Energy changes, and reversible reactions

9.6 Shifting the equilibrium

The challenge
Reversible reactions present a challenge to industry, because they never
complete. Let’s look at that reaction between nitrogen and hydrogen again:
  N2 ( g) 1 3H2 ( g) 2NH3 ( g)

HH N H H
HH N N H
H H H
HH HH H N H H H
N N H
H H H
HH H H N
H N N H H
N H N 100% 100%
HH H H H H
N H N HH H reactants product
N H H HH
H N N H N N
HH H H
H N N
H H N H H
H HH
N H H H HH N
N H H
H

H H H HH
H

This represents the reaction mixture ... because every time a new Here the red triangle represents
at equilibrium. The amount of molecule of ammonia forms, the equilibrium mixture. It is only
ammonia in it will not increase … another breaks down. part way along the scale. Why?

What can be done?

You want as much ammonia as possible. So how can you increase the yield? 100% 100%
This idea, called Le Chatelier’s principle, will help you: reactants product

When a reversible reaction is in equilibrium and you make a change,

the system acts to oppose the change, and restore equilibrium.
A new equilibrium mixture forms.
A reversible reaction always reaches equilibrium, in a closed system.   A change in reaction conditions has
But by changing conditions, you can shift equilibrium, so that the mixture led to a new equilibrium mixture, with
contains more product. Let’s look at four changes you could make. more ammonia. Equilibrium has shifted
to the right, to favour the product.
1  Change the temperature
Will raising the temperature help you obtain more ammonia? Let’s see.

H
N N 100% 100%
heat out N
N2 1 3H3 2NH3 H H reactants product
heat in H HH
2NH3 N2 1 3H3 N H
heat
H used HH
up HH

The forward reaction is exothermic 2 … but if you heat the equilibrium So the reaction reaches equilibrium
it gives out heat. The back reaction mixture, it acts to oppose the change. faster 2 but the new equilibrium
is endothermic 2 it takes it in. More ammonia breaks down in mixture has less ammonia.
Heating speeds up both reactions … order to use up the heat you add. So you are worse off than before.

What if you lower the temperature? The system acts to oppose the change:
more ammonia forms, giving out heat. Great! But if the temperature is too
low, the reaction takes too long to reach equilibrium. Time is money, in a
factory. So it is best to choose a moderate temperature.

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 Energy changes, and reversible reactions

2  Change the pressure

H
H
HH H N
H H 100% 100%
N N reactants product
H
N
HH H
H

4 molecules 2 molecules

Pressure is caused by the gas When you increase the pressure, So the amount of ammonia in the
molecules colliding with the walls the equilibrium mixture acts to mixture has increased. Equilibrium
of the container. The more molecules oppose this. More ammonia forms, has shifted to the right. Well done.
present, the higher the pressure. which means fewer molecules. You are on the right track.

3  Remove the ammonia

The equilibrium mixture is a balance between nitrogen, hydrogen, and
ammonia. Suppose you cool the mixture. Ammonia condenses first, so A summary, for reversible !
you can run it off as a liquid. Then warm the remaining nitrogen and reactions of gases
hydrogen again. More ammonia will form, to restore the balance.  Forward reaction exothermic:
temperature ↑ means yield ↓.
4  Add a catalyst  Forward reaction endothermic:
Iron is a catalyst for this reaction. temperature ↑ means yield ↑.
A catalyst speeds up the forward and back reactions equally.  Fewer molecules on the right-
So the reaction reaches equilibrium faster, which saves you time. hand side of the equation:
But the amount of ammonia does not change. pressure ↑ means yield ↑.

Choosing the optimum conditions

So to get the best yield of ammonia, it is best to:
 use high pressure, and remove ammonia, to improve the yield
 u
 se a moderate temperature, and a catalyst, to get a decent rate.
Page 227 shows how these ideas are applied in an ammonia factory. What about solutions? !
Many reversible reactions take place
A note about rate in solution:
By now, you should realize that: reactants (aq)    products (aq)
You can shift the equilibrium:
 a change in temperature always shifts equilibrium.
 by adding more of a reactant
 a change in pressure will shift equilibrium only if the number of
(increasing its concentration).
molecules is different on each side of the equation.
So more product will form to
But how do these changes affect the rate? Raising the temperature or oppose this change.
pressure increases the rate of both the forward and back reactions, so  by changing the temperature.
equilibrium is reached faster. (A temperature rise gives the molecules A rise in temperature will favour
more energy. An increase in pressure forces them closer. So in both cases, the endothermic reaction.
the number of successful collisions increases.)

Q
1 The reaction between nitrogen and hydrogen is reversible. 4 Sulfur dioxide (SO2) and oxygen react exothermically to form
This causes a problem for the ammonia factory. Why? sulfur trioxide (SO3). The reaction is reversible.
2 What is Le Chatelier‘s principle? Write it down. a Write the symbol equation for this reaction.
3 In manufacturing ammonia, explain why: b What happens to the yield of sulfur trioxide if you:
a high pressure is used  b ammonia is removed i  increase the pressure?  ii  raise the temperature?

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