The term `-(dx)/(dt)` in the rate expression refers to the :

Options

A. Concentration of the reactants
B. Increase in concentration of the reactants
C. Instantaneous rate of the reaction
D. Average rate of the reaction

Explanation

To solve the question regarding the term `-(dx)/(dt)` in the rate expression, we can break it down step by step.

### Step-by-Step Solution:

1. **Understanding the Expression**: The term `-(dx)/(dt)` represents the rate of change of concentration of a reactant with respect to time. Here, `x` typically denotes the concentration of a reactant.

2. **Concept of Rate**: In chemical kinetics, the rate of a reaction is often expressed as the change in concentration of reactants or products over time. The negative sign indicates that the concentration of reactants decreases as the reaction proceeds.

3. **Instantaneous vs Average Rate**:
- **Instantaneous Rate**: This refers to the rate of reaction at a specific moment in time. It is calculated by taking the derivative of concentration with respect to time, which is what `-(dx)/(dt)` signifies.
- **Average Rate**: This is calculated over a finite interval of time and does not reflect the changes at a specific instant.

4. **Why the Negative Sign?**: The negative sign is used because the concentration of reactants decreases as the reaction progresses. If the concentration of a reactant decreases from a higher value to a lower value, the change is negative. To express the rate as a positive value, we multiply by -1.

5. **Conclusion**: Therefore, the term `-(dx)/(dt)` refers to the instantaneous rate of the reaction, indicating how quickly the concentration of a reactant is changing at a specific moment in time.

### Final Answer:
The term `-(dx)/(dt)` in the rate expression refers to the **instantaneous rate of the reaction**.

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