R2.3.1 Dynamic equilibrium

Dynamic Equilibrium: A Balance in Motion

1. You’re standing in a busy train station.
2. Passengers are constantly boarding and exiting a train, yet the number of people on board remains unchanged.
3. It might seem puzzling at first, but this happens because the rate at which people board matches the rate at which they leave.

This is a perfect analogy for dynamic equilibrium in chemistry—a state where change is ongoing, but the system as a whole appears stable.

What is Dynamic Equilibrium?

At equilibrium, the rates of the forward and reverse reactions are equal, creating a balance.

Key Characteristics of Dynamic Equilibrium:

• Constant Concentrations:
  • While reactions continue to occur, the concentrations of reactants and products remain constant over time.
  • However, this does not mean the concentrations are equal—only that they are unchanging.
• Dynamic Nature:
  • At the molecular level, particles are continuously reacting.
  • The system is not static; it is in constant motion.
• Closed System:
  • The system must be closed, meaning no reactants or products can enter or leave.
  • This ensures equilibrium is maintained.

Examples of Dynamic Equilibrium

1. Phase Equilibrium: Water in a Sealed Container

1. Picture a sealed container with liquid water and its vapor.
2. Initially, water molecules evaporate into the gas phase, increasing the concentration of water vapor.
3. Over time, some vapor molecules condense back into liquid water.
4. Eventually, the rate of evaporation equals the rate of condensation, and the system reaches equilibrium.

Equation:$${\text{H}}_2\text{O}(l)\rightleftharpoons {\text{H}}_2\text{O}(g)$$

2. Chemical Equilibrium: The Haber Process

A classic example of chemical equilibrium is the synthesis of ammonia in the Haber process:

Equation:$${\text{N}}_2(g)+3{\text{H}}_2(g)\rightleftharpoons 2{\text{NH}}_3(g)$$

1. Initially, nitrogen (${\text{N}}_2$) and hydrogen (${\text{H}}_2$) combine to form ammonia (${\text{NH}}_3$).
2. As ammonia accumulates, the reverse reaction begins, where ammonia breaks down into nitrogen and hydrogen.
3. Eventually, the rates of synthesis and decomposition equalize, and the concentrations of all three gases remain constant.

Achieving Equilibrium: A Closer Look

1. Starting with Reactants

• If a reaction begins with only reactants, the forward reaction dominates initially, producing products.
• As products accumulate, the reverse reaction begins.
• Over time, the rates of the forward and reverse reactions equalize, and equilibrium is established.

2. Starting with Products

• Equilibrium can also be reached starting with only products.
• In this case, the reverse reaction dominates initially, converting products into reactants.
• Eventually, the same equilibrium state is reached, regardless of the starting conditions.

Reflection