Decrease in free energy of a reacting system indicates:
an exothermic reaction
an endothermic reaction
a spontaneous reaction
a slow reaction.

Options

A. an exothermic reaction
B. an endothermic reaction
C. a spontaneous reaction
D. a slow reaction.

Explanation

To solve the question regarding the decrease in free energy of a reacting system, we need to analyze the implications of Gibbs free energy (G) in thermodynamics.

### Step-by-Step Solution:

1. **Understanding Gibbs Free Energy (G)**:
- Gibbs free energy is a thermodynamic potential that helps predict the direction of chemical reactions and phase changes at constant temperature and pressure.
- It is represented as G = H - TS, where H is enthalpy, T is temperature, and S is entropy.

2. **Change in Gibbs Free Energy (ΔG)**:
- The change in Gibbs free energy (ΔG) during a reaction is calculated as ΔG = G_final - G_initial.
- A decrease in free energy (ΔG < 0) indicates that the final state of the system has less free energy than the initial state.

3. **Spontaneity of Reactions**:
- A reaction is considered spontaneous if it can occur without any external influence.
- According to thermodynamic principles, a reaction is spontaneous when ΔG is negative (ΔG < 0).

4. **Interpreting the Options**:
- **Exothermic Reaction**: While many spontaneous reactions are exothermic (release heat), not all spontaneous reactions are exothermic.
- **Endothermic Reaction**: An endothermic reaction absorbs heat, and it can be spontaneous if it results in a significant increase in entropy.
- **Spontaneous Reaction**: This is the correct interpretation of a decrease in Gibbs free energy, as it indicates that the reaction can proceed without external energy input.
- **Slow Reaction**: The rate of a reaction does not determine its spontaneity. A reaction can be spontaneous but occur at a slow rate.

5. **Conclusion**:
- Therefore, a decrease in free energy of a reacting system indicates that the reaction is spontaneous.

### Final Answer:
The correct option is **a spontaneous reaction**.

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