Video Transcript
Match each of the following
reactions to its graph. Reaction A: CH4 gas plus H2O gas
are in equilibrium with CO gas plus three H2 gas. Reaction B: two SO2 gas plus O2 gas
are in equilibrium with two SO3 gas. Reaction C: H2 gas plus I2 gas are
in equilibrium with two HI gas.
In this question, we have been
given three different reactions and three different graphs. Each reaction is an equilibrium
reaction where all of the species are gases. During each of these reactions, a
dynamic equilibrium will be formed.
Dynamic equilibrium is an
equilibrium between forward and reverse reactions where both reactions occur at the
same nonzero rate and the concentrations of the reactants and products remain
constant. The concentrations of the products
and reactants at equilibrium, and therefore the equilibrium position, can be
affected by changing the concentration of any species involved in the reaction, the
temperature, or the pressure.
To answer the question, we want to
know how increasing the pressure would affect the percentage of products at
equilibrium for each of the given reactions. To help us understand how pressure
would affect the percentage of products at equilibrium, we can use le Chatelier’s
principle.
Le Chatelier’s principle states
that for dynamic equilibrium, if the conditions change, the position of the
equilibrium will move to counteract the change. If we were to increase the pressure
on a dynamic equilibrium, the equilibrium position would move to counteract the
change or decrease the pressure. For reactions involving gases, the
equilibrium will shift towards the side with the fewest moles of gas in order to
decrease the pressure of the system. With this in mind, let’s take a
look at each of the reactions given.
In reaction A, two moles of gas are
in equilibrium with four moles of gas. If the pressure was increased on
this dynamic equilibrium, the equilibrium would shift towards the reactants, the
side that has the fewest moles of gas. As the pressure continues to
increase, there will be more and more reactants in the reaction vessel and fewer and
fewer products. So the percentage of products at
equilibrium will decrease as the pressure increases. This is shown in Graph 3.
In reaction B, three moles of gas
are in equilibrium with two moles of gas. If the pressure was increased on
this dynamic equilibrium, the equilibrium would shift towards the products, the side
that has the fewest moles of gas. As the pressure continues to
increase, there will be more and more products in the reaction vessel and fewer and
fewer reactants. So the percentage of products at
equilibrium will increase as the pressure increases. This is shown in Graph 1.
In reaction C, two moles of gas are
in equilibrium with two moles of gas. As the number of moles of gas on
both sides of the equilibrium is the same, changing the pressure will have little to
no effect on the equilibrium. So the percentage of products at
equilibrium will remain constant. This is shown in Graph 2.
We’ve now matched each of the
reactions to its graph. Reaction A corresponds to Graph 3,
reaction B corresponds to Graph 1, and reaction C corresponds to Graph 2.
