Lower temperature increases the equilibrium yield of ammonia because its formation is exothermic, but it also slows the reaction. Higher temperature makes the reaction faster while reducing the equilibrium yield. The industrial Haber process therefore uses compromise conditions, while recent computational research is exploring whether programmed heating and cooling could use the advantages of different temperatures at different stages.
Category: Chemistry Learning
The Haber-process equilibrium
Ammonia is formed from nitrogen and hydrogen:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH < 0
The forward reaction is exothermic. Four moles of gaseous reactants produce two moles of gaseous ammonia. These two facts explain the main equilibrium conditions used in ammonia manufacture.
Why lower temperature gives more ammonia
According to Le Chatelier’s principle, lowering the temperature favours the direction that releases heat. Since ammonia formation is exothermic, the equilibrium shifts toward NH₃.
Temperature also changes the equilibrium constant. For this exothermic reaction, increasing temperature decreases the equilibrium constant for ammonia formation. Lower temperature therefore produces a larger equilibrium proportion of ammonia—provided sufficient time is available to reach equilibrium.
Why the reaction becomes slow
A favourable equilibrium does not guarantee a fast reaction. The N≡N bond is exceptionally strong. At low temperature, fewer reacting particles possess enough energy to cross the activation barrier, so ammonia forms too slowly for practical production.
Raising the temperature increases the rate constant and the number of effective collisions. It improves the production rate but makes the equilibrium composition less favourable. Industry must balance rate, yield, cost and equipment limitations rather than simply selecting the condition that maximizes one variable.
What does the iron catalyst do?
Iron provides a surface and an alternative pathway with lower activation energy. This allows equilibrium to be reached faster.
- Iron increases both forward and reverse reaction rates.
- It does not change the equilibrium constant.
- It does not shift equilibrium toward ammonia.
- It does not change the equilibrium yield at a fixed temperature.
This is a common JEE and NEET misconception: catalysts improve rate, not the equilibrium position.
Pressure and equilibrium
Increasing pressure favours the side with fewer gaseous moles. The reactant side has four gaseous moles, while the product side has two, so compression favours ammonia formation. However, pressure does not change the value of the equilibrium constant when temperature is unchanged.
Chemistry in Current Research
A recent first-principles microkinetic study investigates ammonia synthesis on an Fe(110) surface under programmed heating and cooling. Instead of keeping the catalyst at one constant temperature, the model explores whether hotter periods can accelerate difficult surface steps while cooler periods favour ammonia formation or retention.
This is an early-stage computational strategy. It does not prove that the Haber process has been replaced or that a commercially viable reactor already exists. It does, however, provide an excellent illustration of how changing conditions with time may help manage a kinetics–equilibrium trade-off.
For the full research interpretation, read Lalit Kumar Mishra’s educator analysis: Can Changing Temperature Improve Ammonia Synthesis? The original study is listed in the ACS Catalysis recent-articles record.
Common examination mistake
Incorrect statement: “Iron increases the equilibrium yield of ammonia.”
Correct statement: Iron lowers activation barriers and allows the system to reach the same equilibrium composition faster. Only a change in temperature changes the equilibrium constant.
Exam-style question
For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH < 0, which change increases the equilibrium constant?
- Increasing pressure
- Adding finely divided iron
- Lowering temperature
- Removing ammonia at constant temperature
Answer: 3. Only temperature changes the equilibrium constant. Pressure and removal of product can shift the equilibrium composition, while a catalyst changes the rate of attaining equilibrium.
Quick revision table
| Change | Effect on equilibrium ammonia | Effect on K | Effect on rate |
|---|---|---|---|
| Lower temperature | Increases | Increases | Decreases |
| Higher pressure | Increases | No change | Usually increases |
| Iron catalyst | No change | No change | Increases |
For structured preparation, explore Topper Formula courses for CBSE XI–XII and JEE/NEET and our free learning resources.