Question

At any temperature, thermodynamically stable phase is the one which has.

• A. maximum free energy
• B. maximum enthalpy
• C. lowest free energy
• D. lowest entropy

Hint:

Think of a ball rolling down a hill. It is most stable when it reaches the bottom (lowest potential energy). Similarly, a chemical system is most stable when it reaches the state of lowest Gibbs Free Energy.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Thermodynamic stability refers to the state of a system that is in its lowest energy state, or chemical equilibrium, with its environment. The key thermodynamic potential used to determine spontaneity and stability under conditions of constant temperature and pressure is the Gibbs Free Energy (G).

Step 2: Key Formula or Approach:
The Gibbs Free Energy (G) is defined as:
\[ G = H - TS \] where H is enthalpy, T is the absolute temperature, and S is entropy.
A process is spontaneous if the change in Gibbs Free Energy (\(\Delta G\)) is negative. A system will spontaneously evolve towards a state with a lower G.

Step 3: Detailed Explanation:
The fundamental principle of thermodynamics is that systems tend to move towards a state of minimum energy to achieve stability.

• Maximum free energy: A state with maximum free energy is highly unstable and will spontaneously transform into a state with lower free energy.

• Maximum enthalpy: While systems tend to favor lower enthalpy (exothermic processes), this is not the sole determinant of stability. The entropy term (\(TS\)) is also crucial.

• Lowest free energy: At constant temperature and pressure, a system is at equilibrium and most stable when its Gibbs Free Energy is at a minimum. Any deviation from this state will result in a spontaneous process that brings the system back towards this minimum. For a given set of conditions, the phase (solid, liquid, or gas) or crystal structure with the lowest Gibbs Free Energy will be the stable phase.

• Lowest entropy: Systems tend to evolve towards a state of maximum entropy (Second Law of Thermodynamics), not minimum entropy. A state of lowest entropy is highly ordered and generally less stable at higher temperatures.
Therefore, the thermodynamically stable phase is the one that possesses the minimum possible Gibbs Free Energy under the given conditions.

Step 4: Final Answer:
The criterion for thermodynamic stability at constant temperature and pressure is the minimization of Gibbs Free Energy. Thus, the most stable phase is the one with the lowest free energy.