Chapter 13 Reaction Rates and Chemical Equilibrium 13.5 Changing Equilibrium Conditions: Le Châtelier’s Principle Learning Goal Use Le Châtelier’s Principle to describe the changes made in equilibrium concentrations when the reaction conditions change. Hypoxia may occur at high altitudes where the oxygen concentration is lower. © 2014 Pearson Education, Inc.

Le Châtelier’s Principle When the conditions of a reaction at equilibrium are changed, the forward and reverse reactions will no longer be equal. Le Châtelier’s Principle states that when a stress (change in conditions) is placed on a reaction at equilibrium, the equilibrium will shift in a direction that relieves the stress. © 2014 Pearson Education, Inc.

Le Châtelier’s Principle When water is added to one tank, the levels readjust to equalize. © 2014 Pearson Education, Inc.

Changing Equilibrium, Adding Reactant Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) If more reactant (H2 or I2) is added, the rate of the forward reaction increases to form more product until the system is again at equilibrium the effect of adding reactant shifts the equilibrium toward the products Adding reactant © 2014 Pearson Education, Inc.

Changing Equilibrium, Adding Reactant (a) The addition of H2 places stress on the equilibrium system of H2(g) + I2(g) 2HI(g). (b) To relieve the stress, the forward reaction converts some reactants H2 and I2 to product HI. (c) A new equilibrium is established when the rates of the forward reaction and the reverse reaction become equal. © 2014 Pearson Education, Inc.

Changing Equilibrium, Removing Reactant Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) If a reactant (H2 or I2) is removed, the rate of the reverse reaction increases to form more reactant until the equilibrium is reached the effect of removing a reactant shifts the equilibrium toward the reactants Removing reactant © 2014 Pearson Education, Inc.

Changing Equilibrium, Adding Product Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) If more HI is added, the rate of the reverse reaction increases to form more H2 and I2 reactants the effect of adding a product shifts the equilibrium toward the reactants Adding product © 2014 Pearson Education, Inc.

Changing Equilibrium, Removing Product Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) When product (HI) is removed, there is a decrease in collisions of HI molecules the rate of the forward reaction increases and forms more products, HI Removing product © 2014 Pearson Education, Inc.

Effect of Concentration Changes on Equilibrium © 2014 Pearson Education, Inc.

Effect of a Catalyst on Equilibrium Adding a catalyst speeds up a reaction by lowering the activation energy, increasing the rate of the forward and reverse reactions. The time to reach equilibrium is shorter; however, the same ratios of reactants and products are present. The addition of a catalyst does not change the equilibrium mixture. © 2014 Pearson Education, Inc.

Decreasing volume, shifts toward fewer moles Effect of Decreasing Volume on Equilibrium A change in the volume of a gas mixture at equilibrium will change the concentration of the gases in the mixture. 2CO(g) + O2(g) 2CO2(g) Decreasing the volume increases the concentration of the gases. The system shifts in the direction of the smaller number of moles to compensate. Decreasing volume, shifts toward fewer moles © 2014 Pearson Education, Inc.

Increasing volume, shifts toward more moles Effect of Increasing Volume on Equilibrium A change in the volume of a gas mixture at equilibrium will change the concentration of the gases in the mixture. 2CO(g) + O2(g) 2CO2(g) Increasing the volume decreases the concentration of the gases. The system shifts in the direction of the larger number of moles to compensate. Increasing volume, shifts toward more moles © 2014 Pearson Education, Inc.

Volume Changes, Equilibrium (a) A decrease in the volume of the container causes the system to shift in the direction of fewer moles of gas. (b) An increase in the volume of the container causes the system to shift in the direction of more moles of gas. © 2014 Pearson Education, Inc.

Chemistry Link to Health, Hb Oxygen transport involves an equilibrium between hemoglobin (Hb), oxygen, and oxyhemoglobin (HbO2). Hb(aq) + O2(g) HbO2(aq) When there is a high concentration of O2 in the alveoli of the lungs, the reaction shifts to make more oxyhemoglobin. When the concentration of O2 is low in the tissues, the reverse reaction releases O2 from oxyhemoglobin. © 2014 Pearson Education, Inc.

Chemistry Link to Health, Hb Given the reaction of hemoglobin: Hb(aq) + O2(g) HbO2(aq) At normal atmospheric pressure, oxygen diffuses into the blood because the partial pressure of oxygen in the alveoli is higher than that in the blood. At altitudes above 8000 ft, a decrease in atmospheric pressure results in a lower pressure of O2. © 2014 Pearson Education, Inc.

Chemistry Link to Health, Hb At an altitude of 18,000 ft, a person will obtain 29% less oxygen and may experience hypoxia. Hypoxia may occur at high altitudes where the oxygen concentration is lower. © 2014 Pearson Education, Inc.

Chemistry Link to Health, Hb According to Le Châtelier’s Principle, a decrease in oxygen shifts the equilibrium in the direction of the reactants depletes the concentration of HbO2, causing hypoxia Hb(aq) + O2(g) HbO2 (aq) Removing reactant © 2014 Pearson Education, Inc.

Endothermic Reaction Equilibrium and Temperature Decreasing the temperature of an endothermic reaction causes the system to respond by shifting the reaction toward more heat shifts the reaction toward the reactants, increasing heat in the system N2(g) + O2(g) + heat 2NO(g) Decrease temperature © 2014 Pearson Education, Inc.

Endothermic Reaction Equilibrium and Temperature Increasing the temperature of an endothermic reaction causes the system to respond by shifting the reaction to remove heat shifts the reaction toward the products, using up the heat H2(g) + O2(g) + heat 2NO(g) Increase temperature © 2014 Pearson Education, Inc.

Exothermic Reaction Equilibrium and Temperature Decreasing the temperature of an exothermic reaction causes the system to respond by shifting the reaction toward more heat shifts the reaction toward the products, increasing heat in the system 2SO2(g) + O2(g) 2SO3(g) + heat Decrease temperature © 2014 Pearson Education, Inc.

Exothermic Reaction Equilibrium and Temperature Increasing the temperature of an exothermic reaction causes the system to respond by shifting the reaction toward removing heat shifts the reaction toward the reactants, decreasing heat in the system 2SO2(g) + O2(g) 2SO3(g) + heat Increase temperature © 2014 Pearson Education, Inc.

Effects of Changing Conditions on Equilibrium © 2014 Pearson Education, Inc.

Indicate the shift in equilibrium of each change Learning Check Indicate the shift in equilibrium of each change 2NO2(g) + heat 2NO(g) + O2(g) toward reactants 2) toward products A. adding NO B. lowering the temperature C. removing O2 D. increasing the volume E. removing NO © 2014 Pearson Education, Inc.

Indicate the shift in equilibrium of each change Solution Indicate the shift in equilibrium of each change 2NO2(g) + heat 2NO(g) + O2(g) toward reactants 2) toward products A. adding NO (1) toward reactants B. lowering the temperature (1) toward reactants C. removing O2 (2) toward products D. increasing the volume (2) toward products E. removing NO (2) toward products © 2014 Pearson Education, Inc.