Le Chatelier's principle
When an equilibrium is disturbed, it shifts to partly counteract the change. This simple principle predicts how to push reactions toward the products you want.
The principle
Le Chatelier's principle: if a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partly offsets the change. It is a qualitative shortcut to the same answers \(Q\) and \(K\) give.
Changing concentration
Adding a reactant shifts the equilibrium toward products to use some of it up; removing a product also shifts it forward. Industry often removes products continuously to drive reactions toward completion. Adding product shifts the equilibrium back toward reactants.
Changing pressure
For gas reactions, increasing pressure (by reducing volume) shifts the equilibrium toward the side with fewer gas molecules. In \(\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3\), 4 moles of gas become 2, so high pressure favours ammonia. Adding an inert gas at constant volume has no effect.
Changing temperature
Temperature is the only change that alters \(K\). Treat heat as a reactant for endothermic reactions and as a product for exothermic ones. Heating an exothermic reaction shifts it toward reactants and lowers \(K\); heating an endothermic reaction shifts it toward products and raises \(K\).
Catalysts and equilibrium
A catalyst speeds up the forward and reverse reactions equally. It helps a system reach equilibrium faster but does not change the equilibrium position or \(K\).
Worked example: the Haber process
Making ammonia is exothermic and reduces gas moles. High pressure favours ammonia, and low temperature favours it at equilibrium, but low temperature makes the reaction too slow. Industry compromises: around 400-450 \(^\circ\)C, about 200 atm, an iron catalyst, and removal of ammonia as it forms.
Equilibrium in the body
Haemoglobin binds oxygen in the lungs, where oxygen concentration is high, and releases it in tissues, where it is low: Le Chatelier in action. At high altitude, lower oxygen pressure shifts the equilibrium toward release, and over weeks the body adapts by producing more haemoglobin.
Common mistakes
Thinking a catalyst shifts equilibrium. Thinking concentration or pressure changes alter \(K\); only temperature does. Forgetting to count only gas moles when predicting pressure effects.
Key takeaways
Disturbed equilibria shift to partly offset the change. Adding reactant or removing product shifts toward products. Higher pressure favours fewer gas moles. Temperature changes \(K\): heating favours the endothermic direction. Catalysts speed equilibrium but do not move it.
Flashcard
Equilibrium shifts to offset a disturbance
Check your understanding
For the exothermic reaction N2 + 3H2 <=> 2NH3, which change increases the yield of NH3 at equilibrium?