Higher and foundation tier
Consider the following reaction:
In this topic we are not concerned with the reacting chemical or the products of the reaction but
with the arrow, → ,in the above equation. This arrow tells us that the reaction goes to completion.
This means that all of the reactants are turned into products. Most of the equations you have seen in
science will probably have used this type of arrow. However very few reactions
occur where all the reactants are turned into products. Neutralisation and combustion are two examples
of reactions that you will have met that go to completion.
Most chemical equations are written with a different arrow:
This equation might look very similar to the one above except for the arrow, which is obviously different.
However this reaction is very different from the one mentioned above.
You can think of it as two separate reactions that are happening at the same time
The important fact is that these two reactions are happening at the same time. The equation below basically
shows two reactions in one!
A + B ⇌ C + D
As A and B react and turn into C and D, C and D are reacting and
turning back into A and B. The reaction is reversible. Both
these reactions happen at the same time.
We can use the symbols Rf and Rb to represent the rates of the forward and
reverse reactions. It is a common error to assume that these two rates are equal, but this is rarely the
It is important to realise that during this reversible reaction you will end up with a mixture
of A, B, C and D once the reaction gets going. The proportions of A, B, C and D will depend to a large
extend on the rate of the forward and reverse reactions.
The simple diagram below may help to give you a mental picture of what is happening during a reversible reaction.
Here the large glass troughs are filled with water, the water in each trough represents the reactants and products
in a reaction. The amount of water in each beaker will give an indication of the speed or rate of reaction for the forward and reverse reactions. What you have to imagine
is pouring the contents of the two beakers backwards and forwards at the SAME TIME into the two troughs.
Study the diagram below, it explains what is meant by dynamic equilibrium.
- The amounts of reactants and products stay the same despite the fact that the forward and reverse reactions
are still going on.
- The rate of the forward and reverse reactions are the same.
- The amounts of reactants and products are not necessarily equal, they are just constant.
- It is only possible to achieve equilibrium in closed systems, that is systems (reactions) in which no
reactants or products can escape or leave.
These changes at equilibrium are summarised in the graph shown below. Here the green line represents the amount
of reactant and the red line the amount product. To begin with there is
100% reactant present and 0% of the product since the
reaction has not started. However both lines for the reactants and products level out at a constant amount, this is the point at
which the forward and reverse reactions are proceeding at the same rate. The reaction has achieved dynamic equilibrium and the
amount of reactant and product does not change despite the fact that both the forward and reverse reactions are still proceeding..
Examples of reversible reactions
Copper sulfate is a colourless (white) ionic crystalline solid. However most of the time you will
have seen or used it in the lab it is blue not white. The reason for
this is that it absorbs water from the air and this turns it blue. The dry or anhydrous copper sulfate
has the formula CuS04, it is an ionic compounds with a giant ionic lattice structure.
The water it absorbs from the air fits into this lattice structure and is held weakly in place. The
wet or hydrated copper sulfate has the formula: CuS04.5H2O (the .5H2O just
means that it has 5 moles of water associated with it crystal structure). This water can be
easily evaporated from the lattice by heating. This is shown below, once it has evaporated it loses
its blue colour and turns white or colourless to form anhydrous copper sulfate. Addition of
water again forms the hydrated blue form of copper sulfate.
We can show this reversible reaction in an equation as:
hydrated copper sulfate ⇌ anhydrous copper sulfate + water
CuS04.5H2O ⇌ CuS04 + 5H20
The thermal decomposition of ammonium chloride
Ammonium chloride is a colourless solid which will decompose when heated to form a mixture of two gases, ammonia and
hydrogen chloride gas. This reaction is reversible and when cooled the mixture of the basic ammonia and the acidic hydrogen chloride gases will
reform solid crystals of ammonia chloride. This reaction can be shown as:
ammonia chloride(s) ⇌ ammonia(g) + hydrogen chloride(g)
NH4(s) ⇌ NH3(g) + HCl(s)
The diagram opposite shows the reaction. On heating the solid crystals of ammonium chloride decompose to form the
basic gas ammonia and the acidic gas hydrogen chloride. These two gases quickly rise up the boiling tube, and if they
meet a cool surface they will immediate react and reform crystals of ammonium chloride. During the experiment shown if
the boiling tube is gently heated then the middle and top will remain sufficiently cool for crystals of ammonium chloride to
reform on the sides of the boiling tube. Once the two gases leave the boiling tube they will also cool down enough for
a cloud of white solid ammonium chloride to form.
- Reversible reactions are chemical reactions that can be reversed. In a reversible reaction the products can react
with each other and reform the reactants.
- Dynamic equilibrium is the point in a reversible reaction where the rate at which the reactants form products is exactly
matched by the rate at which products reform reactants.
- It is only possible to achieve dynamic equilibrium in reactions which are closed. This means no reactants or products are allowed to escape or leave
the reacting system, e.g. no gases are allowed to escape, otherwise equilibrium will never be achieved.