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Ch.19 - Free Energy & Thermodynamics
All textbooksTro 5th EditionCh.19 - Free Energy & ThermodynamicsProblem 56a
Chapter 19, Problem 56a

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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Identify the substances involved in the reaction: 3 NO_2(g) + H_2O(l) → 2 HNO_3(aq) + NO(g).
Use Appendix IIB to find the standard molar entropy (S°) values for each substance: NO_2(g), H_2O(l), HNO_3(aq), and NO(g).
Calculate the total standard entropy of the reactants by summing the products of the stoichiometric coefficients and their respective S° values: (3 * S°(NO_2) + 1 * S°(H_2O)).
Calculate the total standard entropy of the products in a similar manner: (2 * S°(HNO_3) + 1 * S°(NO)).
Determine the standard entropy change for the reaction (ΔS°_rxn) by subtracting the total entropy of the reactants from the total entropy of the products: ΔS°_rxn = (Total S° of products) - (Total S° of reactants).

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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 as reactants transform into products. A positive ΔS suggests an increase in disorder, while a negative ΔS indicates a decrease in disorder.
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Standard Entropy Values

Standard entropy values (S°) are tabulated values that represent the absolute entropy of a substance at standard conditions (1 atm and 25°C). These values are essential for calculating the change in entropy for a reaction (ΔS°rxn) by using the formula ΔS°rxn = ΣS°products - ΣS°reactants, where the sum is taken over all products and reactants.
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Rationalizing ΔS°rxn Sign

Rationalizing the sign of ΔS°rxn involves analyzing the physical states and the number of moles of reactants and products. Generally, reactions that produce more gas molecules than they consume tend to have a positive ΔS, while those that produce fewer gas molecules or involve a transition from gas to liquid or solid typically have a negative ΔS. Understanding these trends helps predict the entropy change in a reaction.
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Related Practice
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 . b. C(s) + H2O(g) → CO(g) + H2(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)

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)

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. b. Cr2O3(s) + 3 CO(g) → 2 Cr(s) + 3 CO2(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. SO2(g) + 1/2 O2(g) → SO3(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. d. N2O4(g) + 4 H2(g) → N2(g) + 4 H2O(g)