Chemistry: H₂ + I₂ ⇌ 2HI — Predicting the Shift

Le Chatelier’s Principle is the compass chemists use to predict how a system at equilibrium responds to disturbance. In essence, it states that if a dynamic equilibrium is subjected to a change in concentration, pressure, or temperature, the system will shift its position to partially counteract that change, re-establishing a new equilibrium state. For the reaction H₂(g) + I₂(g) ⇌ 2HI(g), this principle explains why injecting extra H₂ into a sealed, constant-temperature container doesn’t simply leave the other concentrations untouched—it triggers a cascade of adjustments. The key relationship here is the balance between reactants and products. Adding H₂ increases its concentration, so the system “fights back” by consuming some of that added H₂, which simultaneously draws down I₂ and generates more HI. This shift to the right is not about kinetics (how fast) but about thermodynamics (how far). The equilibrium constant, Kc = [HI]²/([H₂][I₂]), remains fixed at constant temperature, so the system must adjust concentrations to keep this ratio satisfied. Thus, the immediate effect is a decrease in [I₂] and an increase in [HI], while [H₂] settles at a value higher than its original but lower than its post-injection spike. Understanding this predictive logic is fundamental for mastering reactivity, as it applies to industrial processes like the Haber process and to acid–base equilibria alike.