Dynamic Equilibrium

Full Notes:

What Is Dynamic Equilibrium?

A system is in dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.

Dynamic equilibrium can only be reached in a closed system (no matter can enter or leave).

The concentrations of all species remain constant over time, although particles continue to react.

Dynamic refers to the fact that reactions are ongoing and equilibrium because the system appears unchanged at the macroscopic level.

Physical and Chemical Equilibria

Equilibria can involve either physical changes (like phase changes) or chemical reactions.

• In both cases, the forward and reverse processes occur at equal rates.
• Physical equilibria involve no change in chemical identity.
• Chemical equilibria involve making and breaking chemical bonds, forming new substances in each direction.

Example Physical equilibrium: Water Evaporating

Water ⇌ vapour
H₂O(l) ⇌ H₂O(g)

• Water evaporates and condenses at the same rate.
• The amount of liquid and vapour stays constant.

Example Chemical equilibrium: The Haber Process

• Forward reaction: N₂ + H₂ → NH₃
• Reverse reaction: NH₃ decomposes into N₂ and H₂
• At equilibrium, the forward and reverse reactions occur at the same rate, so concentrations remain constant.

Key Characteristics of Equilibrium

• Closed system: No exchange of matter with surroundings.
• Forward and reverse reactions are continuous and occur at equal rates.
• No net change in concentrations of reactants and products.
• Macroscopic properties (e.g. pressure, colour, concentration) remain constant.
• The system is dynamic, not static.
• Equilibrium can be established from either direction (forward or reverse).

Summary

• Dynamic equilibrium means equal reaction rates in a closed system.
• Applies to both physical and chemical systems.
• Observable properties stay unchanged, even though particles keep reacting.