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Ch.19 - Chemical Thermodynamics
All textbooksBrown 14th EditionCh.19 - Chemical ThermodynamicsProblem 61
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Chapter 19, Problem 61

Without using thermochemical data, predict whether ΔG° for the reaction involving octane (1C8H182) is more negative or less negative than ΔH°.

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Step 1: Understand the relationship between Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) using the equation ΔG° = ΔH° - TΔS°, where T is the temperature in Kelvin.
Step 2: Consider the nature of the reaction involving octane. Combustion reactions, such as the burning of octane, typically result in an increase in entropy (ΔS° > 0) because gases are produced from liquids and solids.
Step 3: Recognize that if ΔS° is positive, the term -TΔS° will be negative, which will make ΔG° more negative than ΔH° at higher temperatures.
Step 4: Analyze the implications: since combustion reactions are exothermic (ΔH° < 0) and increase entropy, the negative contribution of -TΔS° will make ΔG° more negative than ΔH°.
Step 5: Conclude that for the combustion of octane, ΔG° is expected to be more negative than ΔH° due to the positive entropy change and the exothermic nature of the reaction.
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Related Practice
Open Question
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)
Textbook Question

Use data in Appendix C to calculate ΔH°, ΔS°, and ΔG° at 25 °C for each of the following reactions.

c. 2  P(s) + 10  HF(g) → 2  PF5(g) + 5  H2(g)

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Textbook Question

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)

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Textbook Question

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°.

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Textbook Question

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

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Textbook Question

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

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