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- The standard entropies at 298 K for certain group 4A elements are: C(s, diamond) = 2.43 J>mol@K, Si1s2 = 18.81 J>mol@K, Ge1s2 = 31.09 J>mol@K, and Sn1s2 = 51.818 J>mol@K. All but Sn have the same (diamond) structure. How do you account for the trend in the S° values?
Problem 49
- Three of the forms of elemental carbon are graphite, diamond, and buckminsterfullerene. The entropies at 298 K for graphite and diamond are listed in Appendix C. (b) What would you expect for the S° value of buckminsterfullerene (Figure 12.49, p. 509) relative to the values for graphite and diamond? Explain.
Problem 50
Problem 51
Using S° values from Appendix C, calculate ΔS° values for the following reactions. In each case, account for the sign of ΔS°.
(a) C2H4(g) + H2(g) → C2H6(g)
(b) N2O4(g) → 2 NO2(g)
(c) Be(OH)2(s) → BeO(s) + H2O(g)
(d) 2 CH3OH(g) + 3 O2(g) ⟶ 2 CO2(g) + 4 H2O(g)
- Calculate ΔS° values for the following reactions by using tabulated S° values from Appendix C. In each case, explain the sign of ΔS°. (a) HNO3(g) + NH3(g) → NH4NO3(s) (b) 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g) (c) CaCO3(s, calcite) + 2 HCl(g) → CaCl2(s) + CO2(g) + H2O(l) (d) 3 C2H6(g) → C6H6(l) + 6 H2(g)
Problem 52
Problem 53a
(a) For a process that occurs at constant temperature, does the change in Gibbs free energy depend on changes in the enthalpy and entropy of the system?
Problem 53b
(b) For a certain process that occurs at constant T and P, the value of ΔG is positive. Is the process spontaneous?
- (a) Is the standard free-energy change, ΔG°, always larger than ΔG? (b) For any process that occurs at constant temperature and pressure, what is the significance of ΔG = 0? (c) For a certain process, ΔG is large and negative. Does this mean that the process necessarily has a low activation barrier?
Problem 54
Problem 55b
For a certain chemical reaction, ΔH° = -35.4 kJ and ΔS° = -85.5 J/K. (b) Does the reaction lead to an increase or decrease in the randomness or disorder of the system?
Problem 55c,d
For a certain chemical reaction, ΔH° = -35.4 kJ and ΔS° = -85.5 J/K. (c) Calculate ΔG° for the reaction at 298 K. (d) Is the reaction spontaneous at 298 K under standard conditions?
- A certain reaction has ΔH° = +23.7 kJ and ΔS° = +52.4 J>K. (c) Calculate ΔG° for the reaction at 298 K. (d) Is the reaction spontaneous at 298 K under standard conditions?
Problem 56
- Using data in Appendix C, calculate ΔH°, ΔS°, and ΔG° at 298 K for each of the following reactions. (a) H₂(g) + F₂(g) → 2 HF(g) (b) C(s, graphite) + 2 Cl₂(g) → CCl₄(g) (c) 2 PCl₃(g) + O₂(g) → 2 POCl₃(g) (d) 2 CH₃OH(g) + H₂(g) → C₂H₆(g) + 2 H₂O(g)
Problem 57
Problem 58
Use data in Appendix C to calculate ΔH°, ΔS°, and ΔG° at 25 °C for each of the following reactions.
a. 4 Cr(s) + 3 O2(g) → 2 Cr2O3(s)
b. BaCO3(s) → BaO(s) + CO2(g)
c. 2 P(s) + 10 HF(g) → 2 PF5(g) + 5 H2(g)
d. K(s) + O2(g) → KO2(s)
Problem 59
Using data from Appendix C, calculate ΔG° for the following reactions. Indicate whether each reaction is spontaneous at 298 K under standard conditions.
(a) 2 SO2(g) + O2(g) → 2 SO3(g)
(b) NO2(g) + N2O(g) → 3 NO(g)
(c) 6 Cl2(g) + 2 Fe2O3(s) → 4 FeCl3(s) + 3 O2(g)
(d) SO2(g) + 2 H2(g) → S(s) + 2 H2O(g)
Problem 60
Using data from Appendix C, calculate the change in Gibbs free energy for each of the following reactions. In each case, indicate whether the reaction is spontaneous at 298 K under standard conditions.
