Which principle states that a system at equilibrium opposes a change in conditions?

• A. Arrhenius Law
• B. Le Chatelier's Principle
• C. Boyle's Law
• D. Hess's Law

Answer: (B)

Le Chatelier's Principle says that a system at equilibrium will respond to a disturbance by shifting the balance in a way that counteracts the change. If you tweak conditions like concentration, pressure, or temperature, the system moves to reduce that disturbance. For example, adding more of a reactant pushes the reaction toward the products to use up the added substance; increasing pressure favors the side with fewer gas moles; raising temperature drives the system toward the direction that absorbs heat (the endothermic direction), while lowering temperature favors the exothermic direction. This idea captures why the system “opposes” changes.

The other ideas describe different aspects of chemistry. Arrhenius Law deals with how rate constants depend on temperature and how acids dissociate; Boyle's Law relates pressure and volume for gases at a constant temperature; Hess's Law concerns the total enthalpy change of a reaction by summing steps. These are useful concepts, but they don’t describe how a system at equilibrium responds to changes in conditions the way Le Chatelier’s Principle does.