Le Ch â telier’s Principle

1. Le Châtelier’s Principle

2. His principle states: • Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change. • So, what does this really mean?

3. 4 important points to consider • The chemical system MUST be at equilibrium first • Some type of change is involved in a factor that affects equilibrium (we’ll discuss shortly) • The old equilibrium will rearrange itself in response to this change. • Opposite meaning that if something is increased, the change will be for something to decrease. Or the reverse is also true.

4. Modern definition • If a system at equilibrium is subjected to a stress, the equilibrium will shift in attempt to reduce the stress. • The equilibrium position changes but the equilibrium constant does not change UNLESS temperature changes

5. Factors that affect a chem. rxn at equilibrium • Concentration • Temperature • Pressure • Addition of a catalyst http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swf

6. Temperature • Is the reaction exothermic (-ΔH) or endothermic (+ΔH)? • If exo: increase in temp will make rxn proceed to left (reactants) • If endo: increase in temp will make rxn proceed to right (products) • Increasing temperature INCREASES reaction rate • Changing temp will change Kc

7. Concentration • Look at reactants/products • If a [reactant] increases, rxn shifts towards product • If a [product] increases, rxn shifts towards reactants • You can change concentration by • Adding (or removing) a reactant/ product • Adding something that reacts completely with reactant/ product (like something that will cause a precipitate to form)

8. Pressure • Check # of moles of GAS • Pressure changes: • Volume • Decrease in volume increases pressure, cause rxn to proceed towards side with FEWER moles of gas • Decreasing volume increases rxn rate for both forward and reverse rxn • Changing volume does NOT change Kc • Adding inert gas • Changing pressure does NOT change rxn rate • Changing pressure does NOT change Kc

9. Catalyst • Speeds up rate of reaction by lowering Ea • NO EFFECT on equilibrium position • NO EFFECT on Kc

10. Examples For the reaction: N2 +3H2↔2 NH3 • Which way will the equilibrium shift: • If more H2 is added • Some NH3 is removed

11. Examples • For the following reaction: 2 SO2 + O2↔2 SO3 + heat Which way will the equilibrium shift: If the temperature increases? If heat is removed (temp goes down)?

12. Examples For the following reaction: PCl3 + Cl2↔ PCl5 Which way will the equilibrium shift: if a catalyst is added?

13. Haber Process: manufacturing ammonia • N2(g) + 3H2(g) <-> 2NH3(g) • Conditions: • High pressure required (200atm) • High pressure = higher density=more collisions • Low temp required (450C) • Exothermic rxn • Low temp = slower rxn rate, takes longer to reach equilibrium • Finely divided pieces of Iron catalyst • Overall yield: 15%

14. Haber Process Ideal situation: High pressure Intermediate temp Iron catalyst Return unreacted gases to mix w/ fresh

15. Contact Process: production of sulfuric acid • 2SO2(g) + O2(g) <-> 2SO3(g) • Conditions: • High pressure required (2atm) • Low temp required (450C) • Vanadium oxide catalyst used • Overall yield: 99%

16. Haber Process