Video Transcript
Five students are asked to write
the definition of dynamic equilibrium. Which student has not fully
understood the concept? (A) A dynamic equilibrium is a
forward and backward reaction that occurs at the same time with the same rate. (B) A dynamic equilibrium is a
reaction where the speed of the forward reaction equals the speed of the backward
reaction. (C) A dynamic equilibrium is a
reaction that goes in both directions, where the amount of each substance does not
change. (D) A dynamic equilibrium is a
reversible reaction that stops when the amount of each substance is stable. (E) A dynamic equilibrium is a
reversible reaction where the concentration of each substance does not change.
To answer this question, we need to
determine which student has not fully understood the concept of dynamic
equilibrium.
Let’s remove the answer choices for
now and consider the equilibrium between dinitrogen tetroxide and nitrogen
dioxide. For the purposes of this example,
we’ll use a yellow circle to represent a molecule of N2O4 and a pink circle to
represent each molecule of NO2.
Let’s imagine that we have a sealed
vessel that initially only contains N2O4. So, before any reaction takes
place, the concentration of dinitrogen tetroxide in the vessel is high, while the
concentration of nitrogen dioxide is zero.
At first, as there are no products
to compete with the forward reaction, the rate of the forward reaction will be
large, and the rate of the reverse reaction will be zero. As time passes, some of the
reactants will be converted into products. And the rate of the forward
reaction will begin to slow as there are fewer reactant molecules. Simultaneously, as the reaction is
reversible and there are now NO2 molecules present in the vessel, some of the
nitrogen dioxide will be converted into dinitrogen tetroxide. And the rate of the reverse
reaction will begin to increase.
If we monitor the concentrations
during this period of time, we’ll see that the concentration of N2O4 will decrease,
while the concentration of NO2 increases. As more time passes, the
concentration of dinitrogen tetroxide will continue to decrease, while the
concentration of nitrogen dioxide continues to increase. At the same time, the rate of the
forward reaction will continue to slow, while the rate of the reverse reaction will
continue to speed up.
Eventually, a point is reached
where the rate of the forward reaction is the same as the rate of the reverse
reaction. From this point in time on, the
rate of the forward and reverse reactions will stay the same. And the concentrations of N2O4 and
NO2 will remain constant.
This doesn’t mean that the
reactions have stopped. In fact, both reactions continue to
occur. But because the reactions occur at
the same rate, as one molecule of reactant is converted into two molecules of
product, two molecules of product are converted into one molecule of reactant at the
same time. So the concentration of each
species doesn’t change.
The point in time when the rate of
the forward and reverse reactions are equal and the concentrations no longer change
is the point at which dynamic equilibrium is established. So dynamic equilibrium is an
equilibrium between forward and reverse reactions where both reactions are occurring
at the same nonzero rate and the concentrations of the reactants and products remain
constant.
With this in mind, let’s bring back
the answer choices. Now we can evaluate the statements
to determine which student has not fully understood the concept of dynamic
equilibrium.
Looking at the answer choices, we
can determine that the statement in answer choice (D) is not completely correct. Dynamic equilibrium does involve a
reversible reaction, and the amount of each substance is stable once equilibrium is
established. But during dynamic equilibrium, the
reactions do not stop. Both the forward and reverse
reactions continue at the same rate.
So the student that has not fully
understood the concept of dynamic equilibrium is the student that wrote the
statement in answer choice (D). A dynamic equilibrium is a
reversible reaction that stops when the amount of each substance is stable.
