ammonia ( + heat).N2(g) + 3H2(g)
2NH3(g) ( +
heat).gcsescience.com 6 gcsescience.com
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 ammonia ( + heat).
N2(g)
+ 3H2(g)
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).
Links
Reversible Reactions
Revision Questions
gcsescience.com The Periodic Table Index Haber Process Quiz gcsescience.com
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