Chapter 15. Chemical Equilibrium

AP Chemistry Chapter 15 Equilibrium Chapter 15. Chemical Equilibrium

Sample Exercise 15.1 (p. 632)

Write the equilibrium expression for Keq for these three reactions:

a) 2 O3(g) ⇄ 3 O2(g)

b) 2 NO(g) + Cl2(g) ⇄ 2 NOCl(g)

+ + c) Ag (aq) + 2 NH3(g) ⇄ Ag(NH3)2 (aq)

Practice Exercise 15.1

Write the equilibrium expression for Keq for these three reactions:

a) H2(g) + I2(g) ⇄ 2 HI(g)

2+ - 2- b) Cd (aq) + 4 Br (aq) ⇄ CdBr4 (aq

- 1 - AP Chemistry Chapter 15 Equilibrium

In the synthesis of ammonia from nitrogen and hydrogen,

N2(g) + 3 H2(g) ⇄ 2 NH3(g)

o Kc = 9.60 at 300 C. Calculate Kp for this reaction at this temperature.

(4.34 x 10-3)

-3 For the equilibrium 2 SO3(g) ⇄ 2 SO2(g) + O2(g), Kc is 4.08 x 10 at 1000 K.

Calculate the value for Kp.

(0.335)

Sample Exercise 15.3 The following diagrams represent three different systems at equilibrium, all in the same size containers. a) Without doing any calculations, rank the three systems in order of increasing equilibrium constant, Kc.

b) If the volume of the containers is 1.0 L and each sphere represents 0.10 mol, calculate Kc for each system.

The equilibrium constant for the reaction H2(g) + I2(g) ⇄ 2 HI(g) varies with temperature as follows:

Kp = 792 at 298 K; Kp = 54 at 700 K. Is the formation of HI favored more at the higher or lower temperature?

Sample Exercise 15.4 (p. 637) -30 o The equilibrium constant for the reaction of N2 with O2 to form NO equals Kc = 1 x 10 at 25 C. N2(g) + O2(g) ⇄ 2 NO(g) Using this information, write the equilibrium constant expression and calculate the equilibrium constant for the following reaction:

2 NO(g) ⇄ N2(g) + O2(g)

-3 o For the formation of NH3 from N2 and H2, N2(g) + H2(g) ⇄ 2 NH3(g), Kp = 4.34 x 10 at 300 C.

What is the value of Kp for the reverse reaction? (2.30 x 102)

Given the following information, + - -4 HF(aq) ⇄ H (aq) + F (aq) Kc = 6.8 x 10 + 2- -6 H2C2O4(aq) ⇄ 2 H (aq) + C2O4 (aq) Kc = 3.8 x 10 determine the value of Kc for the following reaction: 2- - 2 HF(aq) + C2O4 (aq) ⇄ 2 F (aq) + H2C2O4(aq) (0.12)

Given the following information at 700 K,

H2(g) + I2(g) ⇄ 2HI(g) Kp = 54.0 -4 N2(g) + 3 H2(g) ⇄ 2 NH3(g) Kp = 1.04 x 10 determine the value of Kp (at 700 K)

2 NH3(g) + 3 I2(g) ⇄ 6 HI(g) + N2(g) (1.51 x 109)

Write the equilibrium-constant Kc for each of the following reactions: a) CO2(g) + H2(g) ⇄ CO(g) + H2O(l) Kc =

b) SnO2(s) + 2 CO(g) ⇄ Sn(s) + 2 CO2(g) Kc =

Write the equilibrium-constant expressions for each of the following reactions: + 3+ a) Cr(s) + 3 Ag (aq) ⇄ Cr (aq) + 3 Ag(s) Kc =

b) 3 Fe(s) + 4 H2O(g) ⇄ Fe3O4(s) + 4 H2(g) Kp =

Each of the following mixtures was placed in a closed container and allowed to stand. Which of these mixtures is capable of attaining the equilibrium

CaCO3(s) ⇄ CaO(s) + CO2(g)

a) pure CaCO3

b) CaO and a pressure of CO2 greater than the value of Kp

c) Some CaCO3 and a pressure of CO2 greater than the value of Kp

d) CaCO3 and CaO

When added to Fe3O4(s) in a closed container, which one of the following substances – H2(g), H2O(g), O2(g) - will allow equilibrium to be established in the reaction

3 Fe(s) + 4 H2O(g) ⇄ Fe3O4(s) + 4 H2(g)

A mixture of hydrogen and nitrogen in a reaction vessel is allowed to attain equilibrium at 472oC. The equilibrium mixture of gases was analyzed and found to contain 7.38 atm H2, 2.46 atm N2, and 0.166 atm NH3. From these data calculate the equilibrium constant, Kp, for

N2(g) + 3 H2(g) ⇄ 2 NH3(g)

(2.79 x 10-5)

- 5 - AP Chemistry Chapter 15 Equilibrium Practice Exercise 15.8

An aqueous solution of acetic acid is found to have the following equilibrium concentrations at 25oC: -2 + -4 - -4 [HC2H3O2] = 1.65 x 10 M; [H ] = 5.44 x 10 M; and [C2H3O2 ] = 5.44 x 10 M. Calculate the o equilibrium constant, Kc, for the ionization of acetic acid at 25 C. The reaction is

+ - HC2H3O2(aq) ⇄ H (aq) + C2H3O2 (aq)

(1.79 x 10-5)

-3 -3 o A closed system initially containing 1.000 x 10 M H2 and 2.000 x 10 M I2 at 448 C is allowed to reach equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87 x 10-3 M. o Calculate Kc at 448 C for the reaction taking place, which is

H2(aq) + I2(g) ⇄ 2 HI(g)

(1.81 x 10-5)

Sulfur trioxide decomposes at high temperature in a sealed container:

2 SO3(g) ⇄ 2 SO2(g) + O2(g)

Initially the vessel is charged at 1000 K with SO3(g) at a partial pressure of 0.500 atm. At equilibrium the SO3 partial pressure is 0.200 atm. Calculate the value of Kp at 1000 K.

