About Le Chatelier's Principle
This simulation models a reversible chemical equilibrium and shows how it responds when you disturb it. You pick a reaction such as the Haber process (N₂ + 3H₂ ⇌ 2NH₃), then the page computes the reaction quotient Q from the current concentrations and compares it to the equilibrium constant Kc. An iterative relaxation routine nudges the concentrations of A, B and C until Q matches Kc, illustrating how the balance is restored after each perturbation.
The sliders add or remove reactant A and B, add product C, raise temperature from 200 to 600 K, and scale the pressure up to four-fold. Temperature changes Kc through a simplified van't Hoff relationship using each reaction's enthalpy ΔH, while pressure favours the side with fewer gas moles. Bar charts track concentrations and a Q-versus-Kc track shows the predicted shift direction. The Haber process and contact process for sulfuric acid both rely directly on these ideas to maximise industrial yield.
Frequently Asked Questions
What is Le Chatelier's principle?
It states that if a system at chemical equilibrium is disturbed by a change in concentration, temperature or pressure, the equilibrium shifts in the direction that partially opposes that change. The simulation demonstrates this by letting you apply each kind of disturbance and watch the concentrations re-settle.
How does the simulation decide which way the reaction shifts?
It calculates the reaction quotient Q and compares it with the equilibrium constant Kc. If Q is less than Kc the system shifts right toward more products, and if Q is greater than Kc it shifts left toward more reactants. An iterative routine then adjusts the concentrations step by step until Q equals Kc again.
What do the perturbation sliders actually do?
The Add Reactant A, Add Reactant B and Add Product C sliders push a species concentration up or down by up to one molar, the Temperature slider sets T between 200 and 600 K, and the Pressure slider multiplies pressure from 0.5 to 4 times. Each change triggers a recalculation of Kc and a fresh relaxation to the new equilibrium.
What is the key equation behind the model?
The reaction quotient is Q = [C]^c / ([A]^a [B]^b), using each reaction's stoichiometric coefficients. The system is at equilibrium when Q equals Kc. Temperature dependence uses a simplified van't Hoff form, Kc(T) = Kc0 × exp(−ΔH/R × (1/T − 1/298)), with R = 8.314 J per mole per kelvin.
Why does temperature change the equilibrium constant?
Heat behaves like a reactant or product. For an exothermic reaction such as the Haber process (ΔH = −92 kJ/mol), raising temperature lowers Kc and shifts the balance toward reactants. For an endothermic reaction, raising temperature increases Kc and favours products. The simulation applies this through the van't Hoff term for each preset.
How does pressure affect a gaseous equilibrium?
Increasing pressure favours the side of the reaction with fewer moles of gas. In the Haber process four moles of reactant gas become two moles of ammonia, so higher pressure pushes the equilibrium toward products. The model captures this by scaling the effective constant with the change in gas moles raised against the pressure factor.
Why does the shift only partially counteract the disturbance?
The system relieves part of the imposed change but never fully reverses it, otherwise no net change would ever occur. If you add reactant, some of it is consumed as the reaction shifts forward, yet its concentration still ends up higher than before. The new equilibrium restores the same Kc but with different individual concentrations.
Which reactions can I choose?
Four presets are available: the Haber ammonia synthesis, the contact-process step 2SO₂ + O₂ ⇌ 2SO₃, the dissolution of carbon dioxide CO₂ + H₂O ⇌ H₂CO₃, and the iron thiocyanate equilibrium Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺. Each carries its own equilibrium constant, enthalpy and stoichiometry.
Is the model quantitatively accurate?
It is a teaching model rather than a rigorous thermodynamic engine. The Kc values, enthalpies and the van't Hoff temperature dependence are realistic, but concentrations are simplified to a single representative value per species and the relaxation is numerical. It captures the correct qualitative trends and directions reliably, which is the main learning goal.
What is a real-world application of these ideas?
The Haber process for ammonia, the basis of modern fertiliser production, uses high pressure and a carefully chosen temperature to balance yield against reaction rate exactly as Le Chatelier's principle predicts. The contact process for sulfuric acid and many biological and geochemical equilibria follow the same logic of shifting to oppose imposed change.