10.5 Changing Equilibrium Conditions: Le Châtelier’s Principle The transport of oxygen involves an equilibrium between hemoglobin (Hb), oxygen, and oxyhemoglobin (HbO2). Hb(aq) + O2(g) HbO2(aq) For a person living at higher altitudes, hemoglobin concentration increases, causing a shift in the equilibrium back in the direction of HbO2 product. Hypoxia may occur at high altitudes where the oxygen concentration is lower. Learning Goal Use Le Châtelier’s Principle to describe the changes made in equilibrium concentrations when the reaction conditions change.

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 is placed on a reaction at equilibrium, the system responds by changing the rate of the forward or reverse reaction in the direction that relieves that stress.

Le Châtelier’s Principle The stress of adding water to tank A increases the rate of the forward direction to reestablish equal water levels and equilibrium. Core Chemistry Skill Using Le Châtelier’s Principle

Effect of Concentration Change on 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 equilibrium shifts toward the products. Add H2

Effect of Concentration Change on Equilibrium

Effect of Concentration Change on Equilibrium: Removing Reactant Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) If some of a reactant (H2 or I2) is removed, the rate of the reverse reaction increases to form more reactant until the equilibrium is reached. the equilibrium shifts toward the reactants. Remove H2

Effect of Concentration Change on Equilibrium: Adding Product Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) If more product (HI) is added, the rate of the reverse reaction increases to form more H2 and I2 reactants. the equilibrium shifts toward the reactants. Add HI

Effect of Concentration Change on Equilibrium: Removing Product Consider the following reaction at equilibrium: H2(g) + I2(g) 2HI(g) When some of the product (HI) is removed, there is an decrease in collisions of HI molecules. the rate of the forward reaction increases and forms more product (HI). Remove HI

Effect of Concentration Change on Equilibrium

Effect of a Catalyst on Equilibrium Adding a catalyst speeds up a reaction by lowering the activation energy, thus 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.

Effect of Volume Change on Equilibrium: Decreasing Volume 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, and the system shifts in the direction of the smaller number of moles to compensate. Decrease Volume

Effect of Volume Change on Equilibrium: Increasing Volume 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, and the system shifts in the direction of the larger number of moles to compensate. Increase volume

Effect of Volume Change on 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.

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 in the direction of oxyhemoglobin. When the concentration of O2 is low in the tissues, the reverse reaction releases O2 from oxyhemoglobin.

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.

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.

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) Remove O2

Effect of Temperature Change on Equilibrium: Decreasing 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 system. N2(g) + O2(g) + heat 2NO(g) Decrease Temperature

Effect of Temperature Change on Equilibrium: Increasing 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

Effect of Temperature Change on Equilibrium: Decreasing 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 system. 2SO2(g) + O2(g) 2SO3(g) + heat Decrease Temperature

Effect of Temperature Change on Equilibrium: Increasing 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 system. 2SO2(g) + O2(g) 2SO3(g) + heat Increase Temperature

Effects of Changing Conditions on Equilibrium

Study Check Indicate the shift in equilibrium of each change. 2NO2(g) + heat 2NO(g) + O2(g) 1) toward products 2) toward reactants adding more NO decreasing the temperature removing some O2 increasing the volume removing some NO

Study Check Indicate the shift in equilibrium of each change. 2NO2(g) + heat 2NO(g) + O2(g) 1) toward products 2) toward reactants adding more NO 2) toward reactants decreasing the temperature 2) toward reactants removing some O2 1) toward products increasing the volume 1) toward products removing some NO 1) toward products

Concept Map