Using standard free energies of formation to calculate the standard free energy of a reaction is analogous to calculating standard enthalpy changes from standard enthalpies of formation using the familiar minus reactants” rule: “products where m and n are the stoichiometric coefficients of each product and reactant in the balanced chemical equation. A very large negative indicates a strong tendency for products to form ΔG° spontaneously from reactants; it does not, however, necessarily indicate that the reaction will occur rapidly. To make this determination, we need to evaluate the kinetics of the reaction. Note the Pattern The of a reaction can be calculated from tabulated values the minus reactants” ΔG° “products rule. Calculated values of are extremely useful in predicting whether a reaction will occur ΔG° spontaneously if the reactants and products are mixed under standard conditions. We should note, however, that very few reactions are actually carried out under standard conditions, and calculated values of may not tell us whether a given reaction will occur spontaneously under nonstandard ΔG° conditions. What determines whether a reaction will occur spontaneously is the free-energy change (ΔG) under the actual experimental conditions, which are usually different from If the and ΔG°. ΔH TΔS terms for a reaction have the same sign, for example, then it may be possible to reverse the sign of by changing the temperature, thereby converting a reaction that is not thermodynamically ΔG spontaneous, having Keq < 1, to one that is, having a Keq > 1, or vice versa. Because and ΔH ΔS usually do not vary greatly with temperature in the absence of a phase change, we can use tabulated values of and to calculate at various temperatures, as long as no phase change occurs ΔH° ΔS° ΔG° over the temperature range being considered.
Using standard free energies of formation to calculate the standard free energy of a reaction is analogous to calculating standard enthalpy changes from standard enthalpies of formation using the familiar “products minus reactants” rule: where m and n are the stoichiometric coefficients of each product and reactant in the balanced chemical equation. A very large negative ΔG° indicates a strong tendency for products to form spontaneously from reactants; it does not, however, necessarily indicate that the reaction will occur rapidly. To make this determination, we need to evaluate the kinetics of the reaction. Note the Pattern The ΔG° of a reaction can be calculated from tabulated values the “products minus reactants” rule. Calculated values of ΔG° are extremely useful in predicting whether a reaction will occur spontaneously if the reactants and products are mixed under standard conditions. We should note, however, that very few reactions are actually carried out under standard conditions, and calculated values of ΔG° may not tell us whether a given reaction will occur spontaneously under nonstandard conditions. What determines whether a reaction will occur spontaneously is the free-energy change (ΔG) under the actual experimental conditions, which are usually different from ΔG°. If the ΔH and TΔS terms for a reaction have the same sign, for example, then it may be possible to reverse the sign of ΔG by changing the temperature, thereby converting a reaction that is not thermodynamically spontaneous, having Keq < 1, to one that is, having a Keq > 1, or vice versa. Because ΔH and ΔS usually do not vary greatly with temperature in the absence of a phase change, we can use tabulated values of ΔH° and ΔS° to calculate ΔG° at various temperatures, as long as no phase change occurs over the temperature range being considered.
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