Chapter 13 Reaction Rates and Chemical Equilibrium 13.3 Equilibrium Constants Learning Goal Calculate the equilibrium constant for a reversible reaction, given the concentrations of reactants and products at equilibrium. © 2014 Pearson Education, Inc.

Equilibrium Constants For the reaction: aA + bB cC + dD the equilibrium constant expression, Kc, gives the concentrations of the reactants and products, each raised to a power equal to its coefficient in the balanced equation. © 2014 Pearson Education, Inc.

Equilibrium Constants In the equilibrium expression: the square brackets indicate the mol/liter of each substance the coefficients a, b, c, and d are superscripts that raise the mol/liter to a specific power © 2014 Pearson Education, Inc.

Guide to Writing an Equilibrium Expression © 2014 Pearson Education, Inc.

Write an equilibrium expression for the following reaction: Learning Check Write an equilibrium expression for the following reaction: 2CO(g) + O2(g) 2CO2(g) © 2014 Pearson Education, Inc.

Write an equilibrium expression for the following reaction: Solution Write an equilibrium expression for the following reaction: 2CO(g) + O2(g) 2CO2(g) Step 1 Write the balanced chemical equation. 2CO(g) + O2(g) 2CO2(g) © 2014 Pearson Education, Inc.

Write an equilibrium expression for the following reaction: Solution Write an equilibrium expression for the following reaction: 2CO(g) + O2(g) 2CO2(g) Step 2 Write the concentrations of the products as the numerator and the reactants as the denominator. The square brackets indicate the mol/liter of each substance. The coefficients a, b, c, and d are superscripts that raise the mol/liter to a specific power. © 2014 Pearson Education, Inc.

Write an equilibrium expression for the following reaction: Solution Write an equilibrium expression for the following reaction: 2CO(g) + O2(g) 2CO2(g) Step 3 Write any coefficient in the equation as an exponent. © 2014 Pearson Education, Inc.

Heterogeneous Equilibrium In heterogeneous equilibrium, gases, solids, and liquid states are part of the reaction 2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g) solid and liquid concentrations are constant the Kc expression is written with only the compounds that are gases Kc = [CO2][H2O] © 2014 Pearson Education, Inc.

What is the Kc expression for the following reaction? Learning Check What is the Kc expression for the following reaction? 2KClO3(s) 2KCl(s) + 3O2(g) A. B. C. D. © 2014 Pearson Education, Inc.

What is the Kc expression for the following reaction? Solution What is the Kc expression for the following reaction? 2KClO3(s) 2KCl(s) + 3O2(g) The correct answer is D, . Pure solids and pure liquids have a constant concentration and are not included in the equilibrium constant expression. © 2014 Pearson Education, Inc.

Equilibrium Constant Concentrations The equilibrium constant, Kc, is the numerical value obtained by substituting experimental values of molar concentrations at equilibrium. H2(g) + I2(g) 2HI(g) © 2014 Pearson Education, Inc.

Equilibrium Constant Concentrations Experimental concentrations used to calculate the equilibrium constant will always give the same value of Kc, at a specific temperature. A reaction at a specific temperature can have only one value for the equilibrium constant. The units of Kc depend on the specific equation. In this textbook, Kc is given without units. © 2014 Pearson Education, Inc.

Equilibrium Constant Concentrations © 2014 Pearson Education, Inc.

Guide to Calculating the Kc Value © 2014 Pearson Education, Inc.

Learning Check What is the numerical value of Kc at 443 oC if the equilibrium concentrations are: H2(g) + I2(g) 2HI(g) [H2 ] = 1.2 mol/L [I2] =1.2 mol/L [HI] = 0.35 mol/L © 2014 Pearson Education, Inc.

What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g) Solution What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g) Step 1 State the given and needed quantities. © 2014 Pearson Education, Inc.

What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g) Solution What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g) Step 2 Write the Kc expression for the equilibrium. © 2014 Pearson Education, Inc.

What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g) Solution What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g) Step 3 Substitute equilibrium (molar) concentrations and calculate Kc. © 2014 Pearson Education, Inc.