy of hydrolysis of ATP under intracellular conditions (AGP) differs from the standard free-energy change, AG °. We can easily calculate AGP. 

Calculation of the exact value of AGP is perhaps less instructive than the generalization we can make about actual free-energy changes in vivo, the energy released by ATP hydrolysis is greater than the standard free-energy change, AG °. 

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