Free energy standard conditions

Free energy is related to two other energy quantities, the enthalpy (the heat of reaction measured at constant pressure) and the entropy. S. an energy term most simply visualised as a measure of the disorder of the system, the relationship for a reaction taking place under standard conditions being... 

Equilibrium constants for protein-small molecule association usually are easily measured with good accuracy it is normal for standard free energies to be known to within 0.5 kcal/mol. Standard conditions define temperature, pressure and unit concentration of each of the three reacting species. It is to be expected that the standard free energy difference depends on temperature, pressure and solvent composition AA°a also depends on an arbitrary choice of standard unit concentrations. 

The first term, AG°, is the change in Gibb s free energy under standard-state conditions defined as a temperature of 298 K, all gases with partial pressures of 1 atm, all solids and liquids pure, and all solutes present with 1 M concentrations. The second term, which includes the reaction quotient, Q, accounts for nonstandard-state pressures or concentrations. Eor reaction 6.1 the reaction quotient is... 

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

Through all these calculations of the effect of pH and metal ions on the ATP hydrolysis equilibrium, we have assumed standard conditions with respect to concentrations of all species except for protons. The levels of ATP, ADP, and other high-energy metabolites never even begin to approach the standard state of 1 M. In most cells, the concentrations of these species are more typically 1 to 5 mM or even less. Earlier, we described the effect of concentration on equilibrium constants and free energies in the form of Equation (3.12). For the present case, we can rewrite this as... 

Under cellular conditions, this first reaction of glycolysis is even more favorable than at standard state. As pointed out in Chapter 3, the free energy change for any reaction depends on the concentrations of reactants and products. 

Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyrnvate + H9O. The standard free energy change, AG°, for this reaction is +1.8 kj/mol. If the concentration of 2-phosphoglycerate is 0.045 mM and the concentration of phosphoenolpyrnvate is 0.034 mM, what is AG, the free energy change for the enolase reaction, under these conditions ... 

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. 

Calculate the value of A l,/ for the glyceraldehyde-3-phos-phate dehydrogenase reaction, and calculate the free energy change for the reaction under standard-state conditions. 

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

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... 

As a result of the combination of Eqs. (20) and (21), the reaction free energy, AG, and the equilibrium cell voltage, A< 00, under standard conditions are related to the sum of the chemical potentials //,. of the substances involved ... 

Equation (9.5) enables us to calculate ArG for a chemical reaction under a given set of activity conditions when we know the free energy change for the reaction under the standard state condition. Of special interest are the activities when reactants and products are at equilibrium. Under those conditions,... 

Use the standard Gibbs free energies of formation in Appendix 2A to calculate AG° for each of the following reactions at 25°C. Comment on the spontaneity of each reaction under standard conditions at 25°C. 

The reaction Gibbs free energies are for pH = 7 but otherwise standard conditions.) What amount (in moles) of ATP could be formed if all the Gibbs free energy released in the oxidation of... 

In practice, G and H for a substance are defined relative to the G and H for the constituent elements of that substance. These relative values are known as free energy of formation and enthalpy of formation for standard conditions and... 

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. 

N2 + 6 H2 O -> 4 NH3 + 3 O2 Assuming standard conditions (not quite tme but close enough for approximations), how much free energy must the bacteria consume to fix one mole of N atoms ... 

---