The position of equilibrium in a reversible reaction is affected by

Analyze the question: It asks which factor changes the position of equilibrium, meaning the relative amounts of reactants and products at equilibrium.

Concept: Le Chatelier's Principle When a system at equilibrium is disturbed, it shifts in the direction that opposes the change. The factors that shift the position of equilibrium are:

• Concentration of reactants or products
• Temperature
• Pressure (for gaseous systems where the number of moles changes)

Checking each option:

• A. Particle size of the reactants: This affects only the rate of reaction (surface area). It does not change the position of equilibrium. ❌
• B. Vigorous stirring: This only helps mix the reactants and speeds up the approach to equilibrium. The position does not change. ❌
• C. Presence of a catalyst: A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster, but the position stays the same. ❌
• D. Change in concentration of the reactants: Adding more reactant shifts the equilibrium toward the products, and removing reactant shifts it toward the reactants (Le Chatelier's Principle). ✅

Example: For N₂ + 3H₂ ⇌ 2NH₃, increasing the concentration of N₂ or H₂ shifts the equilibrium to the right, giving more NH₃.

Exam tip: Remember that a catalyst, stirring, and particle size affect how fast equilibrium is reached, not where it lies. Only concentration, temperature, and pressure change the position of equilibrium.