AP Chemistry Chapter 19 Chemical Thermodynamics

AP Chemistry Chapter 19 Chemical Thermodynamics

Chapter 19. Chemical Thermodynamics

Sample Exercise 19.1 (p. 803)

Predict whether the following processes are spontaneous as described, are spontaneous in the reverse direction, or are in equilibrium:

a)  When a piece of metal heated to 150oC is added to water at 40oC, the water gets hotter.

b)  Water at room temperature decomposes into H2(g) and O2(g).

c)  Benzene vapor, C6H6(g), at a pressure of 1 atm condenses to liquid benzene at the normal boiling point of benzene, 80.1oC.

Practice Exercise 19.1

Under 1 atm pressure CO2(s) (“dry ice”) sublimes at -78oC. Is the transformation of CO2(s) to CO2(g) a spontaneous process at -100oC and 1 atm pressure?

Sample Exercise 19.2 (p. 807)

The element, Hg, is a silvery liquid at room temperature. The normal freezing point of mercury is -38.9oC, and its molar enthalpy of fusion is DHfusion = 2.29 kJ/mol. What is the entropy change of the system when 50.0 g of Hg(l) freezes at the normal freezing point?

(DSsys = -2.44 J/K)

Practice Exercise 19.2

The normal boiling point of ethanol, C2H5OH, is 78.3oC, and its molar enthalpy of vaporization is 38.56 kJ/mol. What is the change in entropy in the system when 68.3 g of C2H5OH(g) at 1 atm condenses to liquid at the normal boiling point?

(-163 J/K)

Sample Exercise 19.3 (p. 814)

Predict whether DS is positive or negative for each of the following processes, assuming each occurs at constant temperature:

a)  H2O(l) à H2O(g)

b)  Ag+(aq) + Cl-(aq) à AgCl(s)

c)  4 Fe(s) + 3 O2(g) à 2 Fe2O3(s)

d)  N2(g) + O2(g) à 2 NO(g)

Practice Exercise 19.3

Indicate whether each of the following reactions produces an increase or decrease in the entropy of the system:

a)  CO2(s) à CO2(g)

b)  CaO(s) + CO2(g) à CaCO3(s)

c)  HCl(g) + NH3(g) à NH4Cl(s)

d)  2 SO2(g) + O2(g) à 2 SO3(g)

Sample Exercise 19.4 (p. 815)

Choose the sample of matter that has greater entropy in each pair, and explain your choice:

a)  1 mol of NaCl(s) or 1 mol of HCl(g) at 25oC.

b)  2 mol of HCl(g) or 1 mol of HCl(g) at 25oC

c)  1 mol of HCl(g) or 1 mol or Ar(g) at 298 K.

Practice Exercise 19.4

Choose the substance with the greater entropy in each case:

a) 1 mol of H2(g) at STP or 1 mol of H2(g) at 100oC and 0.5 atm

b) 1 mol of H2O(s) at 0oC or 1 mol of H2O(l) at 25oC

c) 1 mol of H2(g) at STP or 1 mol of SO2(g) at STP

d) 1 mol of N2O4(g) at STP or 2 mol of NO2(g) at STP.

Sample Exercise 19.5 (p. 818)

Calculate DSo for the synthesis of ammonia from N2(g) and H2(g) at 298 K.

N2(g) + 3 H2(g) à 2 NH3(g)

(-198.3 J/K)

Practice Exercise 19.5

Using the standard entropies in Appendix C, calculate the standard entropy change, DSo for the following reaction at 298 K:

Al2O3(s) + 3 H2(g) à 2 Al(s) + 3 H2O(g)

(180.39 J/K)

Sample Exercise 19.6 (p. 821)

Calculate the standard free energy change for the formation of NO(g) from N2(g) and O2(g) at 298 K:

N2(g) + O2(g) à 2 NO(g)

Given that DHo = 180.7 kJ and DSo = 24.7 J/K. Is the reaction spontaneous under these circumstances?

Practice Exercise 19.6

A particular reaction has DHo = 24.6 kJ and DSo = 132 J/K at 298 K. Calculate DGo. Is the reaction spontaneous under these conditions?

Sample Exercise 19.7 (p. 823)

a)  Use data from Appendix C to calculate the standard free-energy change for the following reaction at 298 K:

P4(g) + 6 Cl2(g) à 4 PCl3(g)

(-1102.8 kJ)

b)  What is DGo for the reverse of the above reaction?

