Video Transcript
Consider the reaction given
below. Three A gas plus two B gas in
equilibrium with two C gas plus D gas. Δ𝐻 is equal to negative 175
kilojoules per mole. Which of the following changes will
not shift the equilibrium to the left? (A) Removing half of B from the
reaction. (B) Increasing the concentration of
D. (C) Decreasing the pressure. (D) Decreasing the temperature. (E) Increasing the temperature.
This question has given us an
equilibrium reaction. If we want to understand how
changes to conditions will affect an equilibrium reaction, we should refer to Le
Chatelier’s principle. Le Chatelier’s principle states
that for a dynamic equilibrium, if the conditions change, the position of
equilibrium will move to counteract the change. The equilibrium that is reached for
a chemical reaction is a dynamic equilibrium. This means that the forward
reaction, where the reactants react to form the products, and backward reaction,
where the products react to form the reactants, are occurring at the same rate. The forward and backward reaction
balance each other out so no macroscopic change is observed.
When conditions change, Le
Chatelier’s principle tells us the position of equilibrium will move to counteract
the change. Microscopically what’s occurring is
the change in conditions causes either the forward or the backward reaction to
become favored. When the forward reaction is
favored, the position of equilibrium will be more towards the products than it was
before the change. Or using the language in this
question, we can say the position of equilibrium will shift to the right. If the backward reaction is
favored, the position of equilibrium will shift to the left towards the
reactants. Let’s explore this theory by seeing
what effect the changes described by the answer choices will have.
The question wants us to identify
which change will not shift the equilibrium to the left. So the correct answer choice will
be the change that causes the forward reaction to be favored. Let’s look through the changes in
the answer choices now. Statement (A) describes removing
half of B from the reaction. This diagram represents what the
system might look like at equilibrium, with each species represented by a different
colored ball. After half of the B particles have
been removed, the system is no longer in equilibrium. To restore equilibrium, the
backward reaction will be favored so that more particles of B will be produced. So, this change will cause the
equilibrium to shift to the left. So, answer choice (A) is not the
correct answer.
Let’s reset the diagram to the
original equilibrium and take a look at statement (B). Increasing the concentration of D
will put the system out of equilibrium. As a result of this change, the
excess particles of D will react, which will reduce the concentration of D. So, the backward reaction will be
favored in this case. So, this change will also cause the
equilibrium to shift to the left. Answer choice (B) is not
correct.
In answer choice (C), the pressure
is changing. The pressure in a container of gas
is due to the gas particles colliding with the walls of the container. The more collisions there are with
the sides of the container, the higher the pressure will be. This means a container with more
gas particles will have a higher pressure than one that has fewer gas particles. If the pressure is decreased, the
position of equilibrium will shift to increase the pressure. And the pressure will increase if
there are more gas particles in the system. There are five gas particles in
total on the reactants side and three gas particles in total on the products
side. So, equilibrium will be restored if
the backward reaction is favored as the backward reaction will increase the number
of gas particles in the system. So this change will also shift the
equilibrium to the left.
The next two choices have to do
with the temperature, so let’s look at them together. We know that if the temperature
decreases, the equilibrium will shift to increase the temperature, and the opposite
will happen if the temperature increases. But how does a chemical reaction
increase or decrease the temperature? Chemical reactions can release or
absorb energy from the environment in the form of heat. If energy is released into the
surroundings, the temperature of the surroundings will increase. If the system absorbs energy from
the surroundings, the temperature of the surroundings will decrease.
A reaction that releases heat is
called exothermic. Exothermic reactions have a change
in enthalpy, Δ𝐻, that is less than zero. Reactions that absorb energy are
called endothermic. Δ𝐻 here is greater than zero. The change in enthalpy listed here
is negative. This means that the forward
reaction is exothermic. When a reversible reaction is
exothermic in one direction, the opposite direction will be endothermic and vice
versa. So, we know the forward reaction
will increase the temperature of the surroundings. And the backward reaction will
decrease the temperature of the surroundings. So, the forward reaction will be
favored if the temperature decreases. In other words, the equilibrium
will shift to the right. If the temperature increases, the
backward reaction will be favored. So, equilibrium will shift to the
left.
So, for the reaction we were given,
the only change stated in the answer options that would not shift the equilibrium to
the left is decreasing the temperature.
