When the system A + B <--> C + D is at dynamic equilibrium
the forward reaction has stopped.
both the forward and the reverse reactions have stopped.
the reverse reaction has stopped
neither the forward nor the reverse reaction has stopped.
When the system A + B <--> C + D is at dynamic equilibrium, it means that both forward and reverse reactions are still occurring but at the same rate, resulting in no net change in the concentrations of the reactants and products. So, the correct option is "neither the forward nor the reverse reaction has stopped."
When the system A + B <--> C + D is at dynamic equilibrium, it means that the rates of the forward and reverse reactions are equal and there is no net change in the concentrations of the reactants and products.
At dynamic equilibrium, neither the forward nor the reverse reaction has completely stopped. While the rates of the reactions are equal, individual particles of reactants are still undergoing both forward and reverse reactions. Some molecules of A and B are still reacting to form C and D, and at the same time, some molecules of C and D are also reacting to form A and B. This continuous back-and-forth conversion between reactants and products leads to a stable concentration of all species involved.
So, the correct answer is: neither the forward nor the reverse reaction has stopped.