Le Châtelier's principle

Meaning: A system at equilibrium that is disturbed shifts in the direction that partly offsets the disturbance until a new equilibrium forms. Adding reactant or removing product shifts it toward products; the reverse shifts it toward reactants. Decreasing volume (raising pressure) favors the side with fewer moles of gas. Raising temperature favors the endothermic direction. The reasoning: a disturbance makes Q differ from K, and the reaction proceeds until Q = K again.

Equation: Q = [products]/[reactants], each concentration (mol/L) raised to its coefficient. If Q < K the reaction shifts right; if Q > K it shifts left. Q and K are unitless at MCAT level.

On the MCAT: Passages give a reaction, such as N₂ + 3H₂ ⇌ 2NH₃ (exothermic), and ask how yield changes with added gas, compressed volume or heating. It also appears in buffers, the common ion effect on solubility, and hemoglobin-oxygen binding.

Common confusion: Only a temperature change alters K; concentration and pressure changes shift the position but leave K unchanged. A catalyst speeds both directions and does not shift the equilibrium. Adding an inert gas at constant volume typically does not shift it. The shift is partial, so the new equilibrium does not fully undo the change.