Le Chatelier's Principle — How Equilibrium Shifts

Reach for Le Chatelier's principle whenever a system is already at equilibrium and you need to predict which way it shifts after a change in concentration, pressure, volume, or temperature. The disturbed system tends to move in the direction that partly opposes the change. It is a direction tool, not a calculator: it tells you left or right, not the exact new amounts. At equilibrium the forward and reverse reactions still run, just at equal rates, so composition holds steady until conditions change.

The procedure, step by step

Apply the same four steps to every disturbance:

1. Identify the disturbance. Decide whether the change affects concentration, gas pressure or volume, or temperature. Name it before predicting anything.
2. Compare both sides. Ask which side would partly oppose that change under the reaction conditions.
3. Predict the shift. State whether equilibrium moves toward reactants or products, and say the condition out loud.
4. Separate shift from speed. Remember that a catalyst changes how fast equilibrium is reached, not where the equilibrium position sits.

Each disturbance type has its own rule inside step 2. Add a reactant and the system uses some up, shifting toward products; remove a product and the system replaces some, also shifting toward products. For gases, pressure and volume changes matter only when the two sides differ in total moles of gas: decreasing volume raises pressure, so equilibrium shifts toward the side with fewer gas moles, and equal moles means no shift. Temperature is the special case, because for an exothermic forward reaction heat behaves like a product and for an endothermic one it behaves like a reactant, so changing temperature can change which side is favored.

Worked example: compressing the Haber equilibrium

Run all four steps on

with the temperature constant and the container compressed.

1. Identify: compressing lowers volume and raises pressure, a pressure/volume disturbance on a gas equilibrium.
2. Compare: the left side has $1 + 3 = 4$ moles of gas, the right side has $2$, so the sides differ and the rule applies.
3. Predict: shifting right moves toward fewer gas particles, partly relieving the pressure increase, so equilibrium shifts toward ammonia, $NH_3$.
4. Separate shift from speed: a catalyst would speed the approach but would not change this direction.

This is the core pattern for pressure questions: confirm gases are involved, count total gas moles on each side, then decide.

Where each step tends to trip you up, and how to check

• At step 1, watch the temperature case. Treating temperature like just another concentration change misses that temperature alters the equilibrium constant itself, while concentration and pressure changes only move the position. Self-check: did the disturbance change $T$? If so, the constant can change.
• At step 2, the pressure shortcut only applies to gas equilibria with unequal total moles of gas. Self-check: are both sides gases, and are the gas moles actually different?
• At step 3, do not confuse equilibrium with equal amounts. Equilibrium means equal forward and reverse rates, not equal concentrations.
• At step 4, do not credit a catalyst with moving the position. A faster response is not a different equilibrium position.

Le Chatelier's principle gives the direction of the shift, not the final concentrations, pressures, or yield. For those you need an equilibrium expression or an ICE table. It is used in gas-phase reactions, acid-base equilibria, solubility problems, and industrial process design, and is most useful for a fast qualitative prediction or for ruling out wrong answers before a full calculation.

Run the steps on a new reaction

Apply the procedure to

with the volume decreased at constant temperature. Count the gas moles first: the left side has $2 + 1 = 3$, the right side has $2$, so decreasing the volume shifts equilibrium toward the side with fewer gas moles, the $SO_3$ side. For contrast, try a reaction where both sides have equal gas moles and watch step 2 stop the prediction cold, since the pressure shortcut no longer applies.