If a reversible reaction is allowed
to reach equilibrium in a closed system, which of the following will be true? (A) The concentration of the
products will gradually increase. (B) Increasing the pressure inside
the closed system will not affect the equilibrium. (C) Increasing the temperature will
not affect the equilibrium. (D) The concentration of the
reactants and products will be the same. Or (E) the rate of the forward
reaction will be the same as the rate of the backward reaction.
A reversible reaction is one in
which reactants form products and at the same time products are forming
reactants. When this occurs in a closed
system, which is one in which energy can enter or leave but matter cannot, we say an
equilibrium is reached. We can plot the progress of the
reaction in terms of rate of reaction versus time from time equals zero. At time equals zero, reactants form
products very fast. And at time equals zero, before any
products are formed, the backward reaction hasn’t yet begun.
When some products are formed, some
of it will react to reform the reactants. And the rate of the forward
reaction begins to decrease. As more product forms, so the rate
of the backward reaction increases. From a certain point in the
reversible reaction, let’s call it time equals 𝑥, an equilibrium will occur. And the rate of the forward and
backward reactions will be the same. We can see that from time equals
𝑥, both the black and green curves have the same 𝑦-value, in other words, the same
rate of reaction. Option (E) is correct. The rate of the forward reaction
will be the same as the rate of the backward reaction.
Let’s confirm this by having a look
at the other answer options. We can do this by doing another
sketch graph, this time of concentration versus time. At time equals zero, before the
reaction has begun, when there is only a reactant in the vessel, the concentration
of the reactants is very high and the concentration of the products is zero. As the reaction begins to proceed,
so the concentration of the reactants decreases as they are converted into
products. As more reactants are converted to
products, so the concentration of the products increases.
However, from time equals 𝑥, the
concentration of the reactants remains constant and the concentration of the
products remains constant. An equilibrium is formed where the
rate of conversion of reactants to products is the same as the rate of conversion of
products to reactants. If we look carefully, however,
we’ll see that the concentration of the reactants and products are not necessarily
the same at equilibrium. So we can rule out option (D).
We can also rule out option (A)
because the question asks about equilibrium. And at equilibrium, the
concentration of the products will not increase but will remain constant. Now, concentration–time graphs can
look different from the one drawn here. The reactant concentration can be
higher than that of the products at equilibrium, as drawn in this graph. Or the reactants and products can
have the same concentration at equilibrium. Or the products can have a higher
concentration than the reactants.
Taking a ratio of the concentration
of the products to reactants will give us a value either less than one, equal to
one, or greater than one, depending on the graph. These values, which represent the
equilibrium position, depend on the temperature, pressure, or concentration
conditions under which the reaction occurs. In other words, temperature,
pressure, and concentration do influence equilibrium. So we can rule out the remaining
two answer options as they are incorrect.
Finally, if a reversible reaction
is allowed to reach equilibrium in a closed system, the rate of the forward reaction
will be the same as the rate of the backward reaction. And we call this dynamic