What you'll be able to do: Predict the direction an equilibrium shifts when concentration, volume, pressure or temperature is changed, and say whether K changes.
Industrial chemists do not accept the yield a reaction happens to give. They change the conditions to push it. Le Chatelier's principle is the reasoning behind those choices.
These are recommended, not required. You can start this lesson at any time.
If a system at equilibrium is disturbed, it shifts in the direction that partially counteracts the disturbance. Partially matters: the system never fully undoes the change, it only settles at a new position where Q has returned to K.
Adding a reactant makes Q too small, so the system shifts right to consume it. Removing a product also lowers Q and shifts the system right. This is why continuously removing a product, for example condensing out water, drives a reaction to high yield.
Compressing a gaseous system raises every partial pressure, and the system responds by shifting toward whichever side has fewer moles of gas. Expanding does the opposite. If both sides have equal moles of gas, changing the volume causes no shift at all.
Compression shifts toward fewer moles of gas
Adding argon at constant volume raises the total pressure but leaves every partial pressure and every concentration unchanged. Q is untouched, so nothing shifts. This trips up more students than any other case.
Write heat as a product for an exothermic reaction and as a reactant for an endothermic one, then treat a temperature change like adding or removing that term. Heating an exothermic reaction shifts it left and lowers K; cooling it shifts right and raises K. This is the only stress that changes the value of K.
A + B ⇌ C + heat (exothermic)
N₂ + 3 H₂ ⇌ 2 NH₃ is exothermic with four moles of gas becoming two. High pressure and low temperature both favor ammonia, but low temperature makes the reaction unbearably slow, so industry compromises at about 450 C with a catalyst and very high pressure.
At constant volume and temperature, adding an inert gas changes total pressure but not the reacting gases' partial pressures, so their Q remains unchanged. At constant total pressure, adding inert gas expands the container and dilutes each reacting gas; equilibrium can shift toward the side with more gas particles. K changes only with temperature.
A system at equilibrium shifts to partially relieve an applied stress.
Adding a species shifts away from it; removing a species shifts toward it.
Reducing the volume shifts the equilibrium toward the side with fewer moles of gas.
No partial pressure changes, so there is no shift.
The only stress that changes the value of K.
reactants ⇌ products + heat
reactants + heat ⇌ products
For N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g), ΔH = -92 kJ. Predict the effect of (a) adding H₂, (b) decreasing the volume, (c) raising the temperature.
Try it first: For each change, decide whether Q rises, falls, or stays the same.
0 of 3 steps revealed.
Argon is added to the equilibrium N₂O₄(g) ⇌ 2 NO₂(g) at constant volume. What happens?
Try it first: Ask whether any partial pressure in the Q expression changed.
0 of 3 steps revealed.
Cooling a flask of N₂O₄/NO₂ turns the brown color pale. Is the forward reaction N₂O₄ → 2 NO₂ endothermic or exothermic?
Try it first: Decide which species is brown and which way the system moved.
0 of 3 steps revealed.
For N₂O₄(g) ⇌ 2 NO₂(g), compare adding Ar at fixed volume versus fixed total pressure at constant T.
Try it first: Count one gas mole on the left and two on the right.
0 of 3 steps revealed.
Why it's wrong: Only temperature changes K; every other stress moves the system to a new position with the same K.
Check instead: Ask whether the temperature changed.
Why it's wrong: No partial pressure in the expression changes, so Q is unchanged.
Check instead: Test whether Q moved.
Why it's wrong: Pure solids do not appear in the expression.
Check instead: Check whether the species is in the K expression at all.
Why it's wrong: It speeds both directions equally.
Check instead: A catalyst changes only the time taken.
Why it's wrong: Volume changes act on gases.
Check instead: Count only species labelled (g).
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Le Chatelier's principle states that a system at equilibrium responds to a stress by shifting in the direction that partly relieves it. Adding a species shifts the system away from that species and removing it shifts the system toward it. Compressing a gaseous system shifts it toward the side with fewer moles of gas, while adding an inert gas at constant volume changes nothing because no partial pressure changes. Temperature is the one stress that changes K itself: treat heat as a product for an exothermic reaction and as a reactant for an endothermic one. A catalyst changes neither K nor the position of equilibrium.
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