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Ch.18 - Free Energy and Thermodynamics
All textbooksTro 4th EditionCh.18 - Free Energy and ThermodynamicsProblem 52
Chapter 18, Problem 52

For each pair of substances, choose the one that you expect to have the higher standard molar entropy (S°) at 25 °C. Explain your choices. a. NaNO3(s); NaNO3(aq) b. CH4(g); CH3CH3(g) c. Br2(l); Br2(g) d. Br2(g); F2(g) e. PCl3(g); PCl5(g) f. CH3CH2CH2CH3(g); SO2(g)

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Step 1: Understand that standard molar entropy (S°) is a measure of the amount of disorder or randomness in a system. Generally, gases have higher entropy than liquids, which have higher entropy than solids. Additionally, more complex molecules or those with more atoms tend to have higher entropy.
Step 2: For part a, compare NaNO3(s) and NaNO3(aq). The aqueous solution (NaNO3(aq)) will have higher entropy than the solid (NaNO3(s)) because the ions are more dispersed in solution, increasing disorder.
Step 3: For part b, compare CH4(g) and CH3CH3(g). CH3CH3(g) is a larger molecule with more atoms than CH4(g), which generally leads to higher entropy due to increased molecular complexity and more possible microstates.
Step 4: For part c, compare Br2(l) and Br2(g). Br2(g) will have higher entropy than Br2(l) because gases have more freedom of movement and greater disorder than liquids.
Step 5: For part d, compare Br2(g) and F2(g). Although both are gases, Br2(g) is a larger molecule with more electrons than F2(g), which typically results in higher entropy due to more vibrational modes and possible microstates.

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Key Concepts

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

Standard Molar Entropy (S°)

Standard molar entropy (S°) is a measure of the amount of disorder or randomness in a system at a standard state, typically 25 °C and 1 atm. It reflects the number of accessible microstates for a substance, with higher values indicating greater disorder. Entropy increases with temperature, molecular complexity, and the phase of the substance, with gases generally having higher entropies than liquids and solids.
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Phase Changes and Entropy

The phase of a substance significantly affects its entropy. Gases have higher entropy than liquids, which in turn have higher entropy than solids due to the increased freedom of movement and greater number of microstates available in gaseous states. When comparing substances in different phases, the one in the gaseous state will typically have a higher standard molar entropy than its liquid or solid counterpart.
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Molecular Complexity and Entropy

Molecular complexity also influences entropy; larger and more complex molecules tend to have higher entropies than simpler ones. This is because complex molecules can adopt more configurations and have more vibrational modes. When comparing substances, those with more atoms or greater structural diversity will generally exhibit higher standard molar entropy values.
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Related Practice
Textbook Question

Predict the conditions (high temperature, low temperature, all temperatures, or no temperatures) under which each reaction is spontaneous. a. H2O(g) → H2O(l) b. CO2(s) → CO2(g) c. H2(g) → 2 H(g) d. 2 NO2(g) → 2 NO(g) + O2(g) (endothermic)

Textbook Question

How does the molar entropy of a substance change with increasing temperature?

Textbook Question

For each pair of substances, choose the one that you expect to have the higher standard molar entropy (S°) at 25 °C. Explain your choices. a. CO(g); CO2(g) b. CH3OH(l); CH3OH(g) c. Ar(g); CO2(g) d. CH4(g); SiH4(g) e. NO2(g); CH3CH2CH3(g) f. NaBr(s); NaBr(aq)

Textbook Question

Rank each set of substances in order of increasing standard molar entropy (S°). Explain your reasoning. a. NH3(g); Ne(g); SO2(g); CH3CH2OH(g); He(g) c. CH4(g); CF4(g); CCl4(g)

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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)

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)