Which of the following is not the characteristic of chemical equilibrium ?

Options

A. Rate of forward reaction is equal to rate of backward reaction at equilibrium
B. After reaching the chemical equilibrium, the concentrations of reactants and products remain unchanged with time
C. For `A(g) rarr B(g) , K_(c) "is" 10^(-2)` If this reaction is carried out in the presence of catalyst, the value of `K_(c)` decreases
D. After reaching the equilibrium, both forward and backward reactions continue to take place

Explanation

To determine which of the following is not a characteristic of chemical equilibrium, we will analyze each option based on the principles of chemical equilibrium.

### Step-by-Step Solution:

1. **Understanding Chemical Equilibrium**:
Chemical equilibrium is a state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction. At this point, the concentrations of reactants and products remain constant over time.

2. **Analyzing Option A**:
- **Statement**: Rate of forward reaction is equal to the rate of backward reaction at equilibrium.
- **Evaluation**: This is a fundamental characteristic of chemical equilibrium. Therefore, this option is **correct**.

3. **Analyzing Option B**:
- **Statement**: After reaching chemical equilibrium, the concentration of reactants and products remains unchanged with time.
- **Evaluation**: This is also a characteristic of chemical equilibrium. The concentrations do not change once equilibrium is established. Thus, this option is **correct**.

4. **Analyzing Option C**:
- **Statement**: The value of Kc (equilibrium constant) decreases when a catalyst is added.
- **Evaluation**: This statement is incorrect. The presence of a catalyst speeds up both the forward and backward reactions equally but does not change the value of Kc. Therefore, this option is **not a characteristic** of chemical equilibrium.

5. **Analyzing Option D**:
- **Statement**: After reaching equilibrium, both forward and backward reactions continue to take place.
- **Evaluation**: This is true. At equilibrium, both reactions are still occurring, but their rates are equal, leading to no net change in concentration. Thus, this option is **correct**.

### Conclusion:
The option that is **not a characteristic of chemical equilibrium** is **Option C**: "The value of Kc decreases when a catalyst is added."

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