The Haber Process
A high pressure increases the yield as there are fewer moles of gas on the right side of the reaction. Additionally, a high pressure increases the rate of reaction as there are more molecules in the same volume which results in an increased frequency of collisions.
However, any higher than this is too expensive and dangerous. 300 atm is a compromise between rate/yield and cost/danger.
The process normally takes place at 450°C. Fill in the table to state and explain the advantages and disadvantages to increasing or decreasing the temperature.
| Advantages | Disadvantages | |
|---|---|---|
| Decrease | ||
| Increase |
| Advantages | Disadvantages | |
|---|---|---|
| Decrease | Increases the yield – the reaction is exothermic, so decreasing the temperature shifts the equilibrium to the right, resulting in a higher yield. | Decreases the rate of reaction. Lower temperature means the molecules have less kinetic energy and move more slowly. This results in less frequent collisions and a lower proportion of the collisions having at least the activation energy. These result in less frequent successful collisions. |
| Increase | Increases the rate of reaction. Higher temperature means the molecules have more kinetic energy and move more quickly. This results in more frequent collisions and a higher proportion of the collisions having at least the activation energy. These result in more frequent successful collisions. | Decreases the yield – the reaction is exothermic, so increasing the temperature shifts the equilibrium to the left, resulting in a lower yield. |
Nitrogen is obtained from the air. The air is cooled and fractional distillation is used to separate the nitrogen out from the other gases.
OR
The potential energy diagram for the reaction is shown below. Explain how the presence of iron increases the rate of reaction.
The energy required to convert 0.5N2 and 1.5H2 to N and 3H is 1129 kJ mol-1. This is the activation energy for the reaction uncatalyzed.
With the catalyst, the energy change to adsorb the molecules to the catalyst and then form adsorbed H and 3H is -259 kJ mol-1.
The second step has the highest positive enthalpy change, at +106 kJ mol-1.
The mixture is cooled down to below the boiling point of ammonia and the ammonia condenses. The ammonia is then stored at low temperature and high pressure to ensure that the ammonia doesn’t decompose back into nitrogen and hydrogen.
Still gaseous unreacted nitrogen and hydrogen are recycled into the reaction chamber to be converted into ammonia.