At chemical equilibrium, a reaction is: — Chemical Equilibrium Chemistry Question

💡 Solution & Explanation

Step 1 - Define the Thermodynamic Criterion for Spontaneity The spontaneity of a chemical reaction at constant temperature and pressure is governed by the Gibbs free energy change ($\Delta G$): * $\Delta G < 0$: reaction is spontaneous in the forward direction. * $\Delta G > 0$: reaction is spontaneous in the backward direction. * $\Delta G = 0$: system is at chemical equilibrium. Step 2 - Thermodynamics at Chemical Equilibrium At chemical equilibrium, the Gibbs free energy reaches its minimum value along the reaction coordinate. The slope of the free energy curve (which equals $\Delta G$) is zero: \[\Delta G = 0 \text{ at equilibrium}\] Since $\Delta G = 0$: 1. There is no net driving force for the forward direction ($\Delta G \not< 0$). 2. There is no net driving force for the backward direction ($\Delta G \not> 0$). Therefore, the reaction is spontaneous in neither direction at equilibrium. Step 3 - Evaluate the Options * **(A)**: Incorrect. Spontaneous forward requires $\Delta G < 0$. * **(B)**: Incorrect. Spontaneous backward requires $\Delta G > 0$. * **(C)**: Incorrect. $\Delta G$ cannot be simultaneously positive and negative. * **(D)**: Correct. At equilibrium $\Delta G = 0$, so the reaction is spontaneous in neither direction. \[\boxed{\text{D}}\]