Energy changes, and reversible reactions
9.5 Reversible reactions
When you heat copper(II) sulfate crystals …
Water of crystallisation !
The water in blue copper(II)
1 2
sulfate crystals is called
water of crystallisation.
Hydrated means it has water
molecules built into its structure.
Anhydrous means no water is
present.
Two tests for water !
Water will turn:
white anhydrous copper(II)
The blue crystals above are hydrated The reaction is easy to reverse: add sulfate blue
copper(II) sulphate. On heating, water! The anhydrous copper(II) blue cobalt chloride paper pink.
they turn to a white powder. This is sulfate gets hot and turns blue. Both compounds add on water of
anhydrous copper(II) sulfate: The reaction is: crystallisation, giving the colour
CuSO4.5H2O (s) CuSO4 (s) 1 5H2O (l ) change. To reverse, just heat!
CuSO4 (s) 1 5H2O (l ) CuSO4.5H2O (s)
So the reaction can go in either direction: it is reversible.
The reaction we start with (1 above) is called the forward reaction.
Reaction 2 is the back reaction
We use the symbol instead of a single arrow, to show that a reaction is
reversible. So the equation for the reaction above is:
white powder
CuSO4.5H2O (s) CuSO4 (s) 1 5H2O ( l )
energy
energy change
What about the energy change? blue crystals
Reaction 1 above requires heat 2 it is endothermic. In 2, the white
powder gets hot and spits when you drip water on it 2 so that reaction is
exothermic. It gives out the same amount of heat as reaction 1 took in.
A reversible reaction is endothermic in one direction, and exothermic In a reversible reaction, the energy
in the other. The same amount of energy is transferred each time. change is the same in both directions.
Some important reversible reactions
Many important reactions are reversible. Here are some examples:
Reaction Comments
N2 ( g) 1 3H2 ( g) 2NH3 ( g) This is a very important reaction, because ammonia is used to
nitrogen hydrogen ammonia make nitric acid and fertilisers.
2SO2 ( g) 1 O2 ( g) 2SO3 ( g) This is a key step in the manufacture of sulfuric acid.
sulfur dioxide oxygen sulfur trioxide
CaCO3 (s) CaO (s) 1 CO2 ( g) This is a thermal decomposition: it needs heat. Calcium oxide
calcium carbonate calcium oxide carbon dioxide (called lime, or quicklime) has many uses (page 240).
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Energy changes, and reversible reactions
Reversible reactions and equilibrium
As you saw in the last table, the reaction between nitrogen and hydrogen
to make ammonia is reversible:
N2 ( g) 1 3H2 ( g) 2NH3 ( g)
So let’s see what happens during the reaction:
H HH H
N H H N
H
H H H N N H H H N N
H H H N H N H H
H N H N
HH H H H H H H
H N H H H HH
N N H H N H H
N H H N N H H H
N H H HH
H H H N H H N
H H
HH H N H HH
H N H N
N H HH H H
H H H H
Three molecules of hydrogen react ... will it all turn into ammonia? … every time two ammonia
with one of nitrogen to form two of No! Once a certain amount of molecules form, another two
ammonia. So if you put the correct ammonia is formed, the system break down into nitrogen and
mixture of nitrogen and hydrogen reaches a state of dynamic hydrogen. So the level of ammonia
into a closed container … equilibrium. From then on … remains unchanged.
Equilibrium means there is no overall change. The amounts of nitrogen,
hydrogen and ammonia remain steady. But dynamic means there is
continual change: ammonia molecules continually break down, while
new ones form.
In a closed system, a reversible reaction reaches a state of dynamic
equilibrium, where the forward and back reactions take place at
the same rate. So there is no overall change.
The term dynamic equilibrium is usually shortened to equilibrium.
A challenge for industry
Imagine you run a factory that makes ammonia. You want the yield of
ammonia to be as high as possible.
But the reaction between nitrogen and hydrogen is never complete.
Once equilibium is reached, a molecule of ammonia breaks down every
time a new one forms.
This is a problem. What can you do to increase the yield of ammonia? Worried about the yield?
You will find out in the next unit.
Q
1 What is a reversible reaction? 5 Explain the term dynamic equilibrium.
2 Write a word equation for the reaction between solid 6 Nitrogen and hydrogen are mixed, to make ammonia.
copper(II) sulfate and water. a Soon, two reactions are going on in the mixture.
3 How would you turn hydrated copper(II) sulfate into Give the equations for them.
anhydrous copper(II) sulfate? b For a time, the rate of the forward reaction is greater
4 What will you observe if you place pink cobalt chloride than the rate of the back reaction. Has equilibrium
paper in warm oven? been reached? Explain.
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