Dynamic equilibrium and Le Chatelier's principle

Dynamic equilibrium and Le Chatelier's principle

A reversible reaction reaches dynamic equilibrium in a closed system when forward and reverse reaction rates are equal. The concentrations remain constant, but particles continue reacting; equilibrium is not a stopped reaction and does not require equal concentrations. Le Chatelier's principle predicts how an equilibrium system responds to a disturbance: it shifts in the direction that partially opposes a change in concentration, pressure for gaseous systems, or temperature. A catalyst changes the time to reach equilibrium by speeding both directions, but does not shift equilibrium position. Temperature is special because heat behaves as a reactant in an endothermic direction and a product in an exothermic direction.

Work it through

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat, increasing pressure favours the side with fewer gas moles, the ammonia side. Increasing temperature favours the endothermic reverse direction because heat is a product in the forward exothermic reaction. State both the shift and the particle-count or energy reason.

Mastery target

Define dynamic equilibrium, predict a shift from a stated disturbance and justify it using concentration, gas-mole count or energy transfer.