Is Chemical Change Reversible: Detailed Analysis


‘Is chemical change reversible?’ – This question has been asked numerous times and left people scratching their heads.

The Chemical change is reversible at times. Yes, you read it right! Well, a chemical change is usually an irreversible process, but the beauty of chemistry is that – it has exceptions, just like this one.

The reversible chemical reaction has the ability to progress in both ways, which simply means to move forward as well as backward. But this process occurs only till dynamic equilibrium is attained. Equilibrium, in this case, is achieved when two reactions, happening in opposite ways, occur at the same time and at the same rate.

What is a Reversible Process?

Reactions that do not reach to the absolute completion are reversible.

A reversible process or reaction is a reaction in which reactants are converted into products and products are converted into reactants simultaneously.

On occasions, the number of reactants getting converted back from products might vary from the original amount of reactants.

Give some advantages and disadvantages of a reversible chemical reaction.

AdvantagesDisadvantages
Reversible chemical reactions or in general, every chemical reaction help us in understanding the properties of matter and in developing new technologies or new products.Occasionally, the produced outcome may be harmful to the environment as well as to the other life forms on earth (like humans, animals, plants, marine life, etc.).
Increased reaction rates.They cost more in terms of both, money and time.
Improved quality of the product.Finite applications.
Less initial investment.Requirement of complex operations when wanting to reverse the reaction.
Simple reactant to a product mixture.Quite a time, complex product to reactants conversion.
Finer product quality.Considerable experimental development is required.
Enhanced catalyst life.Only a few reactions can be reversed compared to more number of irreversible reactions.

Read more on How Chemical Change Occurs

Give one Example of Reversible Chemical Change.

The reaction of ammonia and hydrogen chloride is one such example of reversible chemical change.

Ammonium chloride, a mixture of ammonia and hydrogen chloride, which is a white solid. When hydrogen chloride and ammonia are mixed at room temperature, i.e., 25°C, they form ammonium chloride.

It again breaks down into ammonia and hydrogen chloride when heated.

[latex]Ammonia \; (NH_{3}) \; + \; Hydrogen \; Chloride \; (HCl) \rightleftharpoons Ammonium \; Chloride \; (NH_{4}Cl)[/latex]

Whenever a reversible chemical reaction takes place, a part of the reaction heading in one direction will always be exothermic, and the reaction heading in another direction will always be endothermic.

Remember, the reaction from left to right doesn’t need to be exothermic, and the reaction from right to left doesn’t need to be endothermic. It can occur in whichever way possible.

What should be the equilibrium position for a reversible reaction?

The equilibrium of a reversible chemical reaction is affected by several factors.

The reaction is in equilibrium when the concentration of reactants and products are equal.

One can change the equilibrium of a reversible chemical reaction through various other conditions like adjusting the temperature, changing the pressure, changing the concentration, etc.

Let us consider a straightforward example to understand the concentration of a reaction with the help of the Haber process.

Say, one part of nitrogen molecules and three parts of hydrogen molecules are mixed together to get two parts of ammonia molecules.

Now, when we say one part means 1 N2 molecule and not 1 N, as Nitrogen (N) alone cannot exist. A single nitrogen atom immediately makes a bond with another nitrogen molecule to fulfill its valance band.

Similarly, three parts of hydrogen mean 3 H2 molecules.

Thus, the reaction for the Haber process is given as follows:

[latex]N_{2} \; + \; 3H_{2} \rightleftharpoons 2NH_{3}[/latex]

Now the question arises that 1N2 + 3H2 should yield 4NH3 but instead, we have 2NH3.

Well, when looked closely, there are 2 N molecules on the left side as well as 2 N molecules on the right side.

Similarly, for hydrogen on the left side, we have 3H2 molecules, which give 3×2 = 6 hydrogen atoms as well as 3×2 = 6 hydrogen atoms on the right side.

When the numbers of atoms or molecules on both sides are equal, the reaction is said to be balanced, and hence, we can say that it is in equilibrium.

is chemical change reversible
General Chemical Reversible Reaction

Read more on Reversible Reaction.

When there is an increase in pressure, the reaction moves in the direction where there are less number of molecules, and when the pressure is decreased, the reaction proceeds in the order where there are more number of molecules.

When there is an increase in concentration, the equilibrium position will move in the opposite direction from the direction of the reactant, of which the concentration is increased. For e.g., if the concentration of reactant ‘A’, which is on the right side, is increased, the equilibrium will move on the left side.

Similarly, when there is an increase in the temperature, the equilibrium will move towards the endothermic process. Thus, if the forward reaction is exothermic, the reverse reaction ought to be endothermic, and if the forward reaction is endothermic, the reverse reaction should essentially be exothermic.

For remembering this information more effectively, it is better to put it in a tabular form.

ChangeEquilibrium
Pressure IncreasedMoves to fewer molecules
Concentration IncreasedMoves to the opposite direction from that reactant
Temperature IncreasedMoves to the endothermic reaction

Examples of Reversible Chemical Reaction

  • Haber process.

[latex]Nitrogen \; (N_{2}) \; + \; Hydrogen \; (3H_{2}) \rightleftharpoons Ammonia \; (2NH_{3})[/latex]

  • Formation of Hydrogen Iodide.

[latex]Hydrogen \; (H_{2}) \; + \; Iodine \; (I_{2}) \rightleftharpoons Hydrogen \; Iodide \; (2HI)[/latex]

  • Decomposition of Calcium Carbonate.

[latex]Calcium \; Carbonate \; (CaCO_{3}) \rightleftharpoons Calcium \; Oxide \; (CaO) \; + \; Carbon \; Dioxide \;(CO_{2})[/latex]

  • The reaction of bromine with water.

[latex]Bromine \; (Br_{2}) \; + \; Water \; (H_{2}O) \rightleftharpoons Hypobromous \; Acid \; (HOBr) \; + \; Hydrobromide \; (HBr)[/latex]

  • The reaction of iron ions with thiocyanate ions.

[latex]Iron (III) \; ions \; (Fe^{3+}) \; + \; Thiocyanate \; ions \; (SCN^{-}) \rightleftharpoons Iron (III) \; thiocyanate \; complex \; ion \; (FeSCN^{2+})[/latex]

Durva Dave

I am Durva Dave, completed my post-graduation in Physics. Physics fascinates me a lot and I like in knowing the ‘Why’ and ‘How’ of everything that unfolds in our universe. I try to write my blogs in simple yet effective language so that it is easier for the reader to understand as well remember. I hope with my curiosity I am able to provide the readers for what their looking for through my blogs. Let’s connect through LinkedIn.

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