Le Chatelier's Principle
Predict how changes in concentration, temperature, and pressure shift a system at equilibrium, and understand how industry uses these principles to maximise yield.
The Principle
Le Chatelier's principle states that if a system at equilibrium is disturbed by a change in conditions, the system will respond by shifting the equilibrium position to partially counteract the change and restore a new equilibrium.
"If a change is imposed on a system at equilibrium, the position of equilibrium will shift in a direction that tends to reduce that change."
-- Henri Louis Le Chatelier, 1884
Remember: The system partially counteracts the disturbance -- it never fully restores the original conditions. Also, a catalyst does NOT shift equilibrium; it only helps equilibrium be reached faster.
Effect of Concentration Changes
Changing the concentration of a reactant or product disturbs the equilibrium. The system shifts to consume the added substance or replenish the removed substance.
Example: N2(g) + 3H2(g) ⇌ 2NH3(g)
Add more N2
Equilibrium shifts right (toward products) to consume the extra N2.
[NH3] increases, [H2] decreases.
Remove NH3
Equilibrium shifts right (toward products) to replace the removed NH3.
[N2] and [H2] decrease.
Keq does not change when concentration changes are made. The system shifts to re-establish the same ratio expressed by Keq.
Effect of Temperature and Pressure
Temperature and pressure changes affect equilibrium differently. Temperature is unique because it is the only factor that changes the value of Keq.
Temperature
Treat heat as a reactant (endothermic) or product (exothermic):
Exothermic (A ⇌ B + heat):
Increase T → shift LEFT (K decreases)
Decrease T → shift RIGHT (K increases)
Endothermic (A + heat ⇌ B):
Increase T → shift RIGHT (K increases)
Decrease T → shift LEFT (K decreases)
Pressure (gases only)
Changing volume/pressure shifts equilibrium toward the side with fewer gas moles:
Increase pressure (decrease volume):
Shifts toward side with fewer moles of gas
Decrease pressure (increase volume):
Shifts toward side with more moles of gas
Equal moles: No shift
Keq does not change with pressure.
Industrial Application: The Haber Process
N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92 kJ mol-1
High pressure (~200 atm): Favours products (4 mol gas → 2 mol gas).
Moderate temperature (~450 °C): A compromise -- low T would favour products but be too slow; high T is fast but favours reactants.
Iron catalyst: Speeds up both forward and reverse rates equally -- does not shift equilibrium but reaches it faster.
Key Vocabulary
Le Chatelier's Principle
When a system at equilibrium is disturbed, it shifts to partially counteract the disturbance and establish a new equilibrium position.
Equilibrium Shift
A change in the relative amounts of reactants and products as the system responds to a disturbance. A shift right favours products; a shift left favours reactants.
Exothermic Reaction
A reaction that releases heat energy to the surroundings (ΔH < 0). Heat can be treated as a product when applying Le Chatelier's principle.
Catalyst
A substance that increases the rate of both forward and reverse reactions equally by lowering the activation energy. It does NOT change the position of equilibrium or the value of K.
Worked Examples
For 2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = -198 kJ mol-1, predict the effect of increasing temperature.
Step 1: The reaction is exothermic (ΔH < 0), so heat is a product.
Step 2: Increasing temperature adds heat. The system shifts to consume heat.
Answer: Equilibrium shifts left (toward reactants). [SO3] decreases, [SO2] and [O2] increase. Keq decreases.
For PCl5(g) ⇌ PCl3(g) + Cl2(g), predict the effect of increasing the total pressure by decreasing volume.
Step 1: Count gas moles: Left = 1, Right = 1 + 1 = 2.
Step 2: Increasing pressure shifts toward fewer gas moles.
Answer: Equilibrium shifts left (toward PCl5) since the left side has fewer gas moles (1 vs 2).
In the contact process: 2SO2(g) + O2(g) ⇌ 2SO3(g), explain why a catalyst is used but does not change the yield.
Step 1: A catalyst (V2O5) speeds up both forward and reverse reactions equally.
Step 2: Since both rates increase by the same factor, the ratio of products to reactants at equilibrium does not change.
Answer: The catalyst allows equilibrium to be reached faster (important for industrial efficiency) but does not change the position of equilibrium or Keq. The yield at equilibrium is the same.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Adding a catalyst to a system at equilibrium will:
Question 2
For the endothermic reaction CaCO3(s) ⇌ CaO(s) + CO2(g), increasing the temperature will:
Question 3
For N2(g) + 3H2(g) ⇌ 2NH3(g), increasing the pressure by decreasing volume will:
Question 4
Which change will increase the value of Keq for an exothermic reaction?
Question 5
For H2(g) + I2(g) ⇌ 2HI(g), increasing the total pressure by decreasing volume will:
Key Concepts Summary
- ●Le Chatelier's principle: A disturbed equilibrium shifts to partially counteract the change.
- ●Concentration: Adding a substance shifts away from it; removing it shifts toward it. K does not change.
- ●Temperature: The only factor that changes K. Increasing T shifts endothermic reactions right and exothermic reactions left.
- ●Pressure: Increasing pressure shifts toward the side with fewer moles of gas. No effect if moles are equal. K does not change.
- ●Catalysts speed up the attainment of equilibrium but do NOT shift it or change K.