Lesson 3

CONCEPT CHECK! A 1.00 mol sample of N2O4(g) is placed in a 10.0 L vessel and allowed to reach equilibrium according to the equation: N2O4(g) 2NO2(g) K = 4.00 × 10-4 Calculate the equilibrium concentrations of: N2O4(g) and NO2(g). Concentration of N2O4 = 0.097 M Concentration of NO2 = 6.22 × 10-3 M Copyright © Cengage Learning. All rights reserved

If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change. Copyright © Cengage Learning. All rights reserved

Le Chatelier’s Principle If a stress is applied to a system at equilibrium, the position of the equilibrium will shift to reduce the stress. 3 Types of stress

1. Change Concentration Change amounts of reactants and/or products Concentration: The system will shift away from the added component. If a component is removed, the opposite effect occurs. Adding product makes Q>K Removing reactant makes Q>K Adding reactant makes Q<K Removing product makes Q<K Determine the effect on Q, will tell you the direction of shift

2. Change Pressure. The system will shift away from the added gaseous component. If a component is removed, the opposite effect occurs. Addition of inert gas does not affect the equilibrium position. Decreasing the volume shifts the equilibrium toward the side with fewer moles of gas. By changing volume System will move in the direction that has the least moles of gas. Because partial pressures (and concentrations) change a new equilibrium must be reached. System tries to minimize the moles of gas.

Partial pressures of reactants and product are not changed 2. Change in Pressure By adding an inert gas Partial pressures of reactants and product are not changed No effect on equilibrium position

3. Change Temperature Temperature: K will change depending upon the temperature (endothermic – energy is a reactant; exothermic – energy is a product). Affects the rates of both the forward and reverse reactions. Doesn’t just change the equilibrium position, changes the equilibrium constant. The direction of the shift depends on whether it is exo- or endothermic

Le Chatelier Example #1 A closed container of ice and water at equilibrium. The temperature is raised. Ice + Energy <-- > Water The equilibrium of the system shifts to the _______ to use up the added energy. right

Le Chatelier Example #2 A closed container of N2O4 and NO2 at equilibrium. NO2 is added to the container. N2O4 (g) + Energy < - - > 2 NO2 (g) The equilibrium of the system shifts to the _______ to use up the added NO2. left

Le Chatelier Example #3 A closed container of water and its vapor at equilibrium. Vapor is removed from the system. water + Energy  vapor The equilibrium of the system shifts to the _______ to replace the vapor. right

Le Chatelier Example #4 A closed container of N2O4 and NO2 at equilibrium. The pressure is increased. N2O4 (g) + Energy < - - > 2 NO2 (g) The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation. left

Think of heat as a product Raising temperature push toward reactants. Exothermic DH<0 Releases heat Think of heat as a product Raising temperature push toward reactants. Shifts to left.

Think of heat as a reactant Raising temperature push toward products. Endothermic DH>0 Produces heat Think of heat as a reactant Raising temperature push toward products. Shifts to right.

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