Higher and foundation tier

### Reversible reactions

Consider the following reaction:

In this topic we are not concerned with the reacting chemical or the products of the reaction but with the type of arrow used in chemical equations, → or ⇌ . What does the use of the → in the above equation tell us?

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 reactions are reversible.

### Reversible reactions

Most chemical equations are written with a different arrow (⇌), this arrow is used to show that the reaction is reversible, :
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. That is 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 case. 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 reactant 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.

What do you think will happen to the amount of reactant and product in each of the troughs in the image below, If you use the two beakers to tip the “water” backwards and forwards between the two troughs at the same time?

Well since each beaker will hold the same volume of water the amount of reactant and product will NOT change with time, even if you spent all day tipping water backwards and forwards between the two troughs.
What we can say here is the rate at which the reactants are turning into product is the same as the rate at which the products are turning back into reactants , or the rate of the forward reaction is the same as the rate of the back reaction.

### Dynamic equilibrium

Study the diagram below, it explains what is meant by the term dynamic equilibrium.

The water in the trough represents the amount of reactants in a chemical reaction. To begin with there are no products. Imagine tipping reactants into the products trough and products back into reactants trough at the same time.
To start with the reactants beaker will be almost full, since the reaction has not started. The products beaker tipping into the reactants trough will be empty to begin with. But as the reaction proceeds, or you keep tipping from one trough to another AT THE SAME TIME the amount of water in each beaker will EVENTUALLY be the same. At this point the volume of the reactant and product in each trough will NOT CHANGE even though you are still tipping water in and out of the troughs.

At this point the reaction is at equilibrium, that is the rate of the forward and reverse reactions are the same or we can say that at equilibrium the amount of reactant and product is not changing. It is important to realise that at equilibrium the reaction has not stopped, it is still continuing, it is just that the rate of the forward and reverse reactions are the same. In our image above this means that the volume of water being poured from the reactant trough into the product trough is the same as the volume of water being poured from the products trough into the reactants trough.

At equilibrium the amount of water transferring between the troughs is the same, that is the rate of the forward and reverse reactions are the same. The amounts of the reactants and products is not changing despite the fact that water is being tipped continually between them.

If this was a chemical reaction it would appear to have stopped since the amount of reactants and product is not changing. However the reaction has NOT stopped, it has achieved dynamic equilibrium. It is called dynamic equilibrium, because dynamic implies movement and the reaction is still going on despite the fact that the amount of reactants and products is not changing. Since the beakers are of a different size, at equilibrium there will NOT be equal amounts of water in each trough.

At 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:

### 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:

##### 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.

## Key Points

• 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.