Gibbs Free Energy and Thermodynamic Favorability - AP Chemistry

1. When is a process never spontaneous at any temperature?

   $\Delta H > 0$ and $\Delta S < 0$. Endothermic with negative entropy makes $\Delta G$ always positive.

2. Identify the units of Gibbs free energy change, $\Delta G$.

   Joules (J) or kilojoules (kJ). Energy units since $\Delta G$ represents energy change per mole of reaction.

3. What does a negative $\Delta G$ indicate about a reaction?

   The reaction is thermodynamically favorable. Negative $\Delta G$ means the process can occur spontaneously without external work.

4. Given $\Delta G = -10 \text{ kJ/mol}$, is the reaction spontaneous?

   Yes, it is spontaneous. Negative $\Delta G$ values always indicate spontaneous processes.

5. What is the expression for $\Delta S_{univ}$ in terms of $\Delta G$?

   $\Delta S_{univ} = -\frac{\Delta G}{T}$. This relates universal entropy change to Gibbs free energy change.

6. How is $\Delta G$ calculated for a reaction not at equilibrium?

   $\Delta G = \Delta G^\circ + RT \ln Q$. This equation accounts for non-standard conditions using the reaction quotient.

7. What is the thermodynamic favorability of a reaction with $\Delta G^\circ = -40 \text{ kJ/mol}$?

   Favorable. Large negative $\Delta G^\circ$ values indicate highly favorable reactions.

8. What is the standard state temperature for $\Delta G^\circ$ calculations?

   $298 \text{ K}$. Standard temperature for thermodynamic calculations is 25°C or 298 K.

9. How does $\Delta G$ relate to the maximum useful work a system can perform?

   $\Delta G$ equals the maximum useful work. The magnitude of $\Delta G$ represents maximum work extractable from the process.

10. In terms of enthalpy and entropy, when is a process always spontaneous?

    $\Delta H < 0$ and $\Delta S > 0$. Exothermic with positive entropy ensures $\Delta G$ is always negative.

11. What does $\Delta G > 0$ indicate about a system's state?

    It is non-spontaneous. Positive $\Delta G$ means the process requires energy input to proceed.

12. Which variable in $\Delta G = \Delta H - T \Delta S$ is affected by temperature changes?

    The $T \Delta S$ term. Only the entropy term $T \Delta S$ contains temperature as a variable.

13. When is $\Delta G = \Delta G^\circ$ true?

    When the reaction is at standard state conditions. Standard conditions mean all species at 1 M concentration or 1 atm pressure.

14. Identify the role of catalysts in altering $\Delta G$.

    Catalysts do not alter $\Delta G$. Catalysts only affect reaction rates, not thermodynamic quantities.

15. What is the relationship between $\Delta G$ and reaction rate?

    $\Delta G$ does not affect reaction rate. $\Delta G$ determines thermodynamic favorability, not kinetic rate.

16. What is the significance of $T\Delta S$ in the Gibbs free energy equation?

    It represents the temperature-dependent entropy term. This term quantifies the entropy contribution weighted by temperature.

17. State the relationship between $K_{eq}$ and $\Delta G^\circ$.

    $\Delta G^\circ = -RT \ln K_{eq}$. This equation connects equilibrium constants to standard free energy changes.

18. What does a positive $\Delta G^\circ$ indicate about a reaction?

    The reaction is non-spontaneous. Positive standard free energy means unfavorable under standard conditions.

19. What is the significance of $\Delta H$ in the Gibbs free energy equation?

    It represents the change in enthalpy. Enthalpy accounts for heat absorbed or released during the reaction.

20. Is a reaction with $K_{eq} > 1$ product-favored or reactant-favored?

    Product-favored. Large equilibrium constants indicate reactions favor product formation.

21. What happens to $\Delta G$ when a reaction is reversed?

    The sign of $\Delta G$ is reversed. Reversing a reaction changes the sign of all thermodynamic quantities.

22. What is the formula for Gibbs free energy change?

    $\Delta G = \Delta H - T\Delta S$. This is the fundamental thermodynamic equation relating free energy, enthalpy, and entropy.

23. Which variable in $\Delta G = \Delta H - T\Delta S$ is affected by temperature changes?

    The $T\Delta S$ term. Only the entropy term $T\Delta S$ contains temperature as a variable.

24. Explain the effect of $\Delta S > 0$ on a reaction's spontaneity.

    It favors spontaneity. Positive entropy change makes the $-T\Delta S$ term negative, favoring spontaneity.

25. What condition must be met for a process to be spontaneous?

    $\Delta G < 0$. Spontaneous processes have negative free energy changes.

26. What is the significance of $\Delta G^\circ = 0$?

    The reaction is in equilibrium under standard conditions. Zero standard free energy means $K_{eq} = 1$ at standard conditions.

27. For which $\Delta H$ and $\Delta S$ values is spontaneity temperature-dependent?

    $\Delta H$ and $\Delta S$ both positive or both negative. Temperature determines which term dominates in the Gibbs equation.

28. Which term in the Gibbs free energy equation accounts for disorder?

    $\Delta S$. Entropy measures the randomness or disorder of the system.

29. What is the value of $R$ in the equation $\Delta G^\circ = -RT \ln K_{eq}$?

    8.314 J/(mol·K). The universal gas constant in SI units for thermodynamic calculations.

30. What does $\Delta G = 0$ signify for a system at equilibrium?

    The system is at equilibrium. Zero $\Delta G$ indicates no net driving force for change in either direction.

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