Home / Chemistry / Dynamic equilibrium

Dynamic equilibrium

Calculate A ⇄ B with RK4 for 60 s. Check whether the forward and reverse reaction speed bars are the same. A If you select the additional amount, it is automatically added in 30 s.

    C16 · Dynamic equilibriumReady
    Calculate A ⇄ B with RK4 for 60 s. Check whether the forward and reverse reaction speed bars are the same. A If you select the additional amount, it is automatically added in 30 s. You can also experiment with the numbers and controls below.

    Chemistry · Reaction rate · Equilibrium / Principles and exploration

    Dynamic equilibrium — principles and investigation

    Dynamic equilibrium is not a state in which the reaction has stopped, but a state in which the speeds of the forward and reverse reactions are equal. Determine the initial concentration and rate constants of A and B and add A after reaching equilibrium. By following the change in concentration and the rate of the two reactions together, we read how a new equilibrium is created.

    How does it work?

    When A is added, the forward reaction becomes faster for a moment, increasing B. In the new equilibrium, the two velocities are the same and the concentration ratio returns to K.

    Key relationship

    d[A]/dt = −kf[A]+kr[B] · K = kf/kr

    Try this investigation

    1. Calculate A ⇄ B with RK4 for 60 s. Check whether the forward and reverse reaction speed bars are the same. A If you select the additional amount, it is automatically added in 30 s.
    2. Run the baseline conditions and record K=2 and the equilibrium concentration of A, approximately 1/3 mol/L.
    3. A 30 s concentration jump and new equilibrium are viewed with the Additional Conditions button. K before and after addition is the same. Conditions that only change the reverse reaction rate constant are also recorded.

    Interpreting your results

    What is the correct meaning of dynamic equilibrium?

    When kf[A]=kr[B], the concentration becomes constant. [B]/[A]=kf/kr=K and the concentration itself does not have to be the same.

    Assumptions and limitations

    Hypothetical A⇄B first-order reaction, constant volume and temperature. Adding A is an instantaneous increase in concentration without a change in volume. RK4 time interval is 1/120 s. 30 s is a state close to equilibrium and a finite time residual remains. K is the concentration ratio [B]/[A] in this model, and activity and multiple reactions are omitted.