Video Transcript
Choose ways, other than changing
the temperature, to increase the amount of PCl5 in the following equilibrium
mixture: PCl5 gas reacts reversibly to form PCl3 gas plus Cl2 gas.
In this question, we want to find
out which of the methods listed would increase the amount of PCl5. PCl5 is phosphorus
pentachloride. The given chemical equation shows
that we have a system in equilibrium and that all of the species are in the gas
phase. To answer this question, we need to
use Le Chatelier’s principle.
Le Chatelier’s principle states
that for a dynamic equilibrium, if the conditions change, the position of the
equilibrium will move, or shift, to counteract the change. Things that affect the position of
a dynamic equilibrium include concentration, pressure, and temperature. Depending on the substances in
equilibrium with each other and the type of change that occurs, the equilibrium may
shift to the left, favoring the formation of reactants. Or the equilibrium will shift to
the right, favoring the formation of products.
We can see from the methods listed
in the table that the only conditions changing are concentration and pressure. We can also see from the chemical
equation that in order to increase the amount of PCl5, we need to change the
conditions to shift the equilibrium to the left-hand side and favor the formation of
the reactant.
The methods listed in G and H will
increase the concentration of one of the products. Le Chatelier’s principle states
that the equilibrium will shift to counteract this change. In the case of adding Cl2 or PCl3,
the equilibrium will shift to favor the reaction that reduces the concentration of
the products. In other words, the equilibrium
will shift to the left, favoring the reactant, and so it will increase the amount of
phosphorus pentachloride. Therefore, both methods G and H
will increase the amount of PCl5.
Methods I and J both relate to a
change in the pressure of the system. When the pressure increases, the
equilibrium shifts towards the side with the fewest moles of gas. Alternatively, when the pressure
decreases, the equilibrium shifts to favor the reaction that produces more moles of
gas. From the chemical equation, we can
determine there to be one mole of gas in the reactants and two moles of gas in the
products.
Therefore, an increase in pressure
will shift the equilibrium to favor the reactant, since there are fewer moles on the
reactant side. And a decrease in pressure will
shift the equilibrium to favor the products, since there are more moles on the
product side. Since we want the method that will
increase the amount of the reactant, PCl5, we can see that increasing the pressure
will favor the formation of PCl5. So, method I will increase the
amount of PCl5, but method J will not.
We can now conclude that the
methods listed in this table that will increase the amount of PCl5 are G, H, and
I.
