AP Chemistry Flashcards: Gibbs Free Energy and Thermodynamic Favorability
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 16 cards to help you master important concepts.
Is a process with a positive ΔH° and a negative ΔS° ever thermodynamically favored?
No, a process that is endothermic (ΔH° > 0) and becomes more ordered (ΔS° < 0) is never thermodynamically favored, as ΔG° will always be positive.
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Is a process with a positive ΔH° and a negative ΔS° ever thermodynamically favored?
No, a process that is endothermic (ΔH° > 0) and becomes more ordered (ΔS° < 0) is never thermodynamically favored, as ΔG° will always be positive.
What is the standard state concentration for a substance in a solution?
For a substance in a solution, the standard state is defined as a concentration of 1.0 M.
When is a process with a negative ΔH° and a negative ΔS° thermodynamically favored?
A process where both ΔH° and ΔS° are negative is thermodynamically favored only at low temperatures, where the unfavorable entropy term is minimized.
How can the signs of ΔH° and ΔS° be used?
The signs of ΔH° and ΔS° can be used to predict the temperature conditions under which a process will be thermodynamically favored (ΔG° < 0).
What is the primary equation used to calculate ΔG° from standard enthalpy and entropy changes?
The standard Gibbs free energy change can be calculated using the equation: ΔG° = ΔH° − TΔS°.
For processes like the dissolution of sodium nitrate, why is considering only enthalpy (ΔH°) insufficient to determine thermodynamic favorability?
For some processes, both enthalpy (ΔH°) and entropy (ΔS°) must be considered together to determine the overall thermodynamic favorability (ΔG°).
What does the term 'standard state' (°) signify in a chemical process?
Standard state implies conditions of pure substances, 1.0 M for solutions, or 1.0 atm (or 1.0 bar) for gases.
Under what temperature conditions is a process with a negative ΔH° and a positive ΔS° thermodynamically favored?
A process where ΔH° < 0 (exothermic) and ΔS° > 0 (increased disorder) is thermodynamically favored at all temperatures.
How is the standard Gibbs free energy change (ΔG°) used to determine if a process is thermodynamically favored?
A process is considered thermodynamically favored if its standard Gibbs free energy change is less than zero (ΔG° < 0).
What is the standard state pressure for a gas?
For a gas, the standard state is defined as a pressure of 1.0 atm or 1.0 bar.
When is a process with a positive ΔH° and a positive ΔS° thermodynamically favored?
A process where both ΔH° and ΔS° are positive is thermodynamically favored only at high temperatures, where the TΔS° term outweighs ΔH°.
What is the formula to calculate the ΔG° for a reaction using the standard Gibbs free energies of formation?
The formula is ΔG°reaction = ΣΔGf°products − ΣΔGf°reactants.
What is the key indicator of thermodynamic favorability for a physical or chemical process?
The key indicator is the evaluation of the standard Gibbs free energy change (ΔG°); a negative value indicates favorability.
If you know the standard enthalpy change (ΔH°) and standard entropy change (ΔS°) for a process, what other variable is required to calculate ΔG°?
To calculate ΔG° using the equation ΔG° = ΔH° − TΔS°, the absolute temperature (T) in Kelvin is also required.
Define Standard Gibbs free energy change (ΔG°).
Standard Gibbs free energy change is a measure of the thermodynamic favorability of a process conducted under standard state conditions.
What is the modern term for a process that was formerly called 'spontaneous'?
The term 'thermodynamically favored' is now used for a process with ΔG° < 0, replacing the older term 'spontaneous'.