AP Syllabus focus: ‘Le Châtelier’s principle predicts how an equilibrium system responds to stresses such as adding/removing a species, changing temperature, changing gas volume/pressure, or diluting a reaction mixture.’
Equilibrium systems are not static: they can be disturbed by external changes.
Le Châtelier’s principle provides a practical, qualitative way to predict how an equilibrium will respond to common laboratory “stresses.”
Le Châtelier’s Principle and “Stresses”
Le Châtelier’s principle: If a system at equilibrium is subjected to a stress, the system shifts in the direction that reduces the effect of the stress, establishing a new equilibrium.
A useful prediction strategy is to identify what was changed, then decide which direction would partially undo that change.
Stress: A change in conditions that affects an equilibrium system (commonly concentration, pressure/volume for gases, temperature, or dilution).
The word shift refers to the net direction the system proceeds as it re-establishes equilibrium; it does not mean the reaction “stops” in one direction.
Shift (of equilibrium): A net change toward products (shift right) or toward reactants (shift left) until equilibrium is re-established.
This figure defines equilibrium “shift” language by explicitly mapping “shift right” to net movement toward products (forward direction) and “shift left” to net movement toward reactants (reverse direction). It supports the qualitative reasoning used throughout Le Châtelier predictions by keeping the terminology consistent. The emphasis on directionality helps link verbal rules to what happens to reactant and product amounts during re-equilibration. Source
Predicting responses to common stresses
Adding or removing a species (concentration changes)
Adding or removing a reactant or product changes how crowded that side of the equilibrium is.
Add reactant: system consumes some of it → shift toward products.
Remove reactant: system replaces it → shift toward reactants.
Add product: system consumes some product → shift toward reactants.
Remove product: system replaces it → shift toward products.
Practical interpretation: the equilibrium mixture will adjust so that the species you added tends to decrease (relative to immediately after the addition), and the species you removed tends to increase.
Changing gas volume or pressure (gaseous equilibria)
Volume/pressure stresses matter when gases are present, because changing volume changes partial pressures.
Decrease volume (compression) or increase pressure:
system shifts to the side with fewer moles of gas (reduces pressure).
Increase volume (expansion) or decrease pressure:
system shifts to the side with more moles of gas (raises pressure).
Important special case:
If the number of moles of gas is the same on both sides, changing volume/pressure causes no shift (though the container conditions have changed, the equilibrium composition does not preferentially move left or right).
Dilution (adding solvent to an aqueous equilibrium)
Dilution lowers the concentrations of all dissolved species at once, so the equilibrium responds by favoring the side that rebuilds dissolved particle concentration more effectively.
Dilute an aqueous equilibrium:
system shifts toward the side with the greater number of aqueous (dissolved) particles.
This is easiest to apply by counting aqueous species in the balanced equation (ignoring pure liquids and solids as they do not represent changing concentrations in the same way).
Changing temperature
Temperature is a stress because heat behaves like a reactant or product depending on whether the forward reaction is endothermic or exothermic.
This diagram treats a temperature increase as adding “heat” to the equilibrium system, then shows how the equilibrium shifts to consume that added heat. For an endothermic forward reaction, added heat drives the equilibrium toward products; for an exothermic forward reaction, added heat drives the equilibrium toward reactants. It visually reinforces the idea that heat can be written on the reactant or product side when applying Le Châtelier’s principle. Source
= treated as a reactant; increasing favours products
= treated as a product; increasing favours reactants
Apply the “add/remove” logic to heat:
Increase temperature:
favours the endothermic direction (consumes heat).
Decrease temperature:
favours the exothermic direction (produces heat).
Unlike concentration or pressure changes, a temperature change can alter the preferred equilibrium mixture because it changes the system’s tendency to absorb or release heat.
FAQ
Immediately after a stress, one direction becomes faster, causing a net change.
As the system re-equilibrates, the rates become equal again.
Count the number of aqueous particles on each side using the balanced equation.
Treat dissociated ions as separate aqueous particles if they appear as such in the equilibrium.
No. The system only shifts enough to reach a new equilibrium.
Both reactants and products remain present at equilibrium.
Pressure/volume shifts rely on changing gas partial pressures.
If no gaseous species are involved, volume/pressure changes typically cause no equilibrium shift.
If “heat” is written on the reactant side, the forward direction is endothermic.
If “heat” is written on the product side, the forward direction is exothermic.
Practice Questions
(2 marks) For the equilibrium , predict the direction of shift when the volume is decreased at constant temperature.
States shift to the left / towards (1)
Correct reasoning: fewer moles of gas on left (1)
(5 marks) Consider an equilibrium mixture for where the forward reaction is exothermic. Predict the equilibrium shift (left/right/no shift) for each stress and justify: (a) Add more . (b) Dilute with water. (c) Increase temperature.
(a) Shift right (1); justification: added reactant is consumed (1)
(b) Shift left (1); justification: dilution favours side with more aqueous particles (reactant side has 2 vs 1) (1)
(c) Shift left (1); justification: increasing favours endothermic direction, so opposes exothermic forward reaction (1)
