If a system at equilibrium is disturbed by a change in conditions, the equilibrium will SHIFT in the direction that OPPOSES the change.
This is a prediction tool โ it allows chemists to predict the effect of changes WITHOUT knowing the detailed kinetics.
Types of changes:
1. Changing CONCENTRATION of a reactant or product.
2. Changing TEMPERATURE.
3. Changing PRESSURE (for gaseous reactions).
The system responds by shifting the equilibrium position LEFT (โ more reactants) or RIGHT (โ more products).
Note: a catalyst DOES NOT shift the equilibrium position โ it speeds up BOTH forward and reverse reactions equally, so equilibrium is reached FASTER but the position is unchanged.
Effect of Concentration and Pressure
CHANGING CONCENTRATION:
INCREASE concentration of REACTANT โ equilibrium shifts RIGHT (โ uses up added reactant, making more product).
INCREASE concentration of PRODUCT โ equilibrium shifts LEFT (โ uses up added product, making more reactant).
REMOVE a product โ equilibrium shifts RIGHT to replace it.
Example: Nโ + 3Hโ โ 2NHโ
Add more Nโ โ equilibrium shifts right โ more NHโ produced.
Remove NHโ โ equilibrium shifts right โ more NHโ produced.
CHANGING PRESSURE (for gaseous reactions):
INCREASE pressure โ equilibrium shifts towards FEWER moles of gas (to reduce pressure).
DECREASE pressure โ equilibrium shifts towards MORE moles of gas (to increase pressure).
Example: Nโ + 3Hโ โ 2NHโ
Left side: 1 + 3 = 4 moles of gas. Right side: 2 moles of gas.
INCREASE pressure โ shifts RIGHT (2 moles) โ more NHโ.
DECREASE pressure โ shifts LEFT (4 moles) โ less NHโ.
Effect of Temperature
CHANGING TEMPERATURE:
INCREASE temperature โ equilibrium shifts in the ENDOTHERMIC direction.
DECREASE temperature โ equilibrium shifts in the EXOTHERMIC direction.
Why: the system absorbs the added heat energy by shifting in the endothermic direction, opposing the temperature increase.
INCREASE temperature โ shifts LEFT (endothermic direction) โ less NHโ.
DECREASE temperature โ shifts RIGHT (exothermic direction) โ more NHโ.
THE HABER PROCESS โ compromise conditions:
High pressure (200 atm) โ favours right (fewer moles of gas) โ more NHโ.
Low temperature โ favours right (exothermic) โ more NHโ BUT reaction too slow.
Compromise temperature ~450ยฐC โ fast enough rate, acceptable yield.
Iron catalyst โ speeds up reaching equilibrium WITHOUT changing position.
This shows the trade-off in industrial chemistry: conditions that maximise yield often slow the rate, requiring a catalyst to compensate.
โ ๏ธ Common Mistake
A catalyst DOES NOT change the equilibrium position โ it only speeds up reaching equilibrium. Equilibrium position is determined by temperature only (not concentration or pressure โ those affect AMOUNTS but not the equilibrium constant). Temperature is the ONLY factor that changes the equilibrium constant itself.
Le Chatelier: equilibrium shifts to OPPOSE any change. Increase reactant conc โ shifts right. Increase pressure โ shifts to fewer gas moles. Increase temperature โ shifts in endothermic direction. Catalyst: faster equilibrium, same position. Haber: high P (more NHโ) + compromise T + Fe catalyst.
๐ฏ Matching Activity โ Le Chatelier's Principle Predictions
For Nโ + 3Hโ โ 2NHโ (forward = exothermic), predict the effect of each change. โ drag the symbols on the right to match the component names on the left.
Equilibrium shifts right โ more NHโ
Drop here
Equilibrium shifts right โ more NHโ
Drop here
Equilibrium shifts left โ less NHโ
Drop here
No change in position
Drop here
Equilibrium shifts right โ more NHโ
Drop here
Increase concentration of Nโ (more reactant added)
Increase temperature (forward is exothermic; reverse is endothermic โ heat drives reverse)
Add iron catalyst โ same equilibrium position, reached faster
Increase pressure (right has fewer moles: 2 vs 4)
Remove NHโ as it forms โ system produces more to replace it
๐ฏ Test Yourself
Question 1 of 2
1. For the reaction 2SOโ(g) + Oโ(g) โ 2SOโ(g), increasing pressure shifts the equilibrium right. Why?
2. The Haber process uses 450ยฐC rather than a lower temperature. Why, given that lower temperature gives more ammonia?
โญ How Well Do You Understand This Topic?
Be honest with yourself โ this helps you know what to revise!
Don't get itGetting thereNailed it!
๐ค Ask MrBadmus
Stuck? Just ask! ๐ฌ
I'll use CFIFA for calculations and flag Higher/Triple content clearly.