Video Transcript
Iodine trichloride is a bright
yellow interhalogen compound formed in the following reaction: ICl plus Cl2 in
equilibrium with ICl3. What effect will removing chlorine
have on the position of the equilibrium? (A) The equilibrium will not
move. (B) The equilibrium will move to
the right. (C) The equilibrium will move to
the left.
This diagram represents the
chemical system in this question at equilibrium. Notice that all three chemical
species involved in the reaction are present. This is because chemical systems
establish a dynamic equilibrium. This means every moment the forward
reaction is occurring, where iodine chloride and chlorine react. This will produce iodine
trichloride. At the same time, the backward
reaction is also occurring. Molecules of iodine trichloride
decompose to form iodine chloride and chlorine. However, the forward and backward
reaction occur at the same rate. So the amount of all of the
products and reactants stay relatively constant over time.
If the molecules of chlorine are
removed, the system will no longer be in equilibrium. Now, the forward reaction can’t
happen because there are no molecules of chlorine to react with the iodine
chloride. So the backward reaction is
favored, where iodine trichloride decomposes. This will restore the chlorine in
the system. Once more chlorine molecules are
formed, the molecules of chlorine can react to form the products. At this point, the rates of the
forward and backward reaction will be equal again and a new equilibrium is
established.
If we compare the equilibrium
composition of our system before and after the change, we can see that the new
equilibrium system has fewer product molecules and more reactant molecules than
before. Another way of saying this is that
the position of equilibrium has moved towards the reactants. We will often say that the position
of equilibrium has shifted to the left as the reactants are on the left-hand side of
the equation. So we know that answer choice (C)
is the correct answer.
This behavior can be summarized by
Le Chatelier’s principle, which states that for a dynamic equilibrium, if the
conditions change, the position of equilibrium will move to counteract the
change. In this case, the change was
removing chlorine from the system. To counteract the change, the
backward reaction was favored to produce more chlorine. As a result, the position of
equilibrium shifted to the left.
So what effect will removing
chlorine have on the position of equilibrium? The correct answer is answer choice
(C). The equilibrium will move to the
left.
