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Reversible Reactions- Definitions, Examples, Practice Problems and FAQs

Reversible Reactions- Definitions, Examples, Practice Problems and FAQs

Every day, we see a variety of things around us changing, such as ice melting and food cooking. Reversing the change to return to the original material is possible in some situations but may not be in others.

The ice in the beaker starts to melt and turn into a liquid by absorbing heat from the environment. But if you place the beaker with the liquid inside of it in the refrigerator, the liquid inside of it begins to freeze and turns back into ice. So, the act of transforming ice into a liquid and then freezing it back into ice is a reversible physical process without any chemical nature change.

There are many more examples of this type but with chemical transformation into new products. Let us discuss the chemistry behind this kind of process and reactions.

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Table of content

  • Reversible reaction
  • Practice Problems
  • Frequently Asked Questions

Reversible reaction

In laboratory you can easily precipitate CaCO3 by mixing aqueous solutions of CaCl2 and Na2CO3.

CaCl2 + Na2CO3 → CaCO3 + 2 NaCl

When CaCO3 is mixed with NaCl solution nothing happens.

CaCO3 + 2 NaCl → X

But such a backward reaction between calcium carbonate rock present on the shores of salt lake water containing NaCl is also naturally occurring in the salt lake of Egypt. Based on this the possibility of a reversible reaction was suggested by Claude Louis Berthollet, a French chemist in the year 1803.

CaCO3 + 2 NaCl ⇌ Na2CO3 + CaCl2

Reversible reactions are those which proceed in both forward and backward directions.

Reversible reactions are a set of two opposing reactions of reactants changing to products with a simultaneous change of products also into reactants at the same time either at the same or different rates(speed).

Some examples of Reversible reactions

  • H2(g)+I2(g) ⇌ 2HI(g)
  • N2(g)+3H2(g) ⇌ 2NH3(g)
  • CaCO3(s) ⇌ CaO(s)+CO2(g)
  • CH3COOH ⇌ CHCOO- + H+

Key points

  1. Reactants combine to form a completely new compound that can then be converted back to a reactant.
  2. A reversible reaction is a two-step reaction denoted by two opposing arrows.
  3. After some time, the rate of the forward and backward reversible reactions becomes same, resulting in the concentrations of reactants and products remaining constant. Reversible reactions are said to have reached equilibrium.
  4. Even when in equilibrium, both forward and backward reactions occur, so the equilibrium is said to be in dynamic equilibrium.
  5. Concentrations of the reactant and products changes initially but come to s constant value after some time governed by Equilibrium constant.
  6. Reversible reactions are never completed.
  7. If the forward reaction is exothermic, the reverse reaction will be endothermic, and vice versa.
  8. Reactions involving gasses in a closed container invariably become a reversible reaction.
  9. Week electrolytes in water are partially ionized and exists in equilibrium between the unionized molecule and ionized ions.
  10. Most of the industrially important reactions, like ammonia, sulphur trioxide and nitrogen oxides are all reversible reactions.

Practice Problems

1. Pick out the correct option for reversible reactions.

  1. the process by which a product becomes a reactant.
  2. the process by which a reactant becomes a product
  3. Both A and B
  4. None of these

Solution: Reactions that can be reversed or Reversible reactions are those in which the byproducts of the forward reaction serve as the reactant for the backward reaction.


In every chemical reaction, reactants will react to give products. Hence, statements A and B both are true. So, the correct answer is an option (C).

2. How do reversible reactions relate to our human body?

Answer: The protein hemoglobin has four binding sites. Carbon dioxide or oxygen molecules can bind to hemoglobin molecules. As blood flows through the lungs, hemoglobin molecules bind to oxygen-rich molecules. As it travels through the body, hemoglobin drops oxygen at the capillaries. After exhaling oxygen, it absorbs carbon dioxide.

As a result, hemoglobin molecules, oxygen, and carbon dioxide act as reactants, and hemoglobin molecules bonded to oxygen or carbon dioxide act as products. In this closed system, some reactants become products while others become reactants, analogous to a reversible reaction.

3. At constant temperature, a reversible reaction has reached dynamic equilibrium. Which of the following sentences is true?

  1. There is an equal amount of reactants and products.
  2. The reaction comes to an end.
  3. The forward reaction's products stop reacting.
  4. The ratio of product concentration to reactant concentration is constant.

Answer: It is not necessary that the amount of reactant and product should be equal. The amount will become constant. (A) is incorrect. The reactions have not stopped; they are still occurring at the same rate. Hence, the reaction does not come to an end. (B) is incorrect. When equilibrium is achieved, whether forward or backward reaction will not stop. Hence, reactant and product both react. (C) is incorrect. As the amount of reactant and product become constant then their ratio will also become constant. (D) is correct. Hence, Statement (D) is correct.

4. Can you explain the backward reaction behavior in the given reaction, MgSO4.6H2O(s)\rlharMgSO4(s)+6H2O(g), when the forward reaction is endothermic?

Answer: It is claimed that the forward reaction is endothermic. This shows that the reaction needs heat energy from the surrounding area to proceed. The amount of heat energy transferred from the environment to the reactant during this dehydration process (forward reaction) is equivalent to the amount of energy transferred from the product to the environment during the hydration reaction of the anhydrous product. This backward process is hence exothermic.

Frequently Asked Questions

Q. Is there an equilibrium state for every reversible reaction at every step?
No, A reversible reaction is not an equilibrium in a closed system at first, but when the concentrations of reactants and products do not change over time, reversible reactions eventually approach equilibrium. However, the forward and reverse reactions have not stopped; they continue to occur concurrently.

Q. Is it possible to roll a paper aeroplane head back?
Yes, rolling a paper aeroplane is a reversible process. Because, if we consider paper to be a reactant and an aeroplane to be a product, the aeroplane can be converted back to paper. As a result, it is a two-way street.

Q. Is it possible to overturn the process of burning a candle?
No, candle burning is an irreversible process. See, melting wax is a reversible process, but when it comes into contact with a flame, it burns and emits carbon dioxide. This CO2 cannot be converted back into wax.

Q. Is stretching a spring a change that can be reversed?
Since there is no change in the chemical components of the spring, extending it can be categorized as a physical change. Additionally, the change is reversible because the spring rebounds to its normal shape when the applied force is released.

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