Applications of Equilibrium Constants

1. Applications of Equilibrium Constants • Kc and Kp can be used to determine the concentration of reactants and/or products at equilibrium.

2. Applications of Equilibrium Constants Example: Calculate the concentration of the ions that are present in a saturated aqueous solution of calcium fluoride at 25oC if Ksp = 3.90 x 10-11.

3. Applications of Equilibrium Constants

4. Applications of Equilibrium Constants Example:At 250oC the reaction PCl5 (g) PCl3 (g) + Cl2 (g) has an equilibrium constant of Kc = 1.80. If the initial concentration of PCl5 in a reactor at 250oC is 0.0400 M, what are the concentrations of PCl5, PCl3, and Cl2 in the mixture at equilibrium?

5. PCl5 (g) PCl3 (g) + Cl2 (g) Applications of Equilibrium Constants Set up a table showing the initial concentrations, changes in concentration, and equilibrium concentrations. Initial 0.0400 M 0.000 M 0.000 M Change Equil.

6. Applications of Equilibrium Constants Substitute the equilibrium values into the expression for Kc Use the quadratic formula to solve for the two possible values of x.

7. Applications of Equilibrium Constants

8. Applications of Equilibrium Constants Only one of the possible values of x will be reasonable. • Determine which one is reasonable • substitute each value of x into the algebraic expression used to represent the equilibrium concentration of reactants and products.

9. Applications of Equilibrium Constants • So: • [PCl5]equil = • [PCl3]equil = • [Cl2]equil = • You can verify your answer by substituting the concentrations into the expression for Kc. • You should get the same (or very close to) the value given for Kc.

10. Le Chatelier’s Principle • Equilbrium reactions such as the one to form NH3 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

11. Le Chatelier’s Principle • Le Chatelier’s Principle explains how 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.

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

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

14. Le Chatelier’s Principle • If you remove two blocks from the right side of the teeter totter, it tips towards the heavier side. • 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.

15. Le Chatelier’s Principle • What happens if you add 2 blocks to the right side of the original teeter totter? • 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.

16. 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 reactant shift toward products • Add product shift toward reactants

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

18. 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?) N2 (g) + 3 H2 (g) 2 NH3 (g)

19. Le Chatelier’s Principle • Effect of Volume and Pressure Changes • Reducing the volume causes the partial pressure of reactants and products in a gaseous system to increase. • The reaction shifts in the direction that reduces the total number of moles of gas. • Increasing the volume causes the partial pressure of reactants and products in a gaseous equilibrium to decrease. • The reaction shifts in the direction that increases the total number of moles of gas.

20. Le Chatelier’s Principle Example:What happens to the system below if the total pressure is increased by reducing the volume? N2 (g) + 3 H2 (g) 2 NH3 (g)

21. 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.

22. 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

23. 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

24. 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

25. Le Chatelier’s Principle Example:Consider the following equilibrium:N2O4 (g) 2 NO2 (g) DH = 58 kJ.In what direction will the equilibrium shift if: • N2O4 is added: • NO2 is removed: • total pressure is increased by adding N2 (g): • volume is increased • temperature is decreased?

26. LeChatelier’s Principle Example: In what direction does the following reaction shift when each of the changes below is made to an equilibrium mixture? • 0.5 g of Na2CrO4 is added. • 0.25 g of K2Cr2O7 is added. • 5 mL of 12 M HCl is added. • 0.5 g of NaOH (s) is added.

27. 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.