Le Châtelier Principle (Gas)
Le Châtelier's principle states that if a system at equilibrium is disturbed, it shifts to partially counteract the disturbance. This simulator uses the gas-phase Haber equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g) as a familiar textbook example: the forward direction is exothermic, so lowering temperature (in this Van't Hoff sketch) increases the equilibrium constant K_c; the reaction reduces the number of gas moles, so decreasing volume at constant temperature raises partial pressures in a way that increases Q relative to the previous state and favors additional conversion to ammonia. Students adjust temperature and volume with sliders, add nitrogen, remove ammonia, or compress and expand the container. The panel shows Q_c in the same grouping as K_c(T)—proportional to n_NH₃²V²/(n_N₂ n_H₂³)—and horizontal bars track mole amounts as the composition relaxes smoothly toward the equilibrium extent for the current T and V. The model is intentionally schematic: K(T) follows a simple integrated Van't Hoff form with a pedagogical ΔH, activities are replaced by mole numbers, non-ideal gas behavior and catalyst kinetics are omitted, and the relaxation is a first-order numerical blend rather than a full rate law.
Who it's for: High school and introductory college chemistry students learning equilibrium constants, reaction quotients, and qualitative Le Châtelier reasoning before rigorous thermodynamics or industrial reactor design.
Key terms
- Le Châtelier principle
- Chemical equilibrium
- Reaction quotient (Q)
- Equilibrium constant (K)
- Van't Hoff equation
- Haber process
- Gas stoichiometry
- Pressure and volume effects
How it works
Qualitative Le Châtelier principle for the gas reaction N₂ + 3H₂ ⇌ 2NH₃ with an exothermic forward step. Sliders change temperature and volume; buttons add nitrogen, remove ammonia, or change pressure via volume. The simulator compares the reaction quotient Q_c in the same form as K_c(T) and relaxes mole amounts toward the equilibrium extent at the current T and V.
Key equations
Frequently asked questions
- Why does compressing the volume favor more NH₃ here?
- At fixed temperature and amounts, shrinking V increases concentrations as n/V. For this stoichiometry, the reaction quotient Q_c scales with V² in the numerator relative to the denominator grouping used here, so compression tends to push Q toward K when the system was not yet at equilibrium, and the equilibrium position itself (for an idealized K_c independent of P) shifts toward the side with fewer gas moles to reduce the pressure increase—here, toward NH₃.
- Is K_c(T) here quantitatively accurate for real ammonia synthesis?
- No. The curve is a teaching sketch with a single exothermic ΔH and a reference K chosen for on-screen behavior. Industrial synthesis uses high pressure, recycling, catalysts, and non-ideal mixtures; quantitative work requires experimentally fitted equilibria and activity models.
- What does the smooth bar motion represent?
- It is a visual relaxation toward the equilibrium extent that solves Q_c = K_c(T) for the current mole vector, not a mechanistic rate law. It helps separate the idea of a driving force (Q versus K) from the speed of approach, which would need forward and reverse rate constants.
- Why can I add N₂ without adding H₂?
- To illustrate a concentration stress that breaks stoichiometric proportions. The solver still moves along the reaction coordinate defined by the single balanced equation until Q matches K at the current T and V, which is how textbooks often discuss adding an excess reactant.
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