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The Haber Process

Chemistry - Pressure.

The industrial conditions are
2) Pressure of 200 atm (200 atmospheres).

Why is such a high pressure used?

nitrogen   +   hydrogen    reversible arrow    ammonia   ( + heat).
N2(g)     +     3H2(g)       reversible arrow      2NH3(g)    ( + heat).

If we look at the reaction, the reactants and products are gases.
One mole of any gas occupies a volume of 24,000 cm3.

On the left side of the equation,
there is one mole of nitrogen, and three moles of hydrogen.
The total is four moles of reactant.
(If you don't know why there are 3 moles of hydrogen
and 1 mole of nitrogen, see moles).

On the right side of the equation (the product),
there are two moles of ammonia.

So, four moles of reactant give two moles of product.
Since one mole of any gas takes up the same volume,
the volume of product is only half the volume of reactants.

Increasing the pressure (from Le Chatelier's principle)
makes the equilibrium mixture have more ammonia.
This is what we want!

What effect does pressure have on reaction rate?
As we can see, increased pressure also
increases the reaction rate. Again, this is what we want!

Q. Why not increase the pressure to 1,000 atm,
and get lots of ammonia really quickly?

A. In the real world, it all comes down to money.
Building high pressure chemical plant is expensive.
Running the reaction at about 200 atm gives
the highest return (the biggest profit) on investment capital
(the amount of money you spend to set up the whole thing).

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