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In today's T-Level Chemistry lesson, we are going to study A twelve point six, The Haber Process.

By the end of this lesson, you should be able to state typical Haber conditions, explain their economic effect, and describe the compromise between equilibrium yield and reaction rate.

The equation is nitrogen plus three hydrogen in equilibrium with two ammonia. The forward reaction has delta H less than zero. It is exothermic.

The industrial aim is ammonia at a useful rate and a useful yield, at an economically acceptable cost.

Typical conditions in the lesson are about four hundred and fifty degrees Celsius, about two hundred atmospheres, and an iron catalyst.

Unreacted nitrogen and hydrogen can be separated from ammonia and recycled. Conditions are not chosen to maximise one factor alone.

Retrieve Le Chatelier. Lower temperature favours the exothermic direction. Increasing pressure favours fewer gas moles. A catalyst lowers activation energy and increases rate, but does not change the equilibrium position.

Count the gas moles. One nitrogen plus three hydrogen is four moles of gas. Two ammonia is two moles of gas. Increasing pressure favours the side with fewer gas moles, so the equilibrium shifts toward ammonia and the equilibrium yield of ammonia increases.

Temperature is a compromise. Lower temperature gives a higher ammonia equilibrium yield, because it favours the exothermic forward reaction, but the rate is slower. Higher temperature gives a faster rate, but a lower equilibrium yield, because it favours the endothermic reverse.

About four hundred and fifty degrees Celsius is fast enough, with a useful equilibrium yield. That is a compromise between rate and yield.

Pressure is also a compromise. Higher pressure increases ammonia equilibrium yield. Extremely high pressure needs more energy for compression, stronger and more expensive equipment, and higher capital and operating costs.

About two hundred atmospheres is high, but economically practical. Exact plant conditions can vary. The specification focus is the compromise, not one global optimum.

The iron catalyst provides an alternative pathway with lower activation energy. It increases the rate of the forward and reverse reactions, so equilibrium is reached faster.

It does not change the equilibrium position, the equilibrium composition, or the equilibrium yield at fixed temperature and pressure. It does not change delta H. It means the plant does not have to raise temperature only to get a useful rate, which would lower the ammonia yield.

Compare the options. Three hundred degrees Celsius at two hundred atmospheres gives a high yield but is too slow for efficient production. Six hundred and fifty degrees Celsius at two hundred atmospheres is fast, but the ammonia equilibrium yield is reduced. Four hundred and fifty degrees Celsius, two hundred atmospheres and iron gives a useful yield and a useful rate.

Plant X is offered three hundred and fifty, four hundred and fifty or six hundred degrees Celsius, and one hundred, two hundred or four hundred atmospheres, with iron available.

Three hundred and fifty degrees Celsius has a higher yield but is slower. Six hundred degrees Celsius has a higher rate but a lower yield. Four hundred atmospheres gives more yield but more compression cost. One hundred atmospheres costs less but gives a lower yield than two hundred atmospheres. The indicative industrial choice is about four hundred and fifty degrees Celsius and about two hundred atmospheres with iron.

Correct the misconceptions. The lowest possible temperature is not chosen, because the rate would be too slow. The highest possible pressure is not chosen, because of energy, equipment and cost. Iron does not increase equilibrium yield. Higher temperature does not always improve production, because for this reaction it decreases the ammonia equilibrium yield.

Examination-style question.

Part a. State typical temperature, pressure and catalyst. Three marks.

About four hundred and fifty degrees Celsius, about two hundred atmospheres, and iron.

Part b. Why is a lower temperature not used, even though yield would be higher? Three marks.

Lower temperature favours the exothermic forward reaction, so yield is higher, but the rate is lower, so production is too slow or less economically efficient.

Part c. Why does higher pressure increase yield? Two marks.

There are two moles of gas against four, so the equilibrium shifts toward ammonia.

Part d. Why is industrial pressure a compromise? Three marks.

Higher pressure increases yield. Very high pressure increases compression, equipment and cost. Industry balances yield against economic and engineering cost.

Part e. What does the iron catalyst do? Two marks.

It provides a lower activation-energy pathway, so rate increases and equilibrium is reached faster, without changing the equilibrium position.

Before we finish, complete the model. Haber ammonia is made at about four hundred and fifty degrees Celsius and about two hundred atmospheres with iron. Temperature and pressure are compromises between yield, rate and cost. The catalyst speeds both directions without changing the equilibrium yield at fixed conditions.

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