Equilibrium Constants General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 9 Reaction Rates and Chemical Equilibrium 9.3 Equilibrium Constants Lectures © 2013 Pearson Education, Inc.

Equilibrium Constants For the reaction, the equilibrium constant expression, Kc, gives the concentrations of the reactants and products at equilibrium, the square brackets indicate the moles/liter of each substance, and the coefficients b and a are written as superscripts that raise the moles/liter to a specific power.

Guide to Writing the Kc Expression

Writing a Kc Expression Write the Kc expression for the following reaction. Step 1 Write the balanced chemical equation. Step 2 Write the concentrations of the products as the numerator and the reactants as the denominator.

Writing a Kc Expression Write the Kc expression for the following reaction. Step 3 Write any coefficient in the equation as an exponent.

Learning Check Which of the following is the correctly written Kc expression for the reaction shown below? A. B. C. D.

Solution Which of the following is the correctly written Kc expression for the reaction shown below? Step 1 Write the balanced chemical equation. Step 2 Write the product concentration in the numerator and the reactant concentration in the denominator.

Solution Which of the following is the correctly written Kc expression for the reaction shown below? Step 3 Write any coefficient in the equation as an exponent.

Homogeneous, Heterogeneous Equilibrium In a homogeneous equilibrium, all reactants and products in the reaction are in the same physical state. In a heterogeneous equilibrium, the reactants and products are in two or more physical states. The concentration of solids and liquids are constant, and therefore omitted from the equilibrium constant expression.

Learning Check Write the equilibrium constant expression for the following reaction at equilibrium.

Solution Write the equilibrium constant expression for the following reaction at equilibrium. Step 1 Write the balanced chemical equation. Step 2 Write the concentrations of the products as the numerator and the reactants as the denominator.

Solution Write the equilibrium constant expression for the following reaction at equilibrium. Step 3 Write any coefficient in the equation as an exponent.

Guide to Calculating the Kc

Calculating Equilibrium Constants What is the Kc for the following reaction at 427 ˚C if the equilibrium concentrations are [H2] = 0.20 M, [I2] = 0.20 M and [HI] = 1.47 M? Step 1 Write the Kc expression for the equilibrium.

Calculating Equilibrium Constants What is the Kc for the following reaction at 427 ˚C if the equilibrium concentrations are [H2] = 0.20 M, [I2] = 0.20 M and [HI] = 1.47 M? Step 2 Substitute equilibrium (molar) concentrations and calculate Kc.

Learning Check Calculate the Kc for the reaction, with the following equilibrium concentrations,

Solution Calculate the Kc for the reaction, with the following equilibrium concentrations, Step 1 Write the Kc expression for the equilibrium.

Solution Calculate the Kc for the reaction, with the following equilibrium concentrations, Step 2 Substitute equilibrium (molar) concentrations and calculate Kc.