The free energy change, `Delta`G = 0, when

Options

A. The reactants are completely consumed
B. A catalyst is added
C. The system is at equilibrium
D. The reactants are initially start reacting

Answer

To solve the question regarding when the free energy change (ΔG) equals zero, we can analyze the options provided and apply our understanding of thermodynamics.

### Step-by-Step Solution:

1. **Understanding ΔG**: The Gibbs free energy change (ΔG) is a thermodynamic quantity that indicates the spontaneity of a process. It can take three forms:
- ΔG < 0: The process is spontaneous.
- ΔG = 0: The system is at equilibrium.
- ΔG > 0: The process is non-spontaneous.

2. **Analyzing the Options**:
- **Option 1**: "The reactants are completely consumed."
- When reactants are completely consumed, the reaction has gone to completion. At this point, the system is not at equilibrium, and ΔG would be greater than zero as the process is no longer spontaneous. **(Incorrect)**

- **Option 2**: "A catalyst is added."
- A catalyst speeds up the reaction but does not change the Gibbs free energy of the system. It does not affect whether the process is spontaneous or at equilibrium. Therefore, ΔG remains unchanged and is not necessarily zero. **(Incorrect)**

- **Option 3**: "The system is at equilibrium."
- At equilibrium, the forward and reverse reactions occur at the same rate, and there is no net change in the concentrations of reactants and products. Here, ΔG is equal to zero. **(Correct)**

- **Option 4**: "The reactants are initially starting to react."
- When reactants begin to react, the process may be spontaneous, but it does not imply that ΔG is zero. In fact, during the initial stages, ΔG is typically less than zero as the reaction proceeds. **(Incorrect)**

3. **Conclusion**: The correct answer is that ΔG = 0 when the system is at equilibrium.

### Final Answer:
The free energy change, ΔG = 0, when the system is at equilibrium.

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