9.1 Reversible Reactions and Dynamic Equilibrium

This topic explains reversible reactions and dynamic equilibrium — reactions that can go in both directions and reach a state of balance.

Definition

Reversible Reaction

A reversible reaction is a reaction in which the products can react with each other to form the original reactants again.

Simple explanation: In a reversible reaction, the reaction can go both ways — forward and backward.

Dynamic Equilibrium

The state of a chemical reaction where the forward and reverse reactions occur at the same rate is called chemical equilibrium (or dynamic equilibrium).

Simple explanation: At equilibrium, the reaction hasn't stopped — it's just that the forward and backward reactions are happening at the same speed, so nothing appears to change.

Main Idea

A complete reaction is a reaction in which all reactants have been converted to products.

However, many important chemical reactions are not completed and a mixture of products and reactants is formed. In such a reaction:

• The products react together to form the reactants again.
• At the same time, reactants form products.

These reactions are called reversible reactions.

Simple explanation: In a reversible reaction, reactants turn into products and products turn back into reactants at the same time.

Core Concept

Forward and Reverse Reactions

Example:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

• Forward reaction: SO₂ + O₂ → SO₃
• Reverse reaction: SO₃ → SO₂ + O₂

Simple explanation: The double arrow (⇌) means the reaction can go both ways.

Dynamic Equilibrium

Chemical equilibrium is dynamic equilibrium because:

• Reactions do not stop when they reach equilibrium.
• Individual molecules are constantly reacting.
• However, the actual quantities of reactants and products do not change.
• This means the concentration of reactants and products becomes constant in the equilibrium state.

Simple explanation: At equilibrium, reactions are still happening, but the amounts of reactants and products stay the same because the rates are equal.

Detailed Explanation

What Happens in a Reversible Reaction?

Consider the reaction between SO₂ and O₂ in a sealed container:

Step 1: Initially, SO₂ and O₂ react to form SO₃ (forward reaction).

2SO_2(g) + O_2(g) → 2SO_3(g)

• At the beginning, there is no SO₃.
• The rate of the reverse reaction is zero.
• Due to the high concentration of reactants, the speed of the forward reaction is the highest.

• The concentration of reactants gradually decreases.
• The speed of the forward reaction decreases.
• The concentration of SO₃ increases.

Step 3: A small amount of SO₃ slowly decomposes back into SO₂ and O₂ (reverse reaction).

2SO_3(g) → 2SO_2(g) + O_2(g)

- As the concentration of SO₃ increases, the reverse reaction accelerates.

• SO₃ decomposes into SO₂ and O₂ as fast as SO₂ and O₂ produce SO₃.
• At this point, the reaction has reached equilibrium.

Simple explanation: At first, only the forward reaction happens. Then the reverse reaction starts and speeds up. Eventually, both reactions happen at the same rate — this is equilibrium.

The Double Arrow (⇌)

A double arrow (⇌) in a chemical equation indicates that the reaction is reversible.

Examples from the book:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

N_2(g) + 2O_2(g) ⇌ 2NO_2(g)

Simple explanation: The double arrow tells you the reaction can go both ways.

Characteristics of Dynamic Equilibrium

• Forward and reverse reactions occur at the same rate.
• Concentrations of reactants and products remain constant.
• The reaction does not stop — it is dynamic.
• Equilibrium can be reached from either direction.
• Equilibrium requires a closed system.

Simple explanation: At equilibrium, the reaction is still happening, but nothing appears to change.

Science Titbit from the Book

Fizzy drinks:

When fizzy drinks are made, CO₂ is dissolved in the liquid drink under pressure and sealed.

When you remove the lid of the bottle, bubbles of CO₂ suddenly appear. When you put the lid back on the bottle, the bubbles stop.

This is due to the following equilibrium:

CO_2(g) ⇌ CO_2(aq)

• The forward reaction happens during manufacturing (CO₂ dissolves in the drink under pressure).
• The reverse reaction happens on opening (CO₂ escapes from the drink).

