CHEMICAL EQUILIBRIUM Chapter 16

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CHEMICAL EQUILIBRIUM Chapter 16 Copyright © 1999 by Harcourt Brace & Company All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida

Properties of an Equilibrium Equilibrium systems are DYNAMIC (in constant motion) REVERSIBLE can be approached from either direction

Properties of an Equilibrium Equilibrium systems are DYNAMIC (in constant motion) REVERSIBLE can be approached from either direction Pink to blue Co(H2O)6Cl2 ---> Co(H2O)4Cl2 + 2 H2O

Properties of an Equilibrium Equilibrium systems are DYNAMIC (in constant motion) REVERSIBLE can be approached from either direction Pink to blue Co(H2O)6Cl2 ---> Co(H2O)4Cl2 + 2 H2O Blue to pink Co(H2O)4Cl2 + 2 H2O ---> Co(H2O)6Cl2

Chemical Equilibrium Fe3+ + SCN- FeSCN2+ Fe(H2O)63+ + SCN- Fe(SCN)(H2O)53+ + H2O

Chemical Equilibrium Fe3+ + SCN- FeSCN2+ After a period of time, the concentrations of reactants and products are constant. The forward and reverse reactions continue after equilibrium is attained.

Examples of Chemical Equilibria Phase changes such as H2O(s) H2O(liq)

Examples of Chemical Equilibria Formation of stalactites and stalagmites CaCO3(s) + H2O(liq) + CO2(g) Ca2+(aq) + 2 HCO3-(aq)

Chemical Equilibria CaCO3(s) + H2O(liq) + CO2(g) Ca2+(aq) + 2 HCO3-(aq) At a given T and P of CO2, [Ca2+] and [HCO3-] can be found from the EQUILIBRIUM CONSTANT.

THE EQUILIBRIUM CONSTANT For any type of chemical equilibrium of the type a A + b B c C + d D the following is a CONSTANT (at a given T)

THE EQUILIBRIUM CONSTANT For any type of chemical equilibrium of the type a A + b B c C + d D the following is a CONSTANT (at a given T) If K is known, then we can predict concs. of products or reactants.

Determining K 2 NOCl(g) 2 NO(g) + Cl2(g) Place 2.00 mol of NOCl is a 1.00 L flask. At equilibrium you find 0.66 mol/L of NO. Calculate K. Solution Set of a table of concentrations [NOCl] [NO] [Cl2] Before 2.00 0 0 Change Equilibrium 0.66

Determining K 2 NOCl(g) 2 NO(g) + Cl2(g) Place 2.00 mol of NOCl is a 1.00 L flask. At equilibrium you find 0.66 mol/L of NO. Calculate K. Solution Set of a table of concentrations [NOCl] [NO] [Cl2] Before 2.00 0 0 Change -0.66 +0.66 +0.33 Equilibrium 1.34 0.66 0.33

Determining K 2 NOCl(g) 2 NO(g) + Cl2(g) [NOCl] [NO] [Cl2] Before 2.00 0 0 Change -0.66 +0.66 +0.33 Equilibrium 1.34 0.66 0.33

Determining K 2 NOCl(g) 2 NO(g) + Cl2(g) [NOCl] [NO] [Cl2] Before 2.00 0 0 Change -0.66 +0.66 +0.33 Equilibrium 1.34 0.66 0.33

Writing and Manipulating K Expressions Solids and liquids NEVER appear in equilibrium expressions. S(s) + O2(g) SO2(g)

Writing and Manipulating K Expressions Solids and liquids NEVER appear in equilibrium expressions. S(s) + O2(g) SO2(g)

Writing and Manipulating K Expressions Solids and liquids NEVER appear in equilibrium expressions. NH3(aq) + H2O(liq) NH4+(aq) + OH-(aq)

Writing and Manipulating K Expressions Solids and liquids NEVER appear in equilibrium expressions. NH3(aq) + H2O(liq) NH4+(aq) + OH-(aq)

Writing and Manipulating K Expressions Adding equations for reactions S(s) + O2(g) SO2(g) K1 = [SO2] / [O2] SO2(g) + 1/2 O2(g) SO3(g) K2 = [SO3] / [SO2][O2]1/2 NET EQUATION S(s) + 3/2 O2(g) SO3(g)

Writing and Manipulating K Expressions Adding equations for reactions S(s) + O2(g) SO2(g) K1 = [SO2] / [O2] SO2(g) + 1/2 O2(g) SO3(g) K2 = [SO3] / [SO2][O2]1/2 NET EQUATION S(s) + 3/2 O2(g) SO3(g)

Writing and Manipulating K Expressions Changing coefficients S(s) + 3/2 O2(g) SO3(g) 2 S(s) + 3 O2(g) 2 SO3(g)

Writing and Manipulating K Expressions Changing coefficients S(s) + 3/2 O2(g) SO3(g) 2 S(s) + 3 O2(g) 2 SO3(g)

Writing and Manipulating K Expressions Changing coefficients S(s) + 3/2 O2(g) SO3(g) 2 S(s) + 3 O2(g) 2 SO3(g)

Writing and Manipulating K Expressions Changing direction S(s) + O2(g) SO2(g) SO2(g) S(s) + O2(g)

Writing and Manipulating K Expressions Changing direction S(s) + O2(g) SO2(g) SO2(g) S(s) + O2(g)

Writing and Manipulating K Expressions Changing direction S(s) + O2(g) SO2(g) SO2(g) S(s) + O2(g)

Writing and Manipulating K Expressions Concentration Units We have been writing K in terms of mol/L. These are designated by Kc But with gases, P = (n/V)•RT = conc • RT P is proportional to concentration, so we can write K in terms of P. These are designated by Kp. Kc and Kp may or may not be the same.

