Ch. 15: Chemical Equilibrium

1. Ch. 15: Chemical Equilibrium Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry

2. I. Chapter Outline • Introduction • The Equilibrium Constant (K) • Le Châtelier’s Principle

3. I. Introduction • Most reactions are reversible, meaning they can proceed in both forward and reverse directions. • This means that as products build up, they will react and reform reactants. • At equilibrium, the forward and backward reaction rates are equal.

4. I. Example Equilibrium

5. II. Equilibrium Concentrations • Equilibrium does not mean that concentrations are all equal!! • However, we can quantify concentrations at equilibrium. • Every equilibrium has its own equilibrium constant.

6. II. The Equilibrium Constant • equilibrium constant: the ratio at equilibrium of the [ ]’s of products raised to their stoichiometric coefficients divided by the [ ]’s of reactants raised to their stoichiometric coefficients. • The relationship between a balanced equation and equilibrium constant expression is the law of mass action.

7. II. The Equilibrium Constant • For a general equilibrium aA + bB  cC + dD, the equilibrium expression is:

8. II. Example Equilibrium Constant

9. II. Sample Problem • Write the equilibrium constant expression for the reaction: 2H2(g) + O2(g) 2H2O(g).

10. II. Physical Meaning of K • Large values of K mean that the equilibrium favors products, i.e. there are high [ ]’s of products and low [ ]’s of reactants at equilibrium. • Small values of K mean that the equilibrium favors reactants, i.e. there are low [ ]’s of products and high [ ]’s of reactants at equilibrium.

11. II. Values of K • Values of K are most easily calculated by allowing a system to come to equilibrium and measuring [ ]’s of the components. • For the equilibrium H2(g) + I2(g) 2HI(g), let’s say equilibrium [ ]’s at 445 °C were found to be 0.11 M, 0.11 M, and 0.78 M for molecular hydrogen, molecular iodine, and hydrogen iodide, respectively.

12. II. Kc for a H2/I2 Mixture • Note that units are not included when calculating K’s. • Thus, equilibrium constants are unitless.

13. II. Equilibrium [ ]’s Vs. K • For any reaction, the equilibrium [ ]’s will depend on the initial [ ]’s of reactants or products. • However, no matter how you set up the reaction, the value of the equilibrium constant will be the same if the temperature is the same.

14. II. Equilibrium [ ]’s Vs. K

15. III. Predicting Qualitative Changes to Equilibrium • If a system is at equilibrium, what happens when it is disturbed? • Le Châtelier’s Principle allow us to make qualitative predictions about changes in chemical equilibria: • When a chemical system at equilibrium is disturbed, the system shifts in the direction that minimizes the disturbance.

16. III. Three Ways to Disturb an Equilibrium • We look at three ways that disturb a system at equilibrium: • Adding/removing a reactant or product. • Changing the volume/pressure in gaseous reactions. • Changing the temperature.

17. III. Adding Reactant/Product • If we add or remove a reactant or product, we’re changing the [ ]’s. • The equilibrium will shift in a direction that will partially consume a reactant or product that is added, or partially replace a reactant or product that has been removed.

18. III. Adding Reactant

19. III. Adding Product

20. III. Changing Volume • Via PV = nRT, changing volume is related to changing pressure and vice versa. • For example, decreasing volume is equivalent to increasing pressure. • Reducing volume of a gaseous reaction mixture shifts the equilibrium in the direction that will, if possible, decrease the # of moles of gas.

21. III. Changing Pressure

22. III. Changing Temperature • To determine the effect of changing the temperature, we need to know the heat of reaction, ΔHrxn. • Once we know if a reaction is exo or endo, we can write “heat” as a product or reactant. • We then apply Le Châtelier’s Principle in the same manner as when we considered the add product/reactant case.

23. III. Changing Temperature

24. III. Sample Problem • For the reaction N2(g) + 3H2(g) 2NH3(g), ΔHrxn = -46.19 kJ/mole. Which way will the equilibrium shift when each of the following occurs? • NH3 is removed. • The reaction vessel is opened. • The reaction vessel is cooled by 25 °C.