Question

At equilibrium, Gibbs free energy change is:

• A. Positive
• B. Negative
• C. Zero
• D. Infinite

Hint:

Do not confuse $\Delta G$ with the standard Gibbs free energy change $\Delta G^\circ$.
At equilibrium, $\Delta G = 0$, but $\Delta G^\circ$ is usually non-zero and is related to the equilibrium constant by $\Delta G^\circ = -RT \ln K$.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the value of the Gibbs free energy change ($\Delta G$) when a thermodynamic system reaches equilibrium at constant temperature and pressure.

Step 2: Key Formula or Approach:
The criterion for equilibrium in a closed system at constant $T$ and $P$ is that the Gibbs free energy $G$ is at a minimum:
\[ dG_{T,P} = 0 \]

Step 3: Detailed Explanation:

• During a spontaneous process, the Gibbs free energy of the system decreases ($\Delta G < 0$).

• The reaction continues to proceed until the system reaches its state of minimum free energy.

• Once this minimum point is reached, any further infinitesimal change in either direction would require an increase in free energy.

• Therefore, at this minimum potential state, the rate of the forward reaction equals the rate of the reverse reaction, and the net change in Gibbs free energy is exactly zero ($\Delta G = 0$).

Step 4: Final Answer:
At equilibrium, the Gibbs free energy change is zero.