Le Chatelier’s Principle says that when you disturb a system at equilibrium, the reaction shifts in whichever direction relieves that disturbance. On the AP Chemistry exam the stress is almost always one of three things: a change in concentration, a change in pressure or volume, or a change in temperature. Only a temperature change actually changes the value of the equilibrium constant K. The other two shift the position of equilibrium while leaving K alone.
What is Le Chatelier’s Principle?
Le Chatelier’s Principle says that if you disturb a system at equilibrium, the reaction will shift in whichever direction relieves that disturbance and moves the system back toward balance. That is the whole principle.
The hard part is not the idea. It is correctly identifying what kind of stress was applied and which direction actually relieves it. That is what the rest of this post walks through. If equilibrium itself is still shaky, start with Introduction to Equilibrium first.
What counts as a stress on an equilibrium?
On the AP exam, a stress is almost always one of three things:
- A change in the concentration of a reactant or product
- A change in pressure or volume, which only matters for gases
- A change in temperature
Each one gets analyzed differently, and mixing them up is the most common source of wrong answers on this topic. There is also a fourth thing students often mistake for a stress, and it is worth knowing early: adding a catalyst does not shift equilibrium at all. It speeds up the forward and reverse reactions equally, so the system reaches the same equilibrium faster.
How do concentration changes shift equilibrium?
If you add more of a reactant, the equilibrium shifts right, toward products, to consume some of that extra reactant. If you remove a product as it forms, the equilibrium also shifts right to replace it.
The system is always trying to partially undo whatever you just did to it. Not fully reverse it, just soften the effect. That word partially matters, because a common exam trap is expecting the system to return to its exact original concentrations. It never does.
Note that K does not change here. Only the position of the equilibrium moves.
How do pressure and volume changes shift equilibrium?
This only matters for reactions involving gases, and it only matters if the number of gas molecules differs between the reactant and product side.
If you decrease the volume of the container, which increases pressure, the equilibrium shifts toward whichever side has fewer moles of gas, since that side takes up less space and partially relieves the added pressure. Increase the volume and it shifts the other way.
If the number of gas moles is equal on both sides, changing pressure or volume has no effect on the equilibrium position at all. That detail catches a lot of students off guard on the exam, so count the gas moles on each side before you answer anything.
One more trap: adding an inert gas at constant volume raises the total pressure but does not change any partial pressure, so it does not shift the equilibrium either.
How do temperature changes shift equilibrium?
Temperature is the one stress that actually changes the value of the equilibrium constant K, not just the position of the equilibrium. This is the single most testable distinction in the topic.
The trick is to treat heat as if it were a reactant or a product:
- Exothermic reaction (heat released, so heat behaves like a product): raising the temperature shifts equilibrium left, back toward reactants, and K decreases.
- Endothermic reaction (heat absorbed, so heat behaves like a reactant): raising the temperature shifts equilibrium right, toward products, and K increases.
This is the one case where you cannot just say the shift relieves the stress in a simple sense. You have to think in terms of heat as part of the equation.
A real world example: hemoglobin and oxygen
Hemoglobin binds oxygen in your lungs and releases it in your tissues, and that whole process is Le Chatelier’s Principle in action.
In your lungs, oxygen concentration is high, which shifts the equilibrium toward the bound hemoglobin-oxygen complex. In your tissues, oxygen concentration is low because it is being used up, which shifts the equilibrium the other way and releases oxygen exactly where your body needs it.
This is also why carbon monoxide poisoning is so dangerous. Carbon monoxide binds hemoglobin far more strongly than oxygen does, which shifts the whole system in a direction that starves your tissues of oxygen even when there is plenty of oxygen in the air.
How does Le Chatelier’s Principle show up on the AP exam?
Expect a free response question that gives you a specific reaction, applies one or more stresses, and asks you to predict the shift and justify it in words, not just state a direction.
Graders want three things: name the stress, explain which side it favors and why, and connect it back to the specific reaction you were given. A generic restatement of the principle does not earn the point. Neither does a correct direction with no reasoning attached.
The four part Le Chatelier sequence in the course works through exactly these question types, starting with LeChatelier’s Principles Part 1.
Frequently asked questions
What is Le Chatelier’s Principle in simple terms?
If you disturb a system that is at equilibrium, the reaction shifts in the direction that partially undoes the disturbance. Add a reactant and it shifts toward products. Remove a product and it shifts toward products. The system always pushes back against whatever you did to it.
Does temperature change the equilibrium constant?
Yes, and it is the only stress that does. Concentration, pressure and volume changes move the position of the equilibrium but leave K the same. Raising the temperature increases K for an endothermic reaction and decreases K for an exothermic one.
Does a catalyst shift equilibrium?
No. A catalyst speeds up the forward and reverse reactions by the same factor, so the system reaches the same equilibrium position faster. It changes the rate, not the outcome, and it does not change K.
When does a pressure change not affect equilibrium?
When both sides of the equation have the same number of moles of gas, and when the pressure increase comes from adding an inert gas at constant volume. In both cases the partial pressures of the reacting species are unchanged, so nothing shifts.
Which way does equilibrium shift if you add a product?
Left, toward the reactants, because the system consumes some of the added product to partially relieve the increase. Removing a product does the opposite and shifts the equilibrium right.
Equilibrium consistently ranks among the topics students find hardest, and Le Chatelier’s Principle is where it starts. It is covered in depth, with worked examples like the ones above, in the full AP and Honors Chemistry course.