8.2 Reversible Reactions and Dynamic Equilibrium

This topic explains reversible reactions and dynamic equilibrium. It describes how reactions can proceed in both directions and how equilibrium is established.

Definition

Reversible Reaction: A reaction in which the products once formed can react to form reactants.

Dynamic Equilibrium: The state of a reversible reaction at which the composition of the reaction mixture does not change, but both forward and reverse reactions continue at equal rates.

Main Idea

A reversible reaction is one in which the products once formed can react to form reactants. Such reactions do not go to completion even if stoichiometric amounts of the reactants are taken. These reactions take place both in the forward and backward directions under the existing conditions. The state of a reversible reaction at which composition of the reaction mixture does not change is called the state of chemical equilibrium.

Core Concept

• Reversible reaction: Products can react to form reactants.
• Dynamic equilibrium: Forward and reverse rates are equal.
• Concentrations: Constant at equilibrium.
• Microscopic activity: Continues at equilibrium.
• Double arrow: Indicates reversible reaction.

Detailed Explanation

What is a Reversible Reaction?

• A reversible reaction is one in which the products once formed can react to form reactants.
• Such reactions do not go to completion even if stoichiometric amounts of the reactants are taken.
• These reactions take place both in the forward and backward directions under the existing conditions.
• Some examples of reversible reactions are given below:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

2NO_2(g) ⇌ N₂O_4(g)

PCl_5(g) ⇌ PCl_3(g) + Cl_2(g)

- The double arrow (⇌) tells that the reaction is reversible.

How Equilibrium is Established

Consider the reaction between steam and carbon monoxide under appropriate conditions.

H₂O_(g) + CO_(g) → H_2(g) + CO_2(g)

• On mixing, macroscopic changes are observed (e.g., changes in concentration).
• Suppose that the reaction is started with same number of moles of both reactants.
• When steam and carbon monoxide are mixed, a maximum number of collisions per second between them will occur.
• Therefore the forward reaction has its maximum speed at the beginning.
• This leads to a decrease in the concentration of the reactants.
• As H₂O and CO are gradually used up, the forward reaction gradually slows down.
• As the molecules of H₂ and CO₂ accumulate, the reverse reaction also starts.
• With the increase in concentration of H₂ and CO₂, more and more collisions per second between these molecules occur.
• Therefore reverse reaction proceeds with increasing speed.

• Forward reaction starts with maximum speed and gradually slows down.
• Reverse reaction starts at zero speed and gradually increases its speed.
• Eventually a time comes when both reactions proceed at the same speed.
• The reaction at this stage is said to be in chemical equilibrium.
• The concentration of reactants and products become constant.

H₂O_(g) + CO_(g) ⇌ H_2(g) + CO_2(g)

Dynamic Equilibrium

• Unless the system is somehow disturbed, no further changes in concentration will occur.
• "The state of a reversible reaction at which composition of the reaction mixture does not change is called the state of chemical equilibrium."
• When the concentration of reactants and products becomes constant, the reaction may appear to have stopped.
• But that is not true.
• At the microscopic level, there is intense activity.
• The individual molecules of the reactants continue to combine.
• Individual product molecules also continue to combine.
• But the speed of one process is exactly balanced by the speed of the other process.
• Therefore, it is a dynamic equilibrium.
• The system is dynamic because the individual molecules are constantly reacting, but the rate of forward and reverse reactions is the same.
• It is balanced.

Intermediate Level

Reversible Reactions

Examples: N₂ + 3H₂ ⇌ 2NH₃ 2NO₂ ⇌ N₂O₄ PCl₅ ⇌ PCl₃ + Cl₂

Dynamic Equilibrium

How Equilibrium is Established

• Forward reaction starts fast.
• Forward reaction slows down as reactants are used.
• Reverse reaction starts and speeds up.
• Both rates become equal → equilibrium.
• Concentrations become constant.

Key Point

• Equilibrium is dynamic, not static.
• Molecules continue to react in both directions.
• Rates of forward and reverse reactions are equal.

Important Points

• 1. Reversible reaction: Products react to form reactants.
• 2. Does not go to completion.
• 3. Double arrow (⇌) indicates reversible.
• 4. Dynamic equilibrium: Forward rate = reverse rate.
• 5. Concentrations constant at equilibrium.
• 6. Microscopic activity continues.
• 7. Examples: N₂ + 3H₂ ⇌ 2NH₃, 2NO₂ ⇌ N₂O₄, PCl₅ ⇌ PCl₃ + Cl₂.

Remember

• - Reversible reaction: Products → reactants
• - Dynamic equilibrium: Forward rate = reverse rate
• - Concentrations: Constant
• - Microscopic activity: Continues
• - Double arrow: ⇌
• - Examples: N₂ + 3H₂ ⇌ 2NH₃, 2NO₂ ⇌ N₂O₄, PCl₅ ⇌ PCl₃ + Cl₂

Common Mistakes

• 1. Thinking equilibrium means reaction stops: It continues at the molecular level.
• 2. Confusing dynamic with static: Dynamic = continues; Static = stops.
• 3. Forgetting the double arrow: Indicates reversible reaction.
• 4. Thinking concentrations are equal: They are constant, not necessarily equal.

Quick Revision

• Reversible reaction: Products react to form reactants.
• Does not go to completion.
• Double arrow (⇌) indicates reversible.
• Dynamic equilibrium: Forward rate = reverse rate.
• Concentrations constant at equilibrium.
• Microscopic activity continues.
• Examples: N₂ + 3H₂ ⇌ 2NH₃, 2NO₂ ⇌ N₂O₄, PCl₅ ⇌ PCl₃ + Cl₂.

60-Second Revision

• Reversible reaction: Products → reactants
• Dynamic equilibrium: Forward rate = reverse rate
• Concentrations: Constant
• Microscopic activity: Continues
• Double arrow: ⇌
• Examples: N₂ + 3H₂ ⇌ 2NH₃, 2NO₂ ⇌ N₂O₄, PCl₅ ⇌ PCl₃ + Cl₂

QUALITY VALIDATION

• Topic: 8.2 Reversible Reactions and Dynamic Equilibrium
• Chapter: 8 (Chemical Equilibrium)

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