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).
Definition: Chemical Reaction
A chemical reaction is a process where one or more substances are converted to one or more different substances.
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.
Definition: Irreversible Reaction
An irreversible reaction is a reaction that proceeds in one direction only; the products do not react together to reform the reactants.
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:
Example 1: Identifying an Irreversible Reaction from Given Observations
An alcohol is ignited and a mixture of two different gases is collected. What conclusion could be drawn from this observation?
- The reaction is irreversible.
- The reaction equation would use the symbol .
- The reaction is endothermic.
- The organic alcohol contains water of crystallization.
- The reaction is reversible.
We are told that the starting substance in this reaction is an alcohol and that it is ignited. When a substance is ignited, we assume that it is burning in air. In other words, we assume that it is undergoing a combustion reaction with oxygen in the air. We are told that there are two different gases collected after the alcohol is ignited. Since the two gases are different from each other and different from the two starting substances (alcohol and oxygen), we can deduce that a reaction did indeed occur. As the initial substance, the alcohol, was not collected at the end of the reaction, we can also assume that all of the reactants were converted to products and that the products did not recombine to form the alcohol. We can conclude that the reaction is irreversible. So, the correct answer is A, the reaction is irreversible.
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.
Definition: Reversible Reaction
A reversible reaction is a reaction that proceeds in both directions; reactants react to form products, and the products react to reform the reactants.
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.
Example 2: Identifying the Statement That Correctly Describes a Reversible Reaction
Which of the following statements about reversible reactions is true?
- A reversible reaction is indicated by the symbol in a chemical equation.
- A reversible reaction is endothermic in both directions.
- A reversible reaction always involves hydrated and anhydrous salts.
- A reversible reaction is often a combustion reaction.
- A reversible reaction is a chemical reaction that can proceed in both directions.
In a reversible reaction, the reactants can react to form products, and the products can react to reform the reactants. This is a general reaction equation for a reversible reaction:
So, A and B can react in the forward reaction to produce C and D, and at the same time, C and D can react in the backward reaction to produce A and B. The correct answer is E, a reversible reaction is a chemical reaction that can proceed in both directions.
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.
Example 3: Understanding How a Forward and a Reverse Reaction are Related in Terms of Energy Transfer
In the following chemical reaction, the forward reaction is endothermic:
What must be true about the reverse reaction?
- The reverse reaction has a lesser overall energy change.
- The reverse reaction has a greater overall energy change.
- The reverse reaction is also endothermic.
- The reverse reaction is exothermic.
- The reverse reaction absorbs heat from the surroundings.
We are told that the forward reaction is endothermic. This means that this reaction absorbs heat energy from the surroundings to occur. The amount of heat energy transferred from the surroundings to the reactant during this dehydration process (forward reaction) is the same as the amount of energy transferred from the product to the surroundings during the hydration reaction of the anhydrous product . The hydration (reverse) reaction is
This backward reaction is therefore exothermic and so the correct answer is D, the reverse reaction is exothermic.
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.
Definition: Hydrated Salt
A hydrated salt is a salt that contains water of crystallization.
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, .
Definition: Anhydrous (Dehydrated) Salt
An anhydrous, or dehydrated, salt is a salt that does not contain water of crystallization.
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.
Example 4: Identifying the Incorrect Statement from a Given List of Statements Describing a Reversible Reaction
Select the statement that does not accurately describe the following chemical reaction:
- The reaction is reversible.
- Blue crystals are formed.
- Anhydrous sodium carbonate is produced.
- The forward reaction is endothermic.
- is formed.
The chemical reaction given has two half-arrows pointing in opposite directions, so we know that the reaction is reversible. The reactant can break apart to form the products and the products combine to form the reactant. Statement A, the reaction is reversible, does correctly describe the reaction.
The reactant contains waters of crystallization, so it is a hydrated compound. When it is heated, the product forms, which does not contain water of crystallization. The product is anhydrous sodium carbonate, where anhydrous means “without water of crystallization.” Statement C, anhydrous sodium carbonate is produced, correctly describes the reaction.
To drive the forward reaction—in other words, to remove the water of crystallization from the hydrated compound—heat energy must be absorbed by the reactant, . Thus, the forward reaction is endothermic. Statement D, the forward reaction is endothermic, accurately describes the reaction.
The reaction equation shows us that gaseous water is one of the products. So, statement E, is formed, is an accurate statement for this reaction.
So far, we have seen that statements A, C, D, and E are correct statements. By the process of elimination, we can deduce that statement B, blue crystals are formed, is not an accurate description of the product . is a white substance. (A commonly known substance that is crystalline and blue is , although there are many other blue crystalline compounds.)
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.
Definition: Dynamic Equilibrium
Dynamic equilibrium is an equilibrium between forward and reverse reactions where both reactions are occurring at the same, nonzero rate.
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.
Definition: Static Equilibrium
Static equilibrium is an equilibrium where 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.
- An irreversible reaction proceeds in one direction only; the products do not react together to reform the reactants:
- A reversible reaction proceeds in both directions; reactants react to form the products and the products react to reform the reactants:
- Conditions, such as temperature, affect the reactions of a reversible reaction.
- If the forward reaction is exothermic, then the reverse reaction is endothermic.
- If the forward reaction is endothermic, then the reverse reaction is exothermic.
- In a reversible reaction, the amount of energy transferred between the system and the surroundings is the same for the forward and the backward reactions.
- In dynamic equilibria, the rate of the forward reaction and the rate of the backward reaction are the same and are nonzero.
- In static equilibria, the rates of the forward reaction and the backward reaction are both zero.