Video Transcript
Which of the following statements
concerning concentrations is always true at equilibrium? (A) The concentrations of the
products are greater than the reactants. (B) The concentrations of reacting
substances are equal. (C) The concentrations of the
reactants are decreasing. (D) The concentrations of reacting
substances do not change. (E) The concentrations of the
reactants and products are constantly changing.
To answer this question, we need to
understand what is occurring during dynamic equilibrium. Dynamic equilibrium is an
equilibrium between forward and reverse reactions where both reactions occur at the
same nonzero rate.
Let’s examine the rate-versus-time
graph for the equilibrium reaction involving dinitrogen tetroxide and nitrogen
dioxide. We can see that initially the rates
aren’t the same. So, at these points in time, the
reaction is not in dynamic equilibrium. Eventually, the rates of the
forward and reverse reactions are the same. This is the point at which dynamic
equilibrium is established.
To answer the question, we need to
know what happens to the concentrations of the reactants and products during dynamic
equilibrium. When a system is at dynamic
equilibrium, both the forward and reverse reactions are still occurring. But, since the rates of the forward
and reverse reactions are the same, as reactants are converted into products,
products are being converted back into reactants at the same rate. This means that although the
reactions are still occurring, the concentrations of the reactants and products
remain constant.
When dynamic equilibrium is
established the concentrations of the reactants and products may be the same or
different. But regardless of whether the
concentrations of the reactants and products are the same or different, at dynamic
equilibrium, the concentrations must remain constant.
Therefore, the statement concerning
concentrations that is always true at equilibrium is answer choice (D). The concentrations of reacting
substances do not change.
