Chapter 9 Chemical Equilibrium 9.4 Using Equilibrium Constants

Reaching Chemical Equilibrium A container initially filled with SO2(g) and O2(g) or only SO3(g) contains mostly SO3(g) and small amounts of O2(g) and SO3(g) at equilibrium reaches equilibrium in both situations

Equilibrium Can Favor Product If equilibrium is reached after most of the forward reaction has occurred, the system favors the products

Equilibrium with a Large Kc At equilibrium, a reaction with a large Kc produces a large amount of product; very little of the reactants remain Kc = [NCl3]2 = 3.2 x 1011 [N2][Cl2]3 a large Kc favors the products N2(g) + 3Cl2(g) 2NCl3(g) When this reaction reaches equilibrium, it will essentially consist of the product NCl3.

Equilibrium Can Favor Reactant If equilibrium is reached when very little of the forward reaction has occurred, the reaction favors the reactants

Equilibrium with a Small Kc At equilibrium, a reaction that produces only a small amount of product has a small Kc Kc = [NO]2 = 2.3 x 10-9 [N2][O2] a small Kc favors the reactants N2(g) + O2(g) 2NO(g) When this reaction reaches equilibrium, it will essentially consist of the reactants N2 and O2.

Summary of Kc Values A reaction that favors products has a large Kc with about equal concentrations of products and reactants has a Kc close to 1 that favors reactants has a small Kc

Summary of Kc Values (continued)

Large and Small Kc Values

Learning Check For each Kc, indicate whether the reaction at equilibrium contains mostly of (R) reactants or (P) products. __A. H2(g) + F2(g) 2HF(g) Kc = 1 x 1095 __B. 3O2(g) 2O3(g) Kc = 1.8 x 10-7

Solution For each Kc, indicate whether the reaction at equilibrium contains mostly of (R) reactants or (P) products. A. P (Kc>>1) H2(g) + F2(g) 2HF(g) Kc = 1 x 1095 B. R (Kc<<1) 3O2(g) 2O3(g) Kc = 1.8 x 10-7

Guide to Using the Kc Value

Using Kc to Solve for an Equilibrium Concentration At equilibrium, the reaction PCl5(g) PCl3(g) + Cl2(g) has a Kc of 4.2 x 10–2 and contains [PCl3] = [Cl2] = 0.10 M. What is the equilibrium concentration of PCl5?

Using Kc to Solve for an Equilibrium Concentration (continued) STEP 1 Write the Kc expression: Kc = [PCl3][Cl2] [PCl5] STEP 2 Solve for the unknown concentration: [PCl5] = [PCl3][Cl2] Kc STEP 3 Substitute the known values and solve: [PCl5] = [0.10][0.10] = 0.24M 4.2 x 10–2 STEP 4 Check answer by placing concentrations in Kc: Kc = [0.10][0.10] = 4.2 x 10–2 [0.24]

Learning Check The Kc is 2.0 for the reaction 2NOBr(g) 2NO(g) + Br2(g) If the equilibrium concentrations are [NOBr] = 0.50 M and [NO] = 0.80 M, what is the equilibrium concentration of Br2? 1) 0.39 M 2) 0.78 M 3) 1.3 M

Solution STEP 1 Write the Kc expression: Kc = [NO]2 [Br2] [NOBr]2 STEP 2 Solve for the unknown concentration: [Br2] = [NOBr]2 Kc [NO]2 STEP 3 Substitute known values and solve: [Br2] = [0.50]2 2.0 = 0.78 M (2) [0.80]2 STEP 4 Check answer by placing concentrations in Kc: Kc = [0.80]2[0.78] = 2.0 [0.50]2