Simple explanation: The CO₂ in fizzy drinks is in equilibrium — when you open the bottle, the pressure changes and CO₂ escapes.

Intermediate Level

Why Equilibrium Is Dynamic

• Both forward and reverse reactions continue to occur.
• Molecules are constantly reacting in both directions.
• The rates of the forward and reverse reactions are equal.
• So the concentrations of reactants and products remain constant.

Simple explanation: It's like a busy escalator — people are constantly moving, but the number of people at the top and bottom stays the same.

Conditions for Equilibrium

• Pure reactants, pure products, or a mixture are placed in a closed container.
• The forward and backward reactions occur at the same speed.
• The concentrations of reactants and products remain constant.

Characteristics of a System at Equilibrium

Why Equilibrium Matters in Industry

• Equilibrium reactions are involved in the commercial production of many important chemicals such as ammonia, sulphuric acid, etc.
• By controlling conditions (temperature, pressure, concentration), industries can shift the equilibrium to maximize product yield.

Simple explanation: Industries use equilibrium principles to make more product.

Important Points

• - Reversible reaction: products can react to form the original reactants.
• - Double arrow (⇌): indicates a reversible reaction.
• - Dynamic equilibrium: forward and reverse reactions occur at the same rate.
• - At equilibrium: concentrations of reactants and products remain constant.
• - Equilibrium is dynamic — reactions do not stop.
• - Forward reaction: reactants → products.
• - Reverse reaction: products → reactants.
• - Examples:
• - 2SO₂ + O₂ ⇌ 2SO₃
• - N₂ + 3H₂ ⇌ 2NH₃
• - CO₂(g) ⇌ CO₂(aq) (fizzy drinks)
• - Equilibrium requires a closed system.

Remember

• - Reversible reaction = can go both ways.
• - ⇌ = reversible reaction symbol.
• - Dynamic equilibrium = forward rate = reverse rate.
• - Concentrations remain constant at equilibrium.
• - Reaction doesn't stop at equilibrium.
• - Examples:
• - 2SO₂ + O₂ ⇌ 2SO₃
• - N₂ + 3H₂ ⇌ 2NH₃
• - CO₂(g) ⇌ CO₂(aq)

Common Mistakes

• 1. Thinking the reaction stops at equilibrium. It doesn't — both reactions continue at the same rate.
• 2. Thinking the amounts of reactants and products are equal at equilibrium. They are constant, not necessarily equal.
• 3. Thinking equilibrium can be reached in an open system. Equilibrium requires a closed system.
• 4. Confusing reversible and irreversible reactions. Reversible reactions can go both ways; irreversible reactions go only one way.
• 5. Thinking equilibrium means equal concentrations. Concentrations are constant, not equal.
• 6. Forgetting the double arrow. The ⇌ symbol indicates a reversible reaction.

Quick Revision

• Reversible reaction: a reaction in which the products can react with each other to form the original reactants again.
• Reversible reactions are shown by the double arrow (⇌) and may not go to completion.
• Chemical equilibrium: the state of a chemical reaction where the forward and reverse reactions occur at the same rate.
• Dynamic equilibrium: reactions do not stop; individual molecules are constantly reacting; the actual quantities of reactants and products do not change; concentrations become constant.
• Forward reaction: reactants → products.
• Reverse reaction: products → reactants.
• Examples:
• 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
• N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
• N₂(g) + 2O₂(g) ⇌ 2NO₂(g)
• CO₂(g) ⇌ CO₂(aq) (fizzy drinks)
• Equilibrium is achieved in a closed container when forward and reverse reactions occur at the same speed.

60-Second Revision

• Reversible reaction = can go both ways.
• Dynamic equilibrium = forward rate = reverse rate.
• Concentrations constant at equilibrium.
• Reaction doesn't stop.
• Examples: 2SO₂ + O₂ ⇌ 2SO₃; N₂ + 3H₂ ⇌ 2NH₃.
• Warning: at equilibrium, the reaction does not stop — it continues in both directions.