(+1102.8 kJ)

Practice Exercise 19.7

By using the data from Appendix C, calculate DGo at 298 K for the combustion of methane:

CH4(g) + 2 O2(g) à CO2(g) + 2 H2O(g)

(-800.7 kJ)

Sample Exercise 19.8 (p. 823)

In Section 5.7 we used Hess’s law to calculate DHo for the combustion of propane gas at 298 K:

C3H8(g) + 5 O2(g) à 3 CO2(g) + 4 H2O(l) DHo = -2220 kJ

a)  Without using data from Appendix C, predict whether DGo for this reaction is more negative or less negative than DHo.

b)  Use data from Appendix C to calculate the standard free-energy change for the reaction at 298 K. Is your prediction from part (a) correct? (-2108 kJ)

Practice Exercise 19.8

Consider the combustion of propane to form CO2(g) and H2O(g) at 298 K:

C3H8(g) + 5 O2(g) à 3 CO2(g) + 4 H2O(g) DHo = -2220 kJ

Would you expect DGo to be more negative or less negative than DHo?

Sample Exercise 19.9 (p. 826)

The Haber process for the production of ammonia involves the following equilibrium:

N2(g) + 3 H2(g) D 2 NH3(g)

Assume that DHo and DSo for this reaction do not change with temperature.

a)  Predict the direction in which DGo for this reaction changes with increasing temperature.

b)  Calculate the values of DGo for the reaction at 25oC and 500oC.

(-33.3 kJ, 61 kJ)

Practice Exercise 19.9

a)  Using standard enthalpies of formation and standard entropies in Appendix C, calculate DHo and DSo at 298 K for the following reaction:

2 SO2(g) + O2(g) à 2 SO3(g).

(DHo = -196.6 kJ, DSo = -189.6 J/K)

b)  Using the values obtained in part (a), estimate DGo at 400 K.

(DGo = -120.8 kJ)

DG = DGo + RTlnQ

Sample Exercise 19.10 (p. 827)

As we saw in Section 11.5, the normal boiling point is the temperature at which a pure liquid is in equilibrium with its vapor at a pressure of 1 atm.

a)  Write the chemical equation that defines the normal boiling point of liquid carbon tetrachloride, CCl4(l).

b)  What is the value of DGo for the equilibrium in part (a)?

c)  Use thermodynamic data in Appendix C and DGo = DHo – TDSo to estimate the normal boiling point of CCl4. (70oC)

Practice Exercise 19.10

Use data in Appendix C to estimate the normal boiling point, in K, for elemental bromine, Br2(l).

(The experimental value is given in Table 11.3). (330 K)

Sample Exercise 19.11 (p. 828)

We will continue to explore the Haber process for the synthesis of ammonia:

N2(g) + 3 H2(g) D 2 NH3(g)

Calculate DG at 298 K for a reaction mixture that consists of 1.0 atm N2, 3.0 atm H2, and 0.50 atm NH3.

(-44.9 kJ/mol)

Practice Exercise 19.11

Calculate DG at 298 K for the reaction of nitrogen and hydrogen to form ammonia if the reaction mixture consists of 0.50 atm N2, 0.75 atm H2, and 2.0 atm NH3.

(-26.0 kJ/mol)

Sample Exercise 19.12 (p. 829)

Use standard free energies of formation to calculate the equilibrium constant Keq at 25oC for the reaction involved in the Haber process:

N2(g) + 3 H2(g) D 2 NH3(g)

(7 x 105)

Practice Exercise 19.12

Use data from Appendix C to calculate the standard free-energy change, DGo, and the equilibrium constant, K, at 298 K for the following reaction:

H2(g) + Br2(l) D 2 HBr(g)

(-106.4 kJ/mol; 4 x 1018)

Sample Integrative Exercise 19 (p. 831)

Consider the simple salts NaCl(s) and AgCl(s). We will examine the equilibria in which these salts dissolve in water to form aqueous solutions of ions:

NaCl(s) D Na+(aq) + Cl-(aq)

AgCl(s) D Ag+(aq) + Cl-(aq)

a)  Calculate the value of DGo at 298 K for each of the preceding reactions.

b)  The two values from part (a) are very different. Is this difference primarily due to the enthalpy term or the entropy term of the standard free-energy change?

c)  Use the values of DGo to calculate Ksp values for the two salts at 298 K.

d)  Sodium chloride is considered a soluble salt, whereas silver chloride is considered insoluble. Are these descriptions consistent with the answers to part (c)?

e)  How will DGo for the solution process of these salts change with increasing T? What effect should this change have on the solubility of the salts?

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