Rate of attainment is how quickly equilibrium is reached

1. The rate of attainment of equilibrium is how long the mixture takes to settle.
2. It is a separate question from the position of the equilibrium, which is what the mixture settles at.
3. A change can affect the rate, the position, or both.
4. Anything that makes particles collide more often, or more energetically, makes equilibrium arrive sooner.
5. An industrial process needs an acceptable yield in an acceptable time, so both questions are asked together.

Temperature and pressure act on rate and on position

1. Raising the temperature gives particles more energy, so equilibrium is reached sooner.
2. Raising the temperature also shifts the position towards the endothermic direction.
3. For the Haber process the forward reaction is exothermic, so a higher temperature lowers the yield.
4. Raising the pressure pushes gas particles closer together, so collisions are more frequent and equilibrium comes sooner.
5. Raising the pressure also shifts the position towards the side with fewer gas molecules, which is ammonia.

Concentration and catalysts act on the rate

1. Raising the concentration of a reactant makes collisions more frequent, so equilibrium is reached sooner.
2. A catalyst provides a different route with a lower activation energy, which speeds the reaction up.
3. It speeds the forward and backward reactions by the same factor.
4. Because both rates rise together, the composition at which they become equal is unchanged.
5. A catalyst therefore brings equilibrium sooner without altering the yield at all.

The Haber conditions are a compromise

1. A low temperature would give the best yield, because the forward reaction is exothermic.
2. At a low temperature the reaction would be far too slow to be worth running.
3. $450{}^{\circ }C$ is chosen as a compromise, accepting a lower yield for a workable rate.
4. A high pressure helps both the yield and the rate, so pressure is pushed as far as is practical.
5. $200$ atmospheres is chosen because higher pressures need stronger, costlier plant and are more dangerous.

Cost of raw materials and energy shapes the choice

1. Maintaining a high temperature and a high pressure both consume energy that has to be paid for.
2. Stronger vessels and compressors raise the capital cost of building the plant.
3. Nitrogen from the air is effectively free, while hydrogen from natural gas carries a real cost.
4. Recycling the unreacted gases means a modest conversion per pass is acceptable.
5. The conditions chosen are the ones that make the product at the lowest overall cost, not the ones that give the highest yield.