Which principle states that a system at equilibrium shifts to oppose the change in conditions such as concentration, temperature, or pressure?

• A. Le Chatelier's Principle
• B. Hess's Law
• C. Avogadro's Law
• D. Boltzmann Distribution

Answer: (A)

Le Chatelier's Principle states that a system at equilibrium shifts to counteract changes in conditions such as concentration, temperature, or pressure. When you add more of a reactant, the equilibrium moves toward more product to consume the added substance; removing a product does the opposite and shifts toward reactants. Temperature effects depend on whether the reaction is exothermic or endothermic: increasing temperature favors the direction that absorbs heat (the endothermic direction), while decreasing temperature favors the exothermic direction. For gas mixtures, increasing pressure shifts the equilibrium toward the side with fewer moles of gas, and decreasing pressure shifts toward the side with more gas. This helps explain how the system adjusts to maintain balance when conditions are changed. The other concepts—Hess's Law deals with enthalpy changes along a reaction path, Avogadro's Law relates volume to amount for gases at fixed temperature and pressure, and Boltzmann distribution describes how molecular energies are populated at a given temperature—do not specify how equilibrium positions respond to perturbations.