For the reaction \(\text{A} \rightleftharpoons \text{B}\), variation of concentration is plotted against time as shown below.
Show 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.
Step 1: Understanding the Question:
The question asks us to identify which region in the concentration-versus-time graph represents the state of chemical equilibrium for the reversible reaction $\text{A} \rightleftharpoons \text{B}$.
Step 2: Detailed Explanation:
Dynamic chemical equilibrium is defined as the state in a reversible chemical reaction where the rates of the forward and backward reactions become equal.
As a direct consequence, the concentrations of both reactants (A) and products (B) become constant over time.
Let us analyze the three regions indicated on the plot:
- In Region (1): The concentration of A is rapidly decreasing, and the concentration of B is rapidly increasing. The concentrations are changing, which shows that the reaction is far from equilibrium.
- In Region (2): The rates of change are slowing down, but the concentrations of both A and B are still changing over time.
- In Region (3): Both curves become completely flat and horizontal (parallel to the time axis). This means that the concentrations of A and B have become completely constant with respect to time, which is the hallmark of a system at equilibrium.
Therefore, Region (3) represents the state of chemical equilibrium.
