Reversible Reactions in Industry

Equilibria

What are the two main aims when choosing conditions for an industrial chemical process?

To convert reactants into products as quickly as possible and as completely as possible.

Why must industrial reaction conditions often be a compromise?

Conditions that give the highest equilibrium yield may produce a reaction that is too slow or be too expensive.

What is synthesis?

The production of chemical compounds by reaction from simpler substances.

What is the equation for the Haber process?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

What is the enthalpy change for the forward reaction in the Haber process?

ΔH = −92 kJ mol⁻¹, so the forward reaction is exothermic.

What conditions would theoretically give the maximum equilibrium yield of ammonia?

A low temperature and high pressure.

Why does a low temperature increase the equilibrium yield of ammonia?

The forward reaction is exothermic, so decreasing temperature shifts equilibrium to the right.

Why does high pressure increase the equilibrium yield of ammonia?

There are 4 moles of gas on the left but only 2 on the right, so increasing pressure shifts equilibrium to the right.

What temperature is typically used in the Haber process?

About 450°C.

Why is a temperature of about 450°C used in the Haber process?

It is a compromise between rate and yield: lower temperatures give a higher equilibrium yield but an unacceptably slow reaction rate.

Why is the reaction between nitrogen and hydrogen slow at low temperatures?

The N≡N bond is very strong, giving the reaction a high activation energy.

What pressure is typically used in the Haber process?

About 250 atm.

Why is a pressure of about 250 atm used in the Haber process?

Higher pressures increase ammonia yield but require more energy to compress the gases and more expensive equipment, so 250 atm is a compromise between yield and cost.

What catalyst is used in the Haber process?

Iron.

Why is a catalyst used in the Haber process?

It increases the reaction rate so ammonia can be produced more quickly without changing the equilibrium position.

Why is iron used rather than some other metal catalysts in the Haber process?

Other metals such as tungsten and platinum can catalyse the reaction but are much more expensive.

Does the Haber process mixture remain in the reaction chamber long enough to reach equilibrium?

No.

Why is the actual yield of ammonia lower than the equilibrium yield predicted for the Haber process conditions?

The reaction mixture does not remain in the reaction chamber long enough for equilibrium to be established.

Approximately what is the actual yield of ammonia in the Haber process?

About 15% per pass.

How is efficiency increased when not all nitrogen and hydrogen react in the Haber process?

Unreacted nitrogen and hydrogen are separated from the ammonia and recycled back into the reaction chamber.

What is the Contact process used to manufacture?

Sulfuric acid.

What is the important reversible reaction in the Contact process?

2SO₂(g) + O₂(g) ⇌ 2SO₃(g).

What is the enthalpy change for the formation of sulfur trioxide in the Contact process?

ΔH = −96 kJ mol⁻¹, so the forward reaction is exothermic.

What temperature would theoretically maximise the equilibrium yield of SO₃?

A low temperature because the forward reaction is exothermic.

Why is a temperature of about 450°C used in the Contact process?

A low temperature would give a higher equilibrium yield but the catalyst would not be sufficiently effective and the reaction would be too slow, so 450°C is a compromise.

What catalyst is used in the Contact process?

Vanadium(V) oxide, V₂O₅.

Why is a catalyst used in the Contact process?

It increases the rate of reaction by providing an alternative pathway with a lower activation energy.

What pressure is used in the Contact process?

About 2 atm.

Why is a relatively low pressure of about 2 atm used in the Contact process?

The yield of SO₃ is already very high at low pressure, so the extra cost of using higher pressures is unnecessary.

What would increasing pressure do to the equilibrium yield of SO₃?

It would increase the yield because there are fewer moles of gas on the product side.

Why is very high pressure not economically worthwhile in the Contact process?

The equilibrium yield is already very high, so the small additional increase in yield would not justify the increased cost.

How does the V₂O₅ catalyst work in the Contact process?

It changes oxidation state during the reaction and is regenerated to its original oxidation state at the end.

What happens to V₂O₅ in the first stage of its catalytic mechanism?

SO₂ reacts with V₂O₅ to form SO₃ and V₂O₄.

What happens to V₂O₄ in the second stage of the Contact process catalytic mechanism?

V₂O₄ reacts with oxygen to regenerate V₂O₅.

Why are industrial conditions not simply chosen to give the maximum possible equilibrium yield?

Maximum-yield conditions may give a reaction that is too slow, require costly equipment or consume too much energy.

What factors should be considered when selecting industrial reaction conditions?

Equilibrium yield, reaction rate, energy costs, equipment costs, catalyst effectiveness and overall profitability.