Question

For the reaction \(\text{A} \rightleftharpoons \text{B}\), variation of concentration is plotted against time as shown below.

• A. Region (1) indicates equilibrium
• B. Region (2) indicates equilibrium
• C. Region (3) indicates equilibrium
• D. Both the Regions (2) and (3) indicate equilibrium

Hint:

On a concentration-time graph for any chemical system, equilibrium is reached at the exact point where the concentration curves of all reactants and products become horizontal straight lines parallel to the time axis.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Dynamic chemical equilibrium is a state in a reversible reaction where the rate of the forward reaction becomes equal to the rate of the backward reaction.
As a direct consequence of equal rates, the concentrations of the reactants and products remain constant over time once equilibrium is established.
On a graph of concentration versus time, this constant concentration is visually represented by the curves becoming perfectly horizontal lines parallel to the time axis.
Step 2: Detailed Explanation:
Let's analyze the regions shown in the plot (C65.png):
- Region (1): In this initial phase, the concentration of reactant A is rapidly decreasing while the concentration of product B is rapidly increasing. The system is just starting, and the forward rate is much higher than the backward rate. No equilibrium here.
- Region (2): The slopes of the curves are becoming shallower, indicating that the rates of change are slowing down as the system approaches equilibrium. However, the concentrations are still changing with time. Equilibrium is not yet reached.
- Region (3): In this final region, the curves for both A and B have become perfectly flat, horizontal lines. This means that for any time $t$ in this region, the concentration $C$ remains exactly the same ($dC/dt = 0$).
Since the Concentrations of A and B have become constant, this region signifies that the system has achieved the state of chemical equilibrium.
Step 3: Final Answer:
The state of equilibrium is indicated by the horizontal portion of the concentration curves, which is Region (3). The correct option is (C).
Step 4: Final Verification:
Check for any slope in Region 3. Since there is none, equilibrium is confirmed.