Equilibrium Le Chatelier's Principle - if a change in conditions, a stress is imposed on a reversible reaction at equilibrium, the equilibrium will shift in the direction that tends to relieve the stress. For any equilibrium system the reaction can be shifted forward or backward by changing the concentration, pressure, or temperature.

Equilibrium Changes in Concentration Narrated animation on each factor in LeChatlier’s Principle Changes in Concentration Adding more of a substance to a reaction will shift the equilibrium to consume some of the added substance. ex: Adding NH3 will shift the equilibrium to the left to consume the ammonia and produce more nitrogen and hydrogen gas. N2(g) + 3H2(g) 2 NH3(g)

Equilibrium Changes in Concentration N2(g) + 3H2(g) 2 NH3(g) Narrated animation on each factor in LeChatlier’s Principle Changes in Concentration N2(g) + 3H2(g) 2 NH3(g) Removing a substance in a reaction will shift the equilibrium to produce more of the substance to replace what was removed. ex: Removing NH3 will shift the equilibrium to the right to consume (use) the nitrogen & hydrogen gas to produce more ammonia.

Equilibrium Changes in Pressure Narrated animation on each factor in LeChatlier’s Principle Changes in Pressure For some gaseous equilibrium systems, the reaction can be shifted by changing the pressure. When substances in a reaction are gases their volumes are in the same ratio as their coefficients in the equation.

Equilibrium Increasing the pressure forces the reaction toward the smaller volume. ex: Increasing the pressure will shift the equilibrium to produce more N2O4. 2 NO2(g) N2O4(g) 2 moles gas 1 mole gas

Equilibrium ex: The pressure is increased for the reaction below. NH4Cl(s) NH3(g) + HCl(g) moles gas 2 moles of gas This causes the reaction to shift to the left, where there are fewer gas molecules.

Equilibrium ex: The pressure is increased for the reaction below. H2(g) + Cl2(g) 2HCl(g) 2 moles gas 2 moles of gas Equal moles of gas so, no effect on the equilibrium

Equilibrium Changes in Temperature Narrated animation on each factor in LeChatlier’s Principle Changes in Temperature 1) When the temperature of an equilibrium system is increased, and the reaction which is endothermic (+∆H) takes place. If the forward reaction is endothermic, more product is formed. If the reverse reaction is endothermic, less product is formed at the higher temperature.

Equilibrium Increases in Temperature PCl5(g) PCl3(g) + Cl2(g) ∆H = +92.9kJ PCl5(g) + 92.9kJ heat PCl3(g) + Cl2(g) If heat is added reaction will shift If heat is removed, reaction will shift

Equilibrium 2) When the temperature of an equilibrium system is decreased, the reaction which is exothermic (-∆H) takes place. N2(g) + 3H2(g) 2NH3(g) ∆H = -91.8kJ N2(g) + 3H2(g) 2NH3(g) + 91.8kJ heat If heat is removed, reaction will shift If heat is added, reaction will shift

Equilibrium The general rule is that an increase in temperature will favor the reaction that needs the addition of energy. Narrated Animation on LeChatelier’s Principle Part 1 04:14 Narrated animation on each factor in LeChatlier’s Principle Narrated Animation on Collision Theory Part 2 03:22

Equilibrium -If the forward reaction is endothermic (+∆H) the formation of more products is favored -If the forward reaction is exothermic (-∆H) the reverse reaction is favored so, less of the products are formed as the reactants are regenerated