Video Transcript
The Haber process involves the
reversible reaction of two gaseous reactants. What is the main disadvantage of
increasing the temperature at which the reaction is performed?
Haber process is an industrial
method to produce ammonia via the reaction of nitrogen and hydrogen. Nitrogen and hydrogen are the two
gaseous reactants referred to in the question. And this is the balanced equation
for their reaction in the Haber process. The question asks what the main
disadvantage of increasing the temperature is. Increasing the temperature of the
reaction is always going to cost a little more. But that’s probably not going to be
the main disadvantage referenced here. To solve this question, we’re going
to need to recall what the entropy change of the reaction is, or at least what its
sign is.
The entropy change for the forward
reaction is negative, meaning the reaction of nitrogen and hydrogen is
exothermic. It might interest you to know that
the entropy change for the forward reaction is actually minus 92.4 kilo joules per
mole. But we don’t need to know this
value in order to answer the question. Le Chatelier’s principle says that,
for a dynamic equilibrium, if the conditions change, the position of equilibrium
will move to counteract the change. In this case, the change is an
increase in temperature. For this reaction, since the
forward reaction is exothermic, the reverse reaction must be endothermic. An endothermic reaction is a
cooling process, which reduces the temperature. So increasing the temperature
favors the reverse reaction.
So if we take our initial
equilibrium and heat it up, our position of equilibrium will move to the left,
favoring the reactants. So increasing the temperature
reduces our equilibrium yield. So we get less ammonia at
equilibrium. So the main disadvantage of
increasing the temperature is that we reduce the maximum percentage yield.
