Chemistry · General chemistry II · Concept
Le Châtelier’s principle and equilibrium shifts
Le Châtelier’s principle predicts the effect of a disturbance on a system at equilibrium: the system shifts in the direction that partly counteracts the change. Adding a reactant or removing a product drives the reaction forward, and compressing a gas mixture favors the side with fewer moles of gas. Both work through the reaction quotient: the change makes Q differ from K, and the reaction runs until Q = K again. A change in temperature is different, because it changes K itself: heating favors the endothermic direction.
Why the shift happens
At equilibrium Q = K. A change in concentration or pressure changes Q but not K, so the system is no longer at equilibrium. It reacts in the direction that brings Q back to K: forward if Q < K, in reverse if Q > K.
Adding or removing a substance
Adding a reactant or removing a product lowers Q below K, so the reaction runs forward; adding a product or removing a reactant does the opposite. A pure solid or liquid does not appear in Q, so adding more of it causes no shift.
Changing the volume
Halving the volume of a gas mixture doubles every concentration. The side with more moles of gas is raised to a higher power in Q, so the equilibrium shifts toward the side with fewer moles of gas. With equal moles on each side there is no shift, and adding an inert gas at constant volume changes no concentration and causes none.
Changing the temperature
Temperature is the one change that alters K. Treat heat as a product of an exothermic reaction and as a reactant of an endothermic one: heating shifts an exothermic reaction toward reactants, so K falls, and an endothermic one toward products, so K rises.
Catalysts
A catalyst speeds up the forward and reverse reactions equally. The system reaches equilibrium sooner, but K and the equilibrium composition are unchanged.
Common mistakes
- Thinking the shift cancels the change completely: it only partly undoes it.
- Expecting added pure solid or liquid to shift the equilibrium.
- Believing that a pressure change or a catalyst changes K: only temperature does.
- Predicting a shift when an inert gas is added at constant volume.
Key terms
- Le Châtelier’s principle
- When an equilibrium is disturbed, by adding a substance or changing pressure or temperature, it shifts in the direction that partly undoes the change.
- Equilibrium constant
- K, the ratio of products to reactants at equilibrium, each raised to its coefficient, for a given equation and temperature. Kc uses concentrations and Kp uses partial pressures.
- Reaction quotient
- Q, calculated like K but with the current concentrations. If Q < K the reaction goes forward, if Q > K it goes in reverse, and if Q = K it is at equilibrium.
- Endothermic reaction
- A reaction that absorbs heat from its surroundings (ΔH > 0).
- Exothermic reaction
- A reaction that releases heat to its surroundings (ΔH < 0).
- Catalyst
- A substance that speeds up a reaction by giving it a lower-energy pathway and is not used up. It does not change the equilibrium constant or where equilibrium lies.
Work through an example
For H₂(g) + I₂(g) ⇌ 2HI(g), Kc = 50.0 at a certain temperature. An equilibrium mixture has [H₂] = 0.100 M, [I₂] = 0.200 M and [HI] = 1.00 M. H₂ is added until [H₂] = 0.300 M. Which way does the reaction shift, and what are the new equilibrium concentrations?
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