Video Transcript
Consider the following reaction
that is exothermic in the forward direction: X plus Y reversibly produces Z plus
W. If the reaction is at equilibrium,
which of the following statements best describes what will happen if the temperature
is increased from 200 kelvin to 250 kelvin? (A) The rate of the forward
reaction will increase. (B) The rate of the reverse
reaction will increase. (C) The rate of the reverse
reaction will decrease. (D) The rate of the reverse
reaction will stay the same. (E) The rate of the forward
reaction will stay the same.
In this question, there is an
increase in temperature, and we must predict what will happen to the rate of
reaction. We should also note that this
reaction is exothermic in the forward direction. This means that energy, often but
not entirely in the form of heat, is released in the reaction that proceeds from
left to right. It also means that the reverse
reaction from right to left is endothermic, and energy, usually in the form of heat,
will be absorbed.
To answer this question, we must be
familiar with Le Chatelier’s principle, which says that for a dynamic equilibrium,
if the conditions change, the equilibrium will shift to counteract that change. In this problem, the changed
condition is the increase in temperature. In order to counteract this change,
the equilibrium will shift to favor the endothermic reaction. When the environment gets hotter,
absorbing energy in the form of heat from the environment counteracts the
change. As we noted earlier, the reverse
reaction, from right to left, is the endothermic direction of this reversible
reaction. When this reaction is favored, X
and Y will be produced at a higher rate than before.
Looking at our answer choices, only
one of them describes favoring the reverse reaction: answer choice (B), the rate of
the reverse reaction will increase. When there is extra energy in the
environment at this new, higher temperature, increasing the rate of the reverse
reaction puts some of that energy back into the reaction, counteracting the
change.
So, what will happen if the
temperature is increased from 200 kelvin to 250 kelvin? Choice (B), the rate of the reverse
reaction will increase.