(0.338)

Sample Exercise 15.10 (p. 645) o At 448 C the equilibrium constant, Kc, for the reaction H2(g) + I2(g)  2 HI(g) is 51.

Predict how the reaction will proceed to reach equilibrium at 448oC if we start with 2.0 x 10-2 mol of HI, -2 -2 1.0 x 10 mole of H2, and 3.0 x 10 mol of I2 in a 2.00-L container. (Q = 1.3, so reaction must proceed from left to right)

At 1000 K the value of Kp for the reaction 2 SO3(g) ⇄ 2 SO2(g) + O2(g) is 0.338. Calculate the value for Qp, and predict the direction in which the reaction will proceed toward equilibrium if the initial partial pressures of reactants are PSO3 = 0.16 atm; PSO2 = 0.41 atm; PO2 = 2.5 atm.

(Qp = 16; Qp > Kp, so reaction will proceed from right to left)

Sample Exercise 15.11 (p. 646) -5 o For the Haber process, N2(g) + 3 H2(g) ⇄ 2 NH3(g), Kp = 1.45 x 10 at 500 C. In an equilibrium mixture of o the three gases at 500 C, the partial pressure of H2 is 0.928 atm and that of N2 is 0.432 atm. What is the partial pressure of NH3 in this equilibrium mixture?

(2.24 x 10-3 atm)

At 500 K the reaction PCl5(g) ⇄ PCl3(g) + Cl2(g) has Kp = 0.497. In an equilibrium mixture at 500 K, the partial pressure of PCl5 is 0.860 atm and that of PCl3 is 0.350 atm. What is the partial pressure of Cl2 in the equilibrium mixture?

(1.22 atm)

o A 1.000-L flask is filled with 1.000 mol of H2 and 2.000 mol of I2 at 448 C. The value of the equilibrium constant, Kc, for the reaction

H2(g) + I2(g) ⇄ 2 HI(g)

o at 448 C is 50.5. What are the partial pressures of H2, I2, and HI in the flask at equilibrium?

([H2] = 0.065 M, [I2] = 1.065 M, [HI] = 1.87 M)

For the equilibrium, PCl5(g) ⇄ PCl3(g) + Cl2(g), the equilibrium constant, Kp, has the value 0.497 at 500 K. A gas cylinder at 500 K is charged with PCl5(g) at an initial pressure of 1.66 atm. What are the equilibrium pressures of PCl5, PCl3, and Cl2 at this temperature?

(PPCl5 = 0.967 atm, PPCl3 = PCl2 = 0.693 atm)

Sample Exercise 15.13 (p. 653) Consider the following equilibrium: o N2O4(g) ⇄ 2 NO2(g) H = 58.0 kJ In what direction will the equilibrium shift when each of the following changes is made to a system at equilibrium:

a) add N2O4

b) remove NO2

c) increase the total pressure by adding N2(g)

d) increase the volume

e) decrease the temperature?

Practice Exercise 15.13 For the reaction o PCl5(g) ⇄ PCl3(g) + Cl2(g) H = 87.9 kJ in what direction will the equilibrium shift when

a) Cl2(g) is removed;

b) The temperature is decreased;

c) The volume of the reaction system is increased;

d) PCl3(g) is added?

Sample Exercise 15.14 (p. 653) a) Using the standard heat of formation data in Appendix C, determine the standard enthalpy change for the reaction

N2(g) + 3 H2(g) ⇄ 2 NH3(g)

b) Determine how the equilibrium constant for this reaction should change with temperature.

Practice Exercise 15.14 Using the thermodynamic data in Appendix C, determine the enthalpy change for the reaction

2 POCl3(g) ⇄ 2 PCl3(g) + O2(g) Use this result to determine how the equilibrium constant for the reaction should change with temperature.

Sample Integrative Exercise 15

At temperatures near 800oC, steam passed over hot coke (a form of carbon obtained from coal) reacts to form CO and H2: C(s) + H2O(g) ⇄ CO(g) + H2(g) The mixture of gases that results is an important industrial fuel called water gas. o a) At 800 C the equilibrium constant for this reaction is Kp = 14.1. What are the equilibrium partial pressures of H2O, CO, and H2 in the equilibrium mixture at this temperature if we start with solid carbon and 0.100 mol of H2O in a 1.00-L vessel? b) What is the minimum amount of carbon required to achieve equilibrium under these conditions? c) What is the total pressure in the vessel at equilibrium? d) At 25oC the value of Keq for this reaction is 1.7 x 10-21. Is the reaction exothermic or endothermic?

e) To produce the maximum amount of CO and H2 at equilibrium, should the pressure of the system be increased or decreased?

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