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The Haber Process

Chemistry - Temperature.

The industrial conditions are
1) Temperature between 450 °C and 500 °C.

The forward reaction (to form ammonia)
is exothermic (it gives out heat).
The backward reaction is endothermic (it takes in heat).

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

If we treat heat as a product,
then removing heat (cooling the reaction down)
will result in the equilibrium mixture
having more ammonia (see Le Chatelier's principle).

Since we want ammonia from the Haber process,
why is the reaction conducted at
450 °C ?
Why don't we cool it with ice,
or at least let it run at room temperature?

If we look at the section on reaction rates,
and the page that deals with the effect of temperature,
we can see that all reactions go faster
when the temperature is raised.

In a reversible reaction like the Haber process,
raising the temperature will make the equilibrium mixture 
have more nitrogen and hydrogen
because forming these from ammonia takes heat in.

If we cool the reaction down, the
amount of ammonia in the equilibrium mixture will increase,
but the rate at which ammonia is formed will decrease
(because the temperature is lower).

It is no good having 90% ammonia in the equilibrium mixture
if it takes all day to make one bucket full.
It is better to have 10% ammonia being made very quickly,
and at the end of the day you can have thousands of litres.

The actual temperature of between 450 °C and 500 °C,
is a compromise between the amount of ammonia
in the equilibrium mixture
(only 15% because of the high temperature)
and the rate at which ammonia is formed
(fast because of the high temperature).

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