Dynamic equilibrium

Dynamic equilibrium occurs in reversible reactions when the forward reaction's rate equals the reverse reaction's rate. Although both reactions continue, their rates remain equal and constant, maintaining the system equilibrium.

1.0What is a Dynamic Equilibrium?

All chemical reactions are reversible, with products reacting to reform the reactants. When the forward and reverse reaction rates equalize, the system reaches chemical equilibrium, where the composition remains constant over time. At equilibrium, both reactions continue at the same rate in opposite directions.

For example:

H₂O(l) ⇌ H⁺(aq) + OH⁻ (aq)

This is a reversible reaction, where both directions co-occur, maintaining balance without net change.

Dynamic equilibrium is not limited to chemistry labs; you encounter it in everyday life, such as in a sealed bottle of soda. Inside the bottle, carbon dioxide exists in both the gaseous phase (bubbles) and the liquid phase. The dynamic equilibrium occurs when the rate at which carbon dioxide gas dissolves into the liquid equals the rate at which carbon dioxide in the liquid converts back into gas.

The equilibrium equation is: CO₂ (g)  ⇌  CO₂ (aq).

Changing a reaction's temperature, pressure, or concentration can disrupt dynamic equilibrium and shift the balance. For example, when you open a soda can and leave it out, it eventually goes "flat" with no more bubbles. This happens because the system is no longer closed, allowing carbon dioxide to escape into the atmosphere. As the carbon dioxide gas is released, the dynamic equilibrium is disturbed, leading to the loss of bubbles.

2.0Characteristics of Dynamic Equilibrium

• Achieved in a closed system.
• Both reactants and products coexist.
• Forward and backward reactions occur at equal rates.
• Equilibrium can be approached from either direction.
• Concentrations, temperature, pressure, and colour remain constant.
• Catalysts speed up the process but don't affect the equilibrium position.
• Changes in temperature, pressure, or concentration shift the equilibrium according to Le Chatelier’s principle.

3.0Comparing Static and Dynamic Equilibrium

• Static Equilibrium occurs when the system's reaction is completely halted, with no movement or exchange between reactants and products.
• Dynamic Equilibrium involves a continuous, reversible reaction where the forward and backward processes occur at equal rates.

Key Differences:

4.0Examples of Dynamic Equilibrium

• Acetic Acid Dissociation: In an aqueous solution, acetic acid dissociates into acetate ions and hydrogen ions, establishing an acid-base equilibrium:CH₃COOH ⇌ CH₃COO⁻ + H⁺
• The dimerisation of Nitrogen Dioxide: In the gaseous phase, nitrogen dioxide dimerises to form dinitrogen tetroxide:2NO₂ ⇌ N₂O₄
• Henry’s Law: The equilibrium concentration of CO2 in a liquid (e.g., soda) is proportional to the partial pressure of CO2 gas above it. This is another example of dynamic equilibrium.
• Haber Process (Ammonia Synthesis): In industrial ammonia synthesis, nitrogen reacts with hydrogen to form ammonia, while ammonia also breaks down into nitrogen and hydrogen, maintaining a dynamic equilibrium:N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
• NaCl in Solution: In a saturated NaCl solution, solid NaCl dissolves and recrystallises simultaneously. The reaction is in dynamic equilibrium when the dissolution rate equals the recrystallisation rate:NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq)
• Nitrogen Dioxide and Carbon Monoxide Reaction: Nitrogen dioxide reacts with carbon monoxide to form nitrogen oxide and carbon dioxide, while the reverse reaction also occurs:NO₂(g) + CO(g) ⇌ NO(g) + CO₂(g)