For the equilibrium A ⇌ B , the variation of the rate of the forward (a) and reverse (b) reaction with time is given by :

Solution

Chemistry ma’am · 3 steps

At equilibrium the forward and reverse rates become equal, so the two curves must meet and run together.

1. At the start. Only A is present. The forward rate (a) is at its highest, and the reverse rate (b) is zero because there is no B yet.
2. As time passes. A is used up, so the forward rate falls. B builds up, so the reverse rate rises. Neither curve turns back, because [A] keeps falling and [B] keeps rising until equilibrium.

   Option	What the curves do	Rates equal at equilibrium?
   (a)	Cross, then reverse rate stays above forward rate	No, they separate again
   (b)	Approach, then turn and move apart	No, they never meet
   (c)	Approach and merge into one level	Yes
   (d)	Level off with a gap between them	No, a gap remains

3. So. Only (c) shows the two rates meeting at one common value and staying there. This is dynamic equilibrium, where the forward rate equals the reverse rate.

The idea to keep. Equilibrium means the forward rate equals the reverse rate, not that the reaction stops. On a rate versus time graph, the two curves must meet and then run as one flat line.

Answer from the NTA final answer key. Solution by PrepWiser.