Chapter 14: Chemical Equilibrium

1. Chapter 14: Chemical Equilibrium CHE 124: General Chemistry II Dr. Jerome Williams, Ph.D. Saint Leo University

2. Overview The Equilibrium Condition: A Second Example Law of Mass Action: Equilibrium Constants Homogeneous Equilibrium

3. H2(g) + I2(g) Û 2 HI(g) 3

4. H2(g) + I2(g) Û 2 HI(g) 4

5. H2(g) + I2(g) Û 2 HI(g) 5

6. Equilibrium ? Equal The rates of the forward and reverse reactions are equal at equilibrium But that does not mean the concentrations of reactants and products are equal Some reactions reach equilibrium only after almost all the reactant molecules are consumed – we say the position of equilibrium favors the products Other reactions reach equilibrium when only a small percentage of the reactant molecules are consumed – we say the position of equilibrium favors the reactants 6

7. An Analogy: Population Changes 7

8. Equilibrium Constant Even though the concentrations of reactants and products are not equal at equilibrium, there is a relationship between them The relationship between the chemical equation and the concentrations of reactants and products is called the Law of Mass Action For the general equation aA + bB Û cC + dD, the Law of Mass Action gives the relationship below the lowercase letters represent the coefficients of the balanced chemical equation always products over reactants K is called the equilibrium constant unitless 8

9. Writing Equilibrium Constant Expressions For the reaction aA(aq) + bB(aq) Û cC(aq) + dD(aq) the equilibrium constant expression is 9

10. What Does the Value of Keq Imply? When the value of Keq >> 1, we know that when the reaction reaches equilibrium there will be many more product molecules present than reactant molecules the position of equilibrium favors products When the value of Keq << 1, we know that when the reaction reaches equilibrium there will be many more reactant molecules present than product molecules the position of equilibrium favors reactants 10

11. A Large Equilibrium Constant 11

12. A Small Equilibrium Constant 12

13. Practice – Write the equilibrium constant expressions, K, and predict the position of equilibrium for the following 13

14. Relationships between K and Chemical Equations When the reaction is written backward, the equilibrium constant is inverted 14

15. Relationships between K and Chemical Equations When the coefficients of an equation are multiplied by a factor, the equilibrium constant is raised to that factor 15

16. Relationships between K and Chemical Equations When you add equations to get a new equation, the equilibrium constant of the new equation is the product of the equilibrium constants of the old equations 16

17. Example 14.2: Compute the equilibrium constant at 25 °C for the reaction NH3(g) ? 0.5 N2(g) + 1.5 H2(g) 17

18. Practice – When the reaction A(aq) Û 2 B(aq) reaches equilibrium [A] = 1.0 x 10-5 M and [B] = 4.0 x 10-1 M. When the reaction 2 B(aq) Û Z(aq) reaches equilibrium [B] = 4.0 x 10-3 M and [Z] = 2.0 x 10-6 M. Calculate the equilibrium constant for each of these reactions and the equilibrium constant for the reaction 3 Z(aq) Û 3 A(aq) 18