Ammonia is produced industrially by reacting nitrogen with hydrogen at 450 degrees C
and 200-atmospheres pressure over an iron catalyst. The equation for this reaction is N₂ plus 3H₂ is in equilibrium with 2NH₃. Figure one shows the flow diagram for this process.
How does the separator remove ammonia from the reaction mixture before it is returned
to the reactor?
Let’s have a look at the flow diagram. Nitrogen and hydrogen are fed into the reactor, and some of it reacts to form
ammonia. This mixture fed into the separator has to be separated somehow. This increases the yield of the reaction because unreacted nitrogen and hydrogen are
fed back into the reactor.
When fed into the separator, nitrogen, hydrogen, and ammonia are all gases. A mixture of gases is most easily separated by condensation, where the mixture is
cooled until the highest-boiling-point fraction starts to condense. Nitrogen and hydrogen both have very low boiling points. It would take extreme cooling in order to liquify them. Ammonia, on the other hand, has a boiling point just above room temperature and is
the first fraction to liquify upon cooling. So in the separator, the gas mixture is cooled until the ammonia condenses.
The forward reaction is exothermic. Using Le Chatelier’s principle, explain how decreasing the temperature affects the
yield of ammonia at equilibrium.
Le Chatelier’s principle is that the position of an equilibrium will shift to resist
a change. Here the change is a decrease in the temperature. Therefore, according to Le Chatelier’s principle, the position of the equilibrium
will shift to increase the heat output.
We can see from the question that the forward reaction is exothermic. As we decrease the temperature, this forward reaction will accelerate, shifting the
equilibrium to the right-hand side. More of the forward reaction means more ammonia. Therefore, the yield of ammonia increases as we decrease the temperature. Decreasing the temperature favours the forward exothermic reaction, as this process
releases heat to counteract the change. Therefore, the equilibrium shifts to the right and the ammonia yield increases.
Explain how decreasing the pressure affects the yield of ammonia at equilibrium.
Once again, we are going to use Le Chatelier’s principle, which is that the position
of an equilibrium will shift to resist a change, to predict what’s going to
happen. Here the change is a decrease in the pressure.
Broadly speaking, the pressure of a gas is proportional to the number of gas
particles. As this is an equilibrium, we have two reactions going on: the forward and the
reverse. In the forward reaction, we have four molecules of gas reacting to form two molecules
of gas. This is a decrease in the number of molecules, and therefore the forward reaction
decreases the pressure. The reverse reaction, on the other hand, involves two molecules of gas forming four
molecules of gas. This is an increase in the overall number of molecules. So the reverse reaction actually increases the pressure.
As the change is a decrease in pressure, this favours the reverse reaction. As the reverse reaction involves the decomposition of ammonia, favouring this
reaction decreases the yield of ammonia. Decreasing the pressure favours the reverse reaction because this process produces
more moles of gas counteracting the change. Therefore, the equilibrium shifts to the left and the yield of ammonia decreases.