Textbook Question
Rank each set of substances in order of increasing standard molar entropy (S°). Explain your reasoning. b. H2O(s); H2O(l); H2O(g)
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Ch.18 - Free Energy and Thermodynamics
Chapter 18, Problem 55b
Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn . b. C(s) + H2O(g) → CO(g) + H2(g)
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Identify the standard entropy values (S°) for each substance involved in the reaction from Appendix IIB.
Write the balanced chemical equation: C(s) + H2O(g) → CO(g) + H2(g).
Use the formula for the standard entropy change of the reaction: ΔS°_{rxn} = ΣS°_{products} - ΣS°_{reactants}.
Calculate the sum of the standard entropy values for the products: S°_{CO(g)} + S°_{H2(g)}.
Calculate the sum of the standard entropy values for the reactants: S°_{C(s)} + S°_{H2O(g)}.
Subtract the sum of the reactants' entropies from the sum of the products' entropies to find ΔS°_{rxn}.
Rationalize the sign of ΔS°_{rxn} by considering the change in the number of gas molecules and the states of matter involved in the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Entropy (ΔS)
Entropy is a measure of the disorder or randomness in a system. In chemical reactions, changes in entropy (ΔS) indicate how the distribution of energy and matter changes. A positive ΔS suggests an increase in disorder, while a negative ΔS indicates a decrease in disorder. Understanding how the states of reactants and products affect entropy is crucial for predicting the sign of ΔS°rxn.
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02:46
Entropy in Thermodynamics
Standard Entropy (S°)
Standard entropy (S°) refers to the absolute entropy of a substance at standard conditions (1 bar, 25°C). Each substance has a specific standard entropy value, which can be found in thermodynamic tables. When calculating ΔS°rxn, the standard entropies of the products and reactants are used to determine the overall change in entropy for the reaction. This concept is essential for quantifying the entropy change in a reaction.
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03:33Standard Molar Entropy
Reaction Direction and Entropy Sign
The sign of ΔS°rxn can often be rationalized by considering the physical states and the number of moles of reactants and products. For example, if a reaction produces more gas molecules than it consumes, it typically results in a positive ΔS°rxn due to increased disorder. In the given reaction, the conversion of solid and gas reactants to gaseous products suggests an increase in entropy, which can help predict the sign of ΔS°rxn.
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Related Practice
Textbook Question
Rank each set of substances in order of increasing standard molar entropy (S°). Explain your reasoning. a. I2(g); F2(g); Br2(g); Cl2(g) b. H2O(g); H2O2(g); H2S(g) c. C(s, graphite); C(s, diamond); C(s, amorphous)
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Open Question
Use data from Appendix IIB to calculate ΔSrxn ° for each of the reactions. In each case, try to rationalize the sign of ΔSrxn ° . a. C2H4(g) + H2(g) → C2H6(g)
Textbook Question
Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn. c. CO(g) + H2O(g) → H2(g) + CO2(g)
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Textbook Question
Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn. d. 2 H2S(g) + 3 O2(g) → 2 H2O(l) + 2 SO2(g)
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Textbook Question
Use data from Appendix IIB to calculate ΔS°rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°rxn . a. 3 NO2(g) + H2O(l) → 2 HNO3(aq) + NO(g)
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