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Dynamic Equilibrium

Chemistry

1. Fundamental Concepts

  • Dynamic Equilibrium: A state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction.

  • At equilibrium:

    • The reaction continues to occur (dynamic, not static).

    • The concentrations of reactants and products remain constant, but are not necessarily equal.

  • Dynamic equilibrium can only be established in a closed system.

2. Key Concepts

  • Reversible Reactions: Represented by the symbol ⇌

  • Rate Equality ≠ Concentration Equality

    • Forward rate = reverse rate

    • Amounts of reactants and products are usually different

  • Equilibrium Position

    • Indicates whether equilibrium favors reactants or products

  • Le Châtelier’s Principle

    • When a system at equilibrium is disturbed (change in concentration, pressure, or temperature), the system shifts to counteract the disturbance.

  • Catalysts

    • Increase the rate at which equilibrium is reached

    • Do not change the equilibrium position or equilibrium constant (K)

3. Examples

Easy

H2(g)+I2(g)⇌2HI(g)\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)

  • At equilibrium, the concentrations of H₂, I₂, and HI remain constant.

  • Both the forward and reverse reactions continue at equal rates.

Medium 

N2(g)+3H2(g)⇌2NH3(g)\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)

Change: Increase the concentration of H₂

  • The system consumes excess H₂.

  • Equilibrium shifts to the right.

  • More NH₃ is produced.

Hard

2NO2(g)⇌N2O4(g)ΔH<02\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g) \quad \Delta H < 0

Change: Increase temperature

  • The forward reaction is exothermic.

  • The system shifts in the endothermic direction to oppose the temperature increase.

  • Equilibrium shifts to the left, producing more NO₂.

4. Problem-Solving Techniques

  • Confirm the reaction is reversible.

  • Use the correct definition of equilibrium

    • Equilibrium means equal rates, not equal concentrations.

  • Le Châtelier’s Three-Step Approach

    1. Identify what change is applied (concentration, pressure, temperature).

    2. Determine how the system resists the change.

    3. Predict the direction of the equilibrium shift.

  • Catalyst Reminder

    • A catalyst affects reaction speed only.

    • It does not change equilibrium position or K.

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