Dynamic equilibrium
A reversible reaction reaches equilibrium when the forward and reverse rates are equal. Concentrations then stay constant, but the reactions have not stopped. Both directions keep going at the same speed.
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Analogy
Two escalators, one up and one down, moving the same number of people each minute: the crowd on each floor stays the same.
Example
Start: only colourless N₂O₄Brown NO₂ builds upColour stops changingBoth reactions still run
In practice
A closed bottle of fizzy drink is at equilibrium: CO₂ leaves the liquid and returns at the same rate.
Practice
Flashcards
- What is true at dynamic equilibrium?Forward and reverse rates are equal, so concentrations stay constant.
Questions people ask
Does equilibrium mean the concentrations are equal?
No. At equilibrium the concentrations are constant, not equal. A reaction can sit at equilibrium with almost all products, or with almost all reactants. The equilibrium constant tells you which side is favoured, and it depends only on temperature. For example, K = 0.001 means almost all reactants, and K = 1000 means almost all products.
Can a reaction reach equilibrium in an open container?
Usually not. If a gas escapes or a product is removed, the reaction keeps moving forward and never settles. Equilibrium needs a closed system where nothing enters or leaves. Baking soda in an open flask keeps fizzing because the CO₂ escapes.