In a reaction,
`aA+bBrarrcC+dD`
rate `=-1/a(d[A])/(dt)` , the - ve sign in it represents

Options

A. The rate of reaction increases with time
B. The rate is negative for the reaction
C. The concentration of the reactant 'A' decreases during the reaction
D. The concentration of reactant 'A' increases in the reaction

Answer

To solve the question regarding the negative sign in the rate expression for the reaction \( aA + bB \rightarrow cC + dD \), we will go through the following steps:

### Step-by-Step Solution:

1. **Understanding the Rate Expression**:
The rate of the reaction is given as:
\[ \text{rate} = -\frac{1}{a} \frac{d[A]}{dt} \]
Here, \([A]\) represents the concentration of reactant A, and \(t\) represents time.

2. **Identifying the Meaning of the Negative Sign**:
The negative sign in the rate expression indicates that the concentration of reactant A is decreasing over time as the reaction proceeds. This is because reactants are consumed during the reaction to form products.

3. **Graphical Representation**:
If we were to plot a concentration vs. time graph, we would see that:
- The concentration of reactants (like A) decreases over time.
- Conversely, the concentration of products (like C and D) increases over time.

4. **Rate of Disappearance vs. Rate of Formation**:
- The rate of disappearance of reactants is expressed with a negative sign to reflect that their concentration is decreasing.
- In contrast, the rate of formation of products is expressed positively, as their concentration is increasing.

5. **Conclusion**:
Thus, the negative sign in the rate expression signifies that the concentration of reactant A decreases as the reaction progresses.

### Final Answer:
The negative sign in the rate expression represents that the concentration of reactant A decreases during the reaction.