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Ch.19 - Chemical Thermodynamics
All textbooksBrown 14th EditionCh.19 - Chemical ThermodynamicsProblem 45
Chapter 19, Problem 45

In each of the following pairs, which compound would you expect to have the higher standard molar entropy: (a) C2H2(g) or C2H6(g)? (b) CO2(g) or CO(g)?

Verified step by step guidance
1
Step 1: Understand that standard molar entropy (S°) is a measure of the amount of disorder or randomness in a system. Generally, more complex molecules and those with more atoms have higher entropy.
Step 2: For part (a), compare C2H2(g) and C2H6(g). Consider the molecular structure and the number of atoms in each compound. C2H6(g) has more atoms and is more complex than C2H2(g), which suggests it might have higher entropy.
Step 3: For part (b), compare CO2(g) and CO(g). Look at the molecular structure and the number of atoms. CO2(g) has more atoms than CO(g), which typically leads to higher entropy due to increased complexity and more possible microstates.
Step 4: Consider the phase of the compounds. Both pairs are in the gaseous state, so phase does not affect the comparison in this case.
Step 5: Conclude that in both comparisons, the compound with more atoms and greater molecular complexity is expected to have the higher standard molar entropy.

Key Concepts

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

Standard Molar Entropy

Standard molar entropy is a measure of the amount of disorder or randomness in a system at standard conditions (1 bar, 25°C). It reflects the number of accessible microstates for a given amount of substance. Generally, larger and more complex molecules have higher entropies due to increased molecular motion and greater vibrational modes.
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Molecular Complexity

Molecular complexity refers to the size and structure of a molecule, including the number of atoms and the types of bonds present. More complex molecules, such as those with more carbon and hydrogen atoms, typically have higher entropies because they can adopt more configