Chemistry
Chemical equilibrium & Le Chatelier — quick study summary
A reversible reaction reaches equilibrium when forward and reverse rates are equal — concentrations stop changing but reactions don't stop. The equilibrium constant K_eq tells us how far the reaction favours products (K > 1) or reactants (K < 1). Le Chatelier's principle predicts how a system at equilibrium responds to stress: if you push it (change concentration, pressure, or temperature), it shifts to oppose the change and restore equilibrium.
Key points
- Equilibrium ≠ stopped: forward and reverse rates are equal and ongoing
- K_eq = [products] / [reactants], each raised to their coefficient
- Add reactant or remove product → shifts right (toward products)
- Increase pressure → shifts toward fewer gas moles
- For exothermic: heating shifts toward reactants (backward); endothermic the opposite
Practice quiz
Click each question to reveal the answer.
1. At equilibrium, how do the forward and reverse reaction rates compare?
- Forward is faster
- Reverse is faster
- Equal
- Both are zero
Answer: Equal
At equilibrium they're equal — concentrations are constant, but molecules keep reacting in both directions.
2. In an exothermic reaction at equilibrium, what happens if you increase the temperature?
Answer: The equilibrium shifts toward reactants (backward)
Heat is effectively a product in exothermic reactions, so adding heat shifts the equilibrium to consume it — backward.
3. What does a very large K_eq (e.g. 10⁶) tell you about a reaction at equilibrium?
Answer: Products are strongly favoured — reaction essentially goes to completion
K_eq is the ratio of product to reactant concentrations. K = 10⁶ means products vastly outnumber reactants.
Last reviewed: May 2026