(a) 2 Ag(s) + Cl2(g) → 2 AgCl(s)
(b) P4O10(s) + 16 H2(g) → 4 PH3(g) + 10 H2O(g)
(c) CH4(g) + 4 F2(g) → CF4(g) + 4 HF(g)
(d) 2 H2O2(l) → 2 H2O(l) + O2(g)
- Without using thermochemical data, predict whether ΔG° for the reaction involving octane (1C8H182) is more negative or less negative than ΔH°.
Problem 61
- Sulfur dioxide reacts with strontium oxide as follows: SO2(g) + SrO(g) → SrSO3(s). (b) If you had only standard enthalpy data for this reaction, how would you estimate the value of ΔG° at 298 K, using data from Appendix C on other substances?
Problem 62
Problem 62a
Sulfur dioxide reacts with strontium oxide as follows: SO2(g) + SrO(g) → SrSO3(s) (a) Without using thermochemical data, predict whether ΔG° for this reaction is more negative or less negative than ΔH°.
Problem 63a,b
Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontanous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T; (iv) spontaneous at high T but not spontaneous at low T.
(a) N2(g) + 3 F2(g) → 2 NF3(g) ΔH° = -249 kJ; ΔS° = -278 J/K
(b) N2(g) + 3 Cl2(g) → 2 NCl3(g) ΔH° = 460 kJ; ΔS° = -275 J/K
Problem 63c
Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontaneous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T; (iv) spontaneous at high T but not spontaneous at low T.
(c) N2F4(g) ⟶ 2 NF2(g) ΔH° = 85 kJ; ΔS° = 198 J/K
Problem 64
From the values given for ΔH° and ΔS°, calculate ΔG° for each of the following reactions at 298 K. If the reaction is not spontaneous under standard conditions at 298 K, at what temperature (if any) would the reaction become spontaneous?
a. 2 PbS(s) + 3 O2(g) → 2 PbO(s) + 2 SO2(g) ΔH° = −844 kJ; ΔS° = −165 J/K
b. 2 POCl3(g) → 2 PCl3(g) + O2(g) ΔH° = 572 kJ; ΔS° = 179 J/K
- A particular constant-pressure reaction is barely spontaneous at 390 K. The enthalpy change for the reaction is +23.7 kJ. Estimate ΔS for the reaction.
Problem 65
Problem 66
A certain constant-pressure reaction is barely nonspontaneous at 45 °C. The entropy change for the reaction is 72 J/K. Estimate ΔH.
- For a particular reaction, ΔH = -32 kJ and ΔS = -98 J>K. Assume that ΔH and ΔS do not vary with temperature. (a) At what temperature will the reaction have ΔG = 0? (b) If T is increased from that in part (a), will the reaction be spontaneous or nonspontaneous?
Problem 67
Problem 68
Reactions in which a substance decomposes by losing CO are called decarbonylation reactions. The decarbonylation of acetic acid proceeds according to: CH3COOH(l) → CH3OH(g) + CO(g) By using data from Appendix C, calculate the minimum temperature at which this process will be spontaneous under standard conditions. Assume that ΔH° and ΔS° do not vary with temperature.
Problem 69a
Consider the following reaction between oxides of nitrogen: NO2(g) + N2O(g) → 3 NO(g) (a) Use data in Appendix C to predict how ΔG for the reaction varies with increasing temperature.
Problem 69b
Consider the following reaction between oxides of nitrogen: NO2(g) + N2O(g) → 3 NO(g) (b) Calculate ΔG at 800 K, assuming that ΔH° and ΔS° do not change with temperature. Under standard conditions is the reaction spontaneous at 800 K?
Problem 69c
Consider the following reaction between oxides of nitrogen: NO2(g) + N2O(g) → 3 NO(g) (c) Calculate ΔG at 1000 K. Is the reaction spontaneous under standard conditions at this temperature?
Problem 70b
Methanol (CH3OH) can be made by the controlled oxidation of methane: CH4(g) + 12 O2(g) → CH3OH(g) (b) Will ΔG for the reaction increase, decrease, or stay unchanged with increasing temperature?
- (a) Use data in Appendix C to estimate the boiling point of benzene, C6H6. (b) Use a reference source, such as the CRC Handbook of Chemistry and Physics, to find the experimental boiling point of benzene.
Problem 71
Problem 72
(a) Using data in Appendix C, estimate the temperature at which the free-energy change for the transformation from I2(s) to I2(g) is zero. (b) Use a reference source, such as Web Elements (www.webelements.com), to find the experimental melting and boiling points of I2. (c) Which of the values in part (b) is closer to the value you obtained in part (a)?