Le Chatelier's Principle Calculator

Le Chatelier’s Principle states that when a system at equilibrium is disturbed, it shifts to partially counteract that disturbance. The word “partially” is doing real work in that sentence. The equilibrium never fully undoes the change; it moves in the direction that reduces the stress and then stops. Double the concentration of a reactant and you do not get the original concentration back, you get something between the two.

Concentration changes are the most intuitive. Add more reactant and the system consumes some of it by shifting toward products. Remove product and the system makes more of it, also shifting forward. The equilibrium constant K does not change when you adjust concentrations. Only the position of equilibrium shifts.

Temperature is different: it actually changes K. For an endothermic reaction (heat is a reactant), raising temperature favors the forward reaction, increasing K. For an exothermic reaction, raising temperature shifts the equilibrium backward, decreasing K.

Pressure changes only affect equilibria that involve gases with unequal total moles on each side. Increasing pressure favors the side with fewer gas molecules. If both sides have the same number of gas moles, pressure has no effect on the equilibrium position.

A catalyst speeds up both the forward and reverse reactions equally. It lowers the activation energy but does not change the equilibrium constant or the equilibrium position. The system reaches equilibrium faster, but arrives at the same concentrations.

Adding an inert gas at constant volume does not affect the partial pressures of the reacting gases, so nothing shifts. The same argon added at constant pressure does shift the equilibrium, because the vessel has to expand and every partial pressure falls. Constant volume is the qualifier that decides the answer, and exam questions leave it out on purpose.

The principle applies to all reversible equilibria: acid-base, solubility, redox, and biochemical reactions like hemoglobin-oxygen binding.

Where the principle quietly misleads

Le Chatelier is a rule of thumb, not a law, and it has known counterexamples. Add a reactant to a gas-phase equilibrium at constant volume and the shift usually goes the way the rule predicts. Do it in some multi-mole systems and the fraction converted can move the other way, even though the absolute amount of product rises. If you want the answer that is always right, compare Q against K with the reaction quotient calculator rather than reasoning about stress.

The other thing worth holding onto: only temperature changes K. Everything else on the list below moves Q and lets the system chase K back down. That single distinction resolves most of the confusion this topic generates.