Section 4d: equilibria
Reversible reactions
4.22 recall that some reactions are reversible and are indicated by the symbol ⇌ in equations
4.23 describe reversible reactions such as the dehydration of hydrated copper (II) sulfate and the effect of
heat on ammonium chloride
4.24 understand the concept of dynamic equilibrium
4.25 predict the effects of changing the pressure and temperature on the equilibrium position in reversible
reactions
In some chemical reactions, the products can be changed back into reactants.


The reaction from reactant into products is called the forward reaction
The reaction which changes the products back into reactants is called the reverse reaction.
Chemical reactions in which products can be turned back into reactants are called reversible reactions
and they are indicated by the symbol ⇌.
Examples of reversible reactions
Hydrating and dehydrating copper sulfate is a reversible reaction
heating
CuSO4.5H2O (s)

hydrated, blue crystals
cooling
CuSO4 (s)
+

anhydrous, white crystals
5H2O (l)

droplets on test tube
When we heat hydrated blue copper sulphate it decomposes into anhydrous white copper sulphate and
water. We can now get the hydrated copper sulphate back by allowing the products to cool as the
anhydrous copper sulphate absorbs water from the air to change back to blue hydrated copper sulfate.
The reverse reaction can also be achieved by adding water to the white anhydrous copper sulfate.
Heating the blue hydrated copper sulfate again will again bring back the anhydrous white copper
sulphate.
So by changing the temperature we can make either the forward or the reverse reaction happen.
Another reversible reaction: decomposition of ammonium chloride
heating
NH4Cl (s)

white powder
HCl (g)
+
NH3 (g)
cooling
Heating ammonium chloride favours the forward reaction and the white powder decomposes in the two
colourless gases, NH3 and HCl.
Allowing the mixture of the two gases to cool causes them to form ammonium chloride again which is
what happens in the cooler parts of the test tube.
Section 4 d equilibria
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Dynamic equilibrium
4.24 understand the concept of dynamic equilibrium
4.25 predict the effects of changing the pressure and temperature on the equilibrium position in reversible
reactions
In fact, in a reversible reaction both the forward and reverse reaction go on at the same time. This
means that in such a system we always have reactant particles as well as product particles.
In such a reaction the forward reaction is always the first reaction to happen but as soon as some
product is formed the reverse reaction will start.
At some point in time during the reaction a steady state is reached when both the forward and reverse
reaction go on at the same speed or rate. This means that the reactants are changed into products as
fast as the products are changed back into reactants. As a result the amounts of reactant and product
remains the same.
When this is the case a dynamic equilibrium is reached. We call it a dynamic because both reactions
are still happening although we cannot see any change happening to the reacting mixture e.g. the
colour remains the same.
As chemists we are now interested in how much reactant and how much product is present at
equilibrium. We indicate the relative amounts of each by referring to the position of the equilibrium.
The position of the equilibrium tells us how the amounts of reactant and product compare with each
other.
If there is more reactant than product we say the equilibrium is towards the
reactants/right or the reverse reaction is favoured. This is usually not want we
want!!
If there is more product than reactant we say the equilibrium is towards the
products/left or the forward reaction is favoured. In most reactions this is what.
we want.
As chemists we can change the position of the equilibrium i.e. increase the amount of product relative
to the amount of reactant. We can do this by making the forward reaction go faster again or as we call
this, by favouring the forward reaction or by moving the equilibrium to the right.
We can favour a forward or reverse reaction by changing the conditions such as pressure and
temperature.
Heating of hydrated copper sulphate: changing temperature
(1)
CuSO4.5H2O (s)

hydrated, blue crystals
(2)
CuSO4 (s)
+

anhydrous, white crystals
5H2O (l)

droplets on test tube
In the above reaction,
 the forward or dehydration reaction (1) is an endothermic reaction ( we need to supply heat
energy)
 therefore the reverse reaction (2) must be exothermic
Section 4 d equilibria
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We can favour a reaction by doing the opposite to it. This is because a reversible reaction system will
oppose any change which we do to it.
For example, when we heat a reversible system, it will respond by using up that heat by favouring the
endothermic reaction whichever one that is. If we cool a reversible system it will release heat energy by
favouring the exothermic reaction.
In our copper sulphate reversible reaction, heating the system will favour the forward reaction as it is
the endothermic reaction. The position of the equilibrium changes to the right and more hydrated
copper sulfate is produced making the system more blue.
Cooling it favours the exothermic reaction which is the reverse reaction; the equilibrium shifts to the left
making more anhydrous copper sulphate and water and the system looks more white.
Production of ammonia (Haber process): changing the pressure.
N2 (g)
+
3H2 (g)
2 NH3 (g)
As the reaction is reversible, some ammonia is decomposed as soon as it is formed, reducing the yield.
For the manufacturers it is important to create the conditions which favour the forward reaction.
These conditions are:

Low temperature. Lowering the temperature of a reversible reaction always fvaours the exothermic
reaction which in the case of the ammonia reaction is the forward reaction.

Very high pressure. This is because the system opposes the high pressure by favouring the
reaction which produces the lowest number of moles of particles. This is because if there are fewer
particles in a system there are fewer collisions with the walls of the containers which causes
pressure. In this reversible system the forward reaction produces 2 moles as opposed to the
reverse reaction which produces 4 moles of particles. So a high pressure favours the forward
reaction shifting the equilibrium towards the product side and forming more ammonia gas. A high
pressure has the added advantage of increasing the rate of the reaction as there will be more
successful collisons per unit of time as the particles are brought closer together i.e. in a smaller
space.
Changing pressure only works with reversible systems which consist of gas particles only.
However, in reality the manufacturers of ammonia does not make ammonia in the suggested conditions
because:


at low temperatures the reaction would be too slow (see rates of reactions);
it is too expensive too achieve and maintain very high pressure;
Therefore, the process is carried out at high pressure and moderate temperatures.
SUMMARY:
Lowering the temperature always favours/moves the equilibrium towards the exothermic
reaction.
Increasing the temperature always favours the endothermic reaction.
Section 4 d equilibria
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Increasing pressure favours the side with the lowest number of moles.
Lowering the pressure favours the side with the highest number of moles.
Section 4 d equilibria
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