The Haber and Contact processes: equilibrium in industry

10 minAS Level7.1.10

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can state the conditions used in the Haber and Contact processes, justify each one with Le Chatelier's principle, and explain why the temperature chosen is not the one that gives the highest yield.

Three pressures on an industrial chemist

A plant wants three things that pull against each other:

a high equilibrium yield, so little reactant is wasted;
a fast reaction, so the reactor does not have to be enormous;
low cost, including energy and equipment.

Equilibrium theory controls the first. Kinetics controls the second. The conditions actually used are the compromise that makes the most money.

The Haber process: making ammonia

Nitrogen from the air is combined with hydrogen from natural gas:

$\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \qquad \Delta H = -92\ \text{kJ mol}^{-1}$

The forward reaction is exothermic and reduces the number of gas molecules from 4 to 2.

Those two facts set the whole design.

Justifying each Haber condition

Condition	Value	Reason
Pressure	about $200\ \text{atm}$	4 mol of gas become 2 mol, so high pressure shifts the equilibrium right and raises the yield
Temperature	about $450\ ^\circ\text{C}$	a compromise: lower would raise the yield but make the rate far too slow
Catalyst	iron	speeds up both directions, so equilibrium is reached quickly without changing the yield
Product removal	ammonia liquefied out	removing a product shifts the equilibrium right
Recycling	unreacted $\text{N}_2$ and $\text{H}_2$ returned	avoids wasting raw materials and improves the overall conversion

The pressure is not pushed even higher because thicker reactor walls and more powerful compressors cost more than the extra ammonia is worth.

Why the temperature is a compromise

Because the forward reaction is exothermic, cooling the reactor would shift the equilibrium to the right and give a higher percentage of ammonia.

But at a low temperature the reaction is so slow that the gas would pass through the catalyst almost unchanged. The plant would make very little ammonia per hour, however favourable the equilibrium.

So $450\ ^\circ\text{C}$ is chosen: hot enough for a usable rate with the iron catalyst, cool enough to keep a worthwhile yield.

The Contact process: making sulfuric acid

Sulfur dioxide is oxidised to sulfur trioxide, which is then converted into sulfuric acid:

$2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \qquad \Delta H = -197\ \text{kJ mol}^{-1}$

Again the forward reaction is exothermic, and again 3 mol of gas become 2 mol, so the same reasoning applies.

Pressure: about $1$ – $2\ \text{atm}$ — only slightly above atmospheric.
Temperature: about $450\ ^\circ\text{C}$ .
Catalyst: vanadium(V) oxide, $\text{V}_2\text{O}_5$ .
Product removal: the $\text{SO}_3$ is absorbed in concentrated sulfuric acid as it forms.

Why the Contact process uses a low pressure

This is the comparison examiners love, because it looks inconsistent with the Haber process.

The equilibrium for $\text{SO}_3$ formation already lies very strongly to the right, even at one atmosphere. Raising the pressure would gain only a tiny further improvement, while the compressor and the stronger vessels would cost a great deal.

Ammonia's equilibrium is much less favourable, so there the extra pressure genuinely buys a much better yield and the cost is worth paying.

The rule is not "high pressure is good". It is "pay for pressure only when it buys enough product".

A shop that refuses to overstock

Think of a plant as a shop that must decide how hard to push each lever. Every lever has a price, and the aim is not the perfect mixture but the best profit.

A yield of $99\%$ in a reactor that takes a week to get there is worse than a yield of $25\%$ in a reactor that cycles every minute — especially when the unreacted gas is recycled.

Common mistakes

The Haber process uses the lowest possible temperature because that gives the highest yield.

A low temperature gives a high yield but an unusably slow rate, so a compromise of about $450\ ^\circ\text{C}$ is used.

The Contact process uses low pressure because pressure has no effect on that equilibrium.

Pressure does favour the products, but the equilibrium is already so far to the right that the extra yield is not worth the cost.

The iron catalyst increases the equilibrium yield of ammonia.

In the exam

Give the condition and the reason, every time: "about 200 atm, because 4 mol of gas form 2 mol so the equilibrium shifts right".
For temperature, use the word compromise and explain both halves: yield falls as temperature rises, but rate falls as temperature falls.
Compare the two processes explicitly when asked: the Contact equilibrium is already far to the right, so high pressure is not worth the cost.
Do not claim the catalyst changes the position or the yield.
Remember to mention product removal and recycling for the Haber process; they are often worth marks.

Checking the idea

Multiple choiceQuestion 1 · Apply

Multiple choiceQuestion 2 · Apply

Answer the 3 questions above to continue.