Lesson Explainer: Reversible and Irreversible Reactions Chemistry

In this explainer, we will learn how to define reversibility in chemical reactions and identify examples of reversible and irreversible processes.

We are all familiar with processes in our daily lives that are reversible and irreversible, for example, water freezing to form ice and ice melting to form liquid water, which are opposite and reversible processes:

However, baking a cake involves irreversible processes. The processes and reactions that occur between the ingredients of a cake mixture while baking in the oven cannot be reversed.

Let us have a look at chemical reactions that are reversible or irreversible. A chemical reaction is a process where one or more substances (called reactants) are converted to one or more different substances (called products).

We tend to think of reactions progressing in one direction only:

When a reaction goes forward in one direction only, we say that it is irreversible.

An example of this is when the fuel in a Bunsen burner undergoes combustion. The fuel is often a mixture of propane and butane. These hydrocarbons react with oxygen in the air when they are ignited.

The reaction for the combustion of propane is

It is an irreversible reaction.

The reaction is complete, meaning that all of the reactant particles reacted and were converted into product. The products, water, and carbon dioxide are stable, and so under normal conditions, they will not spontaneously react to form propane and oxygen again. The reaction is irreversible.

We can write a general equation for an irreversible reaction:

However, in reality, many reactions can also proceed in the opposite direction. The products can interact to reform the reactants by undergoing the backward reaction: or

So, we can write the equation with two half-arrows instead, each pointing in an opposite direction. These arrows show that the reaction can proceed in both the forward and backward directions: or

We call this a reversible reaction. In such a reaction, both the forward and reverse reactions occur simultaneously. A and B react to form C and D at the same time that C and D react together to form A and B.

However, it is important to note that, in principle, all reactions are reversible. However, the conditions necessary for the backward reaction to occur are often very difficult to achieve. For example, the backward reaction for the combustion of propane would need very specific conditions to occur. These conditions would not be easily achievable in a standard laboratory, and that is why we say that only the forward reaction occurs for combustion.

Reversible reactions are more noticeable to us when they are incomplete, in other words, when not all of the reactants are converted to products.

An example of a reversible reaction is when ammonium chloride, a white solid, is heated and decomposes into two different products, ammonia gas and hydrogen chloride gas. These two products can easily react with each other when cool and produce the initial reactant, ammonium chloride:

Sometimes the mouth of the test tube is plugged with cotton wool to prevent cooling vapors from escaping, but allowing lighter gases in the air to escape. In this way, the reaction components are confined within the test tube and the gaseous products can react with each other.

This reaction shows us that heat energy (input or removal) is one of the conditions that can influence the direction of a reversible reaction. If a forward reaction is endothermic, as in the following case, then the input of heat energy (by heating up the chemicals) will drive the forward reaction:

The reverse reaction is then exothermic. Removing heat energy (by cooling the chemicals) will drive the reverse reaction.

It is important to remember, though, that not all forward reactions are endothermic and not all reverse reactions are exothermic.

The reversible reaction

shows that the forward reaction is exothermic and the reverse is endothermic. The amount of energy released by the forward reaction—in other words, the enthalpy change, —is the same as the amount of energy absorbed by the reverse reaction.

The following reaction pathway diagram shows that the amount of energy absorbed by the endothermic reaction of a reversible reaction () or released by the exothermic reaction of a reversible reaction () is the same.

Bond breaking is typically an endothermic process as energy is required to break the bonds in the reactants. The reactant atoms then rearrange to form the products. Forming bonds is typically an exothermic process and energy is released. We call an overall reaction endothermic if the overall amount of energy absorbed is greater than the overall amount of energy released. Conversely, in an exothermic reaction, the overall amount of energy released is greater than the overall amount of energy absorbed.

Let us look at some more examples of reversible reactions.

, hydrated copper(II) sulfate, is a blue crystalline solid. It contains waters of crystallization, and so we call it a hydrated salt.

If we heat some of these hydrated copper sulfate crystals, we would notice a white vapor being given off and the crystals turning into a white powder. The vapor is steam, or gaseous water. The white powder that is left behind is anhydrous, or dehydrated copper(II) sulfate, .

This is the reaction that occurs:

We can observe the reverse reaction too by adding a few drops of water to the white anhydrous powder. The white powder would turn blue again as shown in the photograph below. The anhydrous copper(II) sulfate turns blue again as it becomes hydrated, and because of these vivid differences in color, we can use copper(II) sulfate to test for the presence of water.

The forward reaction is endothermic and is favored by heating. Conversely, when water is added, the reverse reaction gives off heat energy because it is exothermic. This is often the case for hydrated salts.

If both the forward and reverse reactions occur in a closed system, then after some time, the reaction will reach equilibrium. For example, this happens with weak acids and bases in solution. Weak acids and weak bases dissociate in water in a reversible manner. An instance of this is ethanoic acid, , which is not all converted to ions in water, but some ions reassociate to form at the same time that other molecules are dissociating. The reversible reaction, or process, that occurs is

Both the forward and reverse processes occur at the same time and will continue to occur simultaneously until the system reaches an equilibrium. At equilibrium, the concentrations of all the species in the system will remain constant but not necessarily the same. The concentration of on the left-hand side of the equation and the concentrations of the ions on the right-hand side of the equation depend on the conditions, such as temperature. If the conditions change, then the equilibrium will “shift” to either the left or the right side of the equation, changing the concentration of each species. However, both the forward and reverse reactions will continue to occur. They will occur at the same rates, and we call this dynamic equilibrium.

There is another type of equilibrium called static equilibrium. In static equilibria, the rates of the forward reaction and the backward reaction are both zero.

Dynamic and static equilibria are covered in more depth in another explainer.