The Meaning of K 1. Can tell if a reaction is product-favored or reactant-favored. For N2(g) + 3 H2(g) 2 NH3(g)

The Meaning of K 1. Can tell if a reaction is product-favored or reactant-favored. For N2(g) + 3 H2(g) 2 NH3(g)

The Meaning of K 1. Can tell if a reaction is product-favored or reactant-favored. For N2(g) + 3 H2(g) 2 NH3(g) Conc. of products is much greater than that of reactants at equilibrium.

The Meaning of K 1. Can tell if a reaction is product-favored or reactant-favored. For N2(g) + 3 H2(g) 2 NH3(g) Conc. of products is much greater than that of reactants at equilibrium. The reaction is strongly product-favored.

The Meaning of K For AgCl(s) Ag+(aq) + Cl-(aq) Kc = [Ag+] [Cl-] = 1.8 x 10-5 Conc. of products is much less than that of reactants at equilibrium. The reaction is strongly reactant-favored. Ag+(aq) + Cl-(aq) AgCl(s) is product-favored.

The Meaning of K 2. Can tell if a reaction is at equilibrium. If not, which way it moves to approach equilibrium.

The Meaning of K If [iso] = 0.35 M and [n] = 0.15 M, are you at equilibrium? Which way does the reaction “shift” to approach equilibrium? See Screen 16.9.

The Meaning of K In general, all reacting chemical systems are characterized by their REACTION QUOTIENT, Q. If Q = K, then system is at equilibrium.

The Meaning of K In general, all reacting chemical systems are characterized by their REACTION QUOTIENT, Q. If Q = K, then system is at equilibrium. If [iso] = 0.35 M and [n] = 0.15 M, are you at equilibrium? Q (2.3) < K (2.5).

The Meaning of K In general, all reacting chemical systems are characterized by their REACTION QUOTIENT, Q. If Q = K, then system is at equilibrium. Q (2.33) < K (2.5). Reaction is NOT at equilibrium, so [Iso] must become ________ and [n] must ____________.

Typical Calculations H2(g) + I2(g) ¸ 2 HI(g) PROBLEM: Place 1.00 mol each of H2 and I2 in a 1.00 L flask. Calc. equilibrium concentrations. H2(g) + I2(g) ¸ 2 HI(g)

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 1. Set up table to define EQUILIBRIUM concentrations. [H2] [I2] [HI] Initial 1.00 1.00 0 Change Equilib

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 1. Set up table to define EQUILIBRIUM concentrations. [H2] [I2] [HI] Initial 1.00 1.00 0 Change -x -x +2x Equilib 1.00-x 1.00-x 2x where x is defined as am’t of H2 and I2 consumed on approaching equilibrium.

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 2. Put equilibrium concentrations into Kc expression.

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 3. Solve Kc expression - take square root of both sides.

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 3. Solve Kc expression - take square root of both sides.

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 3. Solve Kc expression - take square root of both sides. x = 0.79

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 3. Solve Kc expression - take square root of both sides. x = 0.79 Therefore, at equilibrium [H2] = [I2] = 1.00 - x = 0.21 M

H2(g) + I2(g) 2 HI(g), Kc = 55.3 Step 3. Solve Kc expression - take square root of both sides. x = 0.79 Therefore, at equilibrium [H2] = [I2] = 1.00 - x = 0.21 M [HI] = 2x = 1.58 M

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g)

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) If initial concentration of N2O4 is 0.50 M, what are the equilibrium concentrations? Step 1. Set up an equilibrium table [N2O4] [NO2] Initial 0.50 0 Change Equilib

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) If initial concentration of N2O4 is 0.50 M, what are the equilibrium concentrations? Step 1. Set up an equilibrium table [N2O4] [NO2] Initial 0.50 0 Change -x +2x Equilib 0.50 - x 2x

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) Step 2. Substitute into Kc expression and solve. Rearrange: 0.0059 (0.50 - x) = 4x2 0.0029 - 0.0059x = 4x2 4x2 + 0.0059x - 0.0029 = 0 This is a QUADRATIC EQUATION ax2 + bx + c = 0 a = 4 b = 0.0059 c = -0.0029

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) Solve the quadratic equation for x. ax2 + bx + c = 0 a = 4 b = 0.0059 c = -0.0029

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) Solve the quadratic equation for x. ax2 + bx + c = 0 a = 4 b = 0.0059 c = -0.0029

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) Solve the quadratic equation for x. ax2 + bx + c = 0 a = 4 b = 0.0059 c = -0.0029 x = -0.00074 ± 1/8(0.046)1/2 = -0.00074 ± 0.027

Nitrogen Dioxide Equilibrium N2O4(g) 2 NO2(g) x = 0.026 or -0.028 But a negative value is not reasonable. Conclusion [N2O4] = 0.050 - x = 0.47 M [NO2] = 2x = 0.052 M