Video Transcript
Which of the following will never
affect the position of an equilibrium? (A) The pressure, (B) a catalyst,
(C) the temperature, (D) the concentration of reactants, or (E) the concentration of
products.
Equilibrium position is the ratio
of the product and reactant concentrations at equilibrium. To understand which of the
conditions will never affect the position of an equilibrium, let’s consider the
equilibrium reaction between dinitrogen tetroxide and nitrogen dioxide.
When this reaction is at dynamic
equilibrium, the rates of the forward and reverse reactions will be the same and the
concentrations of the reactants and products will remain constant. At dynamic equilibrium, this
reaction will have a certain equilibrium position depending on the reaction
conditions. The dynamic equilibrium will
continue and the equilibrium position will remain the same, unless the conditions
are changed. If the pressure is increased, the
equilibrium will shift towards the side of the reaction that has the fewest moles of
gas. So, some of the nitrogen dioxide
will be converted into dinitrogen tetroxide. This will continue until a new
dynamic equilibrium is established.
In this example, when the pressure
was increased, the concentration of the product decreased and the concentration of
the reactant increased. So, changing the pressure can
change the equilibrium position. As we want to know which of the
conditions will never affect to the position of an equilibrium, answer choice (A)
cannot be the answer to this question.
Now, let’s consider what would
happen if we change the temperature. We can see from the provided
positive enthalpy change value that the forward reaction is endothermic, meaning
that heat is absorbed from the surroundings during the reaction. The reverse reaction therefore is
exothermic, meaning that heat is released to the surroundings during the
reaction. Heating the reaction vessel
provides energy to the system, which favors the endothermic reaction. So, some of the dinitrogen
tetroxide will be converted into nitrogen dioxide until dynamic equilibrium is
reestablished.
In this example, when the
temperature was increased, the concentration of the product increased and the
concentration of the reactant decreased. So, changing the temperature can
change the equilibrium position. Therefore, answer choice (C) cannot
be the answer to this question.
Now, let’s consider what would
happen if the concentration of reactants is increased after dynamic equilibrium has
been established. Some of these newly added reactants
would turn into products until dynamic equilibrium is reestablished. Once again, the concentrations of
the reactant and product changed, changing the equilibrium position. So, answer choice (D) is not the
answer to this question.
Now, let’s consider what would
happen if we decreased the concentration of the products by removing them from the
reaction vessel entirely. In order to reestablish
equilibrium, some of the reactants must be converted into products. Thus, changing the concentration of
the products will change the equilibrium position. So, answer choice (E) cannot be the
answer to this question.
This leaves us with answer choice
(B), a catalyst. A catalyst is a substance that
increases the rate of reaction. A catalyst speeds up the rate of
the forward and reverse reactions to the same degree. So, when a catalyst is added,
dynamic equilibrium will be established more quickly, but it will have no effect on
the concentrations of the reactants and products at equilibrium. Therefore, adding a catalyst will
have no effect on the equilibrium position.
In conclusion, the condition that
will never affect the position of an equilibrium is answer choice (B), a
catalyst.
