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The reaction conditions used are chosen to obtain an acceptable yield of product in an acceptable time. For example, there would be little profit in getting a very high yield if it took several weeks or months to achieve.

The Haber process

The Haber process for making ammonia provides a useful example of how this works:

N₂(g) + 3H₂(g) ⇌ 2NH₂(g) (forward reaction is exothermic)

Controlling temperature

If the temperature is increased, the equilibrium position moves in the direction of the endothermic reaction. This means it moves to the left in the Haber process.

You might think that a low temperature is chosen, moving the equilibrium position to the right and making more ammonia. However, the rate of reaction is low at low temperatures. So a compromise temperature of 450 °C is chosen. This is:

• low enough to achieve an acceptable yield
• high enough to do this in an acceptable time

Controlling pressure

If the pressure is increased, the equilibrium position moves in the direction of the fewest molecules of gas. This means it moves to the right in the Haber process.

You might think that a very high pressure is chosen to move the equilibrium position to the right, making more ammonia.

However, it is expensive to achieve very high pressures. Stronger equipment is needed, and more energy is needed to compress the gases. So a compromise pressure of 200 atmospheres is chosen. This is:

• low enough to keep costs down
• high enough to achieve an acceptable yield

Using a catalyst

Catalysts do not change the equilibrium concentrations of reacting substances in reversible reactions. However, they do reduce the time taken to reach equilibrium. Iron is a cheap catalyst used in the Haber process. It helps to achieve an acceptable yield in an acceptable time.

The Contact process

The Contact process is one of the stages involved in the manufacture of sulfuric acid. Sulfur dioxide reacts with oxygen to make sulfur trioxide:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (forward reaction is exothermic)

Controlling temperature

If the temperature is increased, the equilibrium position moves in the direction of the endothermic reaction. This means it moves to the left in the Contact process.

You might think that a low temperature is chosen, moving the equilibrium position to the right and making more sulfur trioxide. However, the rate of reaction is low at low temperatures. So, as in the Haber process, a compromise temperature of 450 °C is chosen.

Controlling pressure

If the pressure is increased, the equilibrium position moves in the direction of the fewest molecules of gas. This means it moves to the right in the Contact process.

You might think that a very high pressure is chosen to move the equilibrium position to the right, making more sulfur trioxide. However, the equilibrium position is already far to the right in this reaction.

There is no need for a high pressure, so a pressure of two atmospheres is used. This is just enough to push the reacting gases through the reactor.

Using a catalyst

Vanadium(V) oxide, V₂O₅, is the catalyst used in the Contact process. It helps to achieve an acceptable yield in an acceptable time.