Explain whether a physical or chemical process is thermodynamically favored based on an evaluation of ΔGo. Thermodynamics and Electrochemistry UNIT 9 • The Gibbs free energy change for a chemical process in which all the reactants and products are present in a standard state (as pure substances, as solutions of 1.0 M concentration, or as gases at a pressure of 1.0 atm (or 1.0 bar)) is given the symbol ΔGo. • The standard Gibbs free energy change for a chemical or physical process is a measure of thermodynamic favorability. Historically, the term “spontaneous” has been used to describe processes for which ΔGo < 0. The phrase “thermodynamically favored” is preferred instead so that common misunderstandings (equating “spontaneous” with “suddenly” or “without cause”) can be avoided. When ΔGo < 0 for the process, it is said to be thermodynamically favored. • The standard Gibbs free energy change for a physical or chemical process may also be determined from the standard Gibbs free energy of formation of the reactants and products. EQN: ΔG°reaction = ΣΔGf °products − ΣΔGf °reactants • In some cases, it is necessary to consider both enthalpy and entropy to determine if a process will be thermodynamically favored. The freezing of water and the dissolution of sodium nitrate are examples of such phenomena. • Knowing the values of ΔH° and ΔS° for a process at a given temperature allows ΔG° to be calculated directly. EQN: ΔG° = ΔH° − T ΔS° • In general, the temperature conditions for a process to be thermodynamically favored (ΔG° < 0) can be predicted from the signs of ΔH° and ΔS° as shown in the table below: ΔH° ΔS° Symbols ΔG° < 0, favored at: < 0 > 0 < > all T > 0 < 0 > < no T > 0 > 0 > > high T < 0 < 0 < < low T In cases where ΔH° < 0 and ΔS° > 0, no calculation of ΔG° is necessary to determine that the process is thermodynamically favored (ΔG° < 0). In cases where ΔH° > 0 ΔS° < 0and , no calculation of ΔG° is necessary to determine that the process is thermodynamically unfavored (ΔG° > 0).

AP Chemistry 9.7 Coupled Reactions Topic Practice

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