# Applications of Equilibrium Constants K c and K p can be used to determine the concentration of reactants and/or products at equilibrium.

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Applications of Equilibrium Constants K c and K p can be used to determine the concentration of reactants and/or products at equilibrium.

Applications of Equilibrium Constants Example: Calculate the concentration of Ca 2+ ions and F - ions that are present in a saturated aqueous solution of CaF 2 if K sp = 3.90 x 10 -11 at 25 o C.

Applications of Equilibrium Constants CaF 2 (s) Ca 2+ (aq) + 2 F - (aq)

Applications of Equilibrium Constants Example: At 250 o C the reaction PCl 5 (g) PCl 3 (g) + Cl 2 (g) has an equilibrium constant K c = 1.80. If the initial concentration of PCl 5 in a reactor at 250 o C is 0.0400 M, what are the concentrations of PCl 5, PCl 3, and Cl 2 in the mixture at equilibrium? Write the expression for K c :

Applications of Equilibrium Constants Set up a table showing the initial concentrations, changes in concentration, and equilibrium concentrations. Initial0.0400 M0.000 M Change Equil. PCl 5 (g) PCl 3 (g) + Cl 2 (g)

Applications of Equilibrium Constants

Substitute the equilibrium values into the expression for K c

Applications of Equilibrium Constants

Only one of the possible values of x will be reasonable. Determine which one is reasonable substitute possible values of x into the algebraic expression used to represent the equilibrium concentration of reactants and/or products.

Applications of Equilibrium Constants

So: [PCl 5 ] equil = [PCl 3 ] equil = [Cl 2 ] equil = You can verify your answer by substituting the concentrations into the expression for K c. You should get the same (or very close to) the value given for K c.

Le Chatelier’s Principle Equilbrium reactions such as the one to form NH 3 tend to stop short of the maximum (theoretical) yield of products. Industrial chemists are always looking for ways to improve the yield of products in a particular reaction. Improves cost effectiveness of process Increases profits for the company Reduces the cost for the consumer

Le Chatelier’s Principle Le Chatelier’s Principle explains the way a system at equilibrium will change in response to changes made in the temperature, pressure or concentration of one of the components of a system at equilibrium.

Le Chatelier’s Principle If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.

Le Chatelier’s Principle How does changing the concentration of a reactant or product impact the equilibrium?

Le Chatelier’s Principle In order to visualize the impact of changing the concentration of a reactant or product, consider how adding or removing weight from one side of a balanced teeter totter impacts the balance. Balanced: At “equilibrium”

Le Chatelier’s Principle If you remove two blocks from the right side of the teeter totter, what happens? What do you have to do to re-balance the teeter totter (re-establish the equilibrium)?

Le Chatelier’s Principle To re-balance the teeter totter (re-establish the equilibrium), you must move one of the blocks from the left side over to the right side.

Le Chatelier’s Principle When material is removed from one side of the teeter totter, the teeter totter is no longer balanced: To re-balance: shift material towards the side where it was removed.

Le Chatelier’s Principle What happens if you add 2 blocks to the right side of the original teeter totter? What do you have to do to re-balance (re- establish the equilibrium) the teeter totter?

Le Chatelier’s Principle To re-balance the teeter totter (re-establish the equilibrium), you must move one of the blocks from the right side to the left side.

Le Chatelier’s Principle When material is added to one side of the teeter totter, the teeter totter is no longer balanced. To re-balance: shift material away from where it was added

Le Chatelier’s Principle Similar trends hold true for chemical reactions at equilibrium: If a reactant or product is added to a system at equilibrium, the system will shift away from the material added. Add reactantshift toward products Add productshift toward reactants

Le Chatelier’s Principle If a reactant or product is removed from a system at equilibrium, the system will shift toward the material removed. Remove reactantshift toward reactants Remove productshift toward products

Le Chatelier’s Principle Example: Give three ways that the total amount of ammonia produced in the following reaction can be increased? (i.e. How can you shift the reaction towards the products?) N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

Le Chatelier’s Principle Effects of Changing Pressure and Volume: If Volume decreases, partial pressures of the reactants and products increase: system shifts to reduce pressure

Le Chatelier’s Principle Reducing the volume (thereby increasing the partial pressures) of a gaseous system at equilibrium causes the reaction to shift in the direction that reduces the total number of moles of gas Increasing the volume (thereby decreasing the partial pressure) of a gaseous equilibrium mixture, cases a shift in the direction that produces more gas molecules.

Le Chatelier’s Principle Decrease Volume shift towards fewer gas molecules Increase Partial Pressure shift towards fewer gas molecules Increase Volume: shift towards more gas molecules Decrease Partial Pressure: shift towards more gas molecules

Le Chatelier’s Principle Example: What happens to the system below if the total pressure is increased by reducing the volume? N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

Le Chatelier’s Principle Effect of Temperature Change: The value of an equilibrium constant depends on temperature. The impact of increasing the temperature of a reaction depends on whether the reaction is exothermic or endothermic.

Le Chatelier’s Principle To understand the impact of increasing temperature, consider heat to be a reactant (endothermic) or product (exothermic). Endothermic Reactions: absorb heat Reactants + heat Products Exothermic Reactions: produce heat Reactants Products + heat

Le Chatelier’s Principle When the temperature is increased, the equilibrium shifts in the direction that absorbs heat (i.e uses up the heat). Endothermic Rxn: Increase T shift towards products Exothermic Rxn: Increase T shift towards reactants

Le Chatelier’s Principle When the temperature is decreased the equilibrium shifts in the direction that produces heat. Endothermic Rxn: As T decreases shift towards reactants Exothermic Rxn: As T decreases shift towards products

Le Chatelier’s Principle Example: Consider the following equilibrium: N 2 O 4 (g) 2 NO 2  H = 58 kJ. In what direction will the equilibrium shift if: N 2 O 4 is added: NO 2 is removed: total pressure is increased by adding N 2 (g): volume is increased temperature is decreased?

Le Chatelier’s Principle Effect of Catalyst Addition of a catalyst does not change the equilibrium. Addition of a catalyst simply increases the rate at which equilibrium is attained by reducing the activation energy for the reaction.

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