Video Transcript
Which of the following statements
best defines Le Chatelier’s principle? (A) If conditions change in a
dynamic equilibrium, the position of the equilibrium moves to increase the
change. (B) If a chemical reaction is at
equilibrium and experiences a change in pressure, temperature, or concentration of
products or reactants, the equilibrium shifts in the opposite direction to increase
the rate of reaction. (C) When a system at equilibrium is
subjected to changes in concentration or volume, the system shifts to a new
equilibrium to counteract the applied change. (D) Changes in the temperature and
pressure in a system will result in changes to the system in order to achieve a new
equilibrium state. Or (E) if a change occurs to an
equilibrium mixture, the system will tend to adjust itself so as to cancel out, as
far as possible, the effect of that change.
Let’s start by clearing a little
space. According to Le Chatelier’s
principle, for a dynamic equilibrium, if the conditions change, the position of
equilibrium will move to counteract the change. Let’s look at an example, where
pale yellow iron three plus reacts with colorless thiocyanate, SCN−, to produce
reddish iron(III) thiocyanate complex, FeSCN2+.
If the concentration of a reactant
is increased, the equilibrium will move to the right to consume additional reactant,
thereby lowering its concentration. If the equilibrium shifts to the
right, in the forward direction, then more of the product is produced. As the product FeSCN2+ is reddish,
the mixture looks more red. Similarly, if the concentration of
a reactant is decreased, the equilibrium will move to the left to generate a higher
concentration of reactant and consequently raising its concentration to counteract
the change.
Le Chatelier’s principle can apply
to concentration, volume, pressures, and temperature. Rather than being specific to a
fixed reaction or set of conditions, Le Chatelier’s principle is a recognition that
systems will always reestablish new equilibria after changes, finding as
energetically stable a position as possible.
Let’s now bring back all of the
answer choices. We can see that the correct answer
is (E). If a change occurs to an
equilibrium mixture, the system will tend to adjust itself so as to cancel out, as
far as possible, the effect of that change.
