Chem 3 Chemical Equilibrium Lecture Notes Part 3 Le Chatelier’s

Chem 3 Chemical Equilibrium Lecture Notes Part 3 LE CHATELIER’S PRINCIPLE & GIBB’S FREE ENERGY (17.6)

c!e-γoT 2010

Demo: Chromate-Dichromate Equilibrium (ADOBCY) 2CrO42- (aq) +2H+ (aq)  Cr2O72- (aq) + H2O (l) 2CrO42- (aq) + H2O (aq)  Cr2O72- (aq) + 2OH- (aq) Stress: Acidity/Basicity Observation: Color Change Le Chatelier’s Principle: When a system at equilibrium is subjected to a stress, the system will adjust so as to relieve the stress

These stresses or factors affecting chemical equilibrium could be:      

Change in concentration of the species Change in pressure Change in volume Change in temperature Effect of catalysis Effect of inert gas

Example: Consider the hypothetical reaction: 2A(g) + B(g)  C(g) + D(g) + heat 1. Adding more A - direction: →, ↑ [A], ↓ [B], ↑[C], ↑ [D], KEQ: no change 2. Removal of B – direction: ←, ↑ [A], ↓ [B], ↓ [C], ↓ [D], KEQ: no change 3. Heating the reaction vessel: direction: ←, ↑ [A], ↑ [B], ↓ [C], ↓ [D], KEQ: ↓ 4. What will be the effect of a + catalyst, - catalyst? No effect 5. Increasing the pressure: direction: →, ↓ [A], ↓ [B], ↑[C], ↑ [D], KEQ: no change 6. Increasing the volume of the reaction vessel: direction: ←, ↑ [A], ↑ [B], ↓ [C], ↓ [D], KEQ: no change 7. Adding inert species X into the vessel will cause which effect on the equilibrium system? ↑ Ptotal, ↓ Χ ∆G = ∆H - T∆S ∆Go = -RTlnKEQ Case 1: Keq > 1, ∆G <0 therefore spontaneous Case 2: Keq < 1, ∆G >0 therefore nonspontaneous Case 3: Keq = 1, ∆G =0 therefore equilibrium