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Ch.19 - Free Energy & Thermodynamics
Chapter 19, Problem 55d

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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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Standard Entropy (ΔS°)

Standard entropy (ΔS°) is a measure of the disorder or randomness in a system at standard conditions (1 atm, 25°C). It quantifies the amount of energy that is unavailable to do work due to the dispersal of energy among the particles in a substance. Higher entropy values indicate greater disorder, while lower values suggest more order.
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Entropy Change in Reactions

The change in entropy (ΔS°rxn) for a chemical reaction is calculated by considering the difference in standard entropies of the products and reactants. It is given by the equation ΔS°rxn = ΣS°(products) - ΣS°(reactants). A positive ΔS°rxn indicates an increase in disorder, while a negative value suggests a decrease in disorder during the reaction.
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Rationalizing the Sign of ΔS°rxn

Rationalizing the sign of ΔS°rxn involves analyzing the physical states and the number of moles of reactants and products. Generally, reactions that produce gases from solids or liquids, or increase the number of gas molecules, tend to have a positive ΔS°rxn. Conversely, reactions that produce fewer gas molecules or convert gases to liquids or solids typically exhibit a negative ΔS°rxn.
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