Activity, Heterogeneous Equilibria and Calculations using the Equilibrium Constant

1. Lecture #18 Activity, Heterogeneous Equilibria and Calculations using the Equilibrium Constant Chemistry 142 B Autumn Quarter 2004 J. B. Callis, Instructor

2. The Concept of Activity • The ‘true’ equilibrium constant expression does not simply involve pressure or concentration for a substance. • Instead, it uses the ratio of the equilibrium pressure (or concentration) for a given substance to a reference pressure (or concentration) for that substance. • This ratio is defined as the activity of the substance.

3. Use of Activities in Equilibrium Expressions

4. The Difference between K and Kp • In general K and Kp are not equal in value. • Exception: when Dn = 0.

5. Heterogeneous Equilibria • Homogeneous equilibria take place in one phase. • Heterogeneous equilibria involve more than one phase.

6. NH4NO2(s) = N2(g) + 2 H2O(g) is an example of a heterogeneous equilibrium The equilibrium constant for this reaction may be expressed as:

7. What We Can Learn About a Reaction from Its Equilibrium Constant • The tendency of the reaction to occur: • A value of K > 1 favors products • A value of K < 1 favors reactants • However, the value of K says nothing about the speed of the reaction. • Whether a given set of concentrations represents an equilibrium condition. • The equilibrium position that will be reached for a given set of initial conditions.

8. The Reaction Quotient, Q

9. Which Direction? • If Q = K, the system is at equilibrium; the concentrations will not change. • If Q > K, the system will shift to the left by converting products back to reactants. • If Q < K, the system will shift to the right by converting reactants to products.

10. Problem 18-1: Predict Reaction Direction using Q • For the following reaction: • CH4(g) + 2 H2S(g) = CS2(g) + 4 H2(g) • 1.00 mol CH4, 1.00 mol CS2, 2.00 mol H2S and 2.00 mol H2 are mixed in a 250 mL vessel at 960oC. At this temperature, K = 0.036. In which direction will the reaction proceed in order to reach equilibrium? • Solution:

11. Solution: Predict Reaction Direction using Q (2) Calculating the value of Q: (3) Comparing Q and K:

12. Solving Equilibrium Problems • Write the balanced equation for the reaction. • Write the equilibrium expression. • List the initial concentrations. • Calculate Q and determine the direction of shift to equilibrium. • Define the change needed to reach equilibrium and define the equilibrium concentrations. • Substitute the equilibrium concentrations into the equilibrium expression and solve for the unknown. • Check the solution by calculating K and making sure it is identical to the original K.

13. Calculating Equilibrium Pressures and Concentrations – Problem 18-2 Consider the equilibrium: CO(g) + H2O(g) = CO2(g) + H2(g) If 0.250 mol CO and 0.250 mol H2O are placed in a 125 mL flask at 900 K, what is the composition of the equilibrium mixture if K = 1.56. Answer:

14. Example 18-2: Continued (a) Construct the reaction table:

15. Example 18-2: Continued (b) Substitute into the equilibrium expression:

16. Answers to Problems in Lecture 18 • Q = 64. Comparing Q and K: Q > K, so the reaction goes to the left. Therefore reactants increase and products decrease their concentrations.