Higher and foundation tier

Reversible reactions

Consider the following reaction:
reversible reaction equations

combustion 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.

Reversible reactions

Most chemical equations are written with a different arrow:
reversible reaction 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 reversible reactions

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. reversible reaction 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 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.

reversible reactions

Dynamic equilibrium

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

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..

graphs to show reactant and product concentrations at equilibrium

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.

heating hydrated copper sulfate

We can show this reversible reaction in an equation as:

hydrated copper sulfateanhydrous copper sulfate + water
CuS04.5H2O CuS04 + 5H20

The thermal decomposition of ammonium chloride

decomposition of ammnonium 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.

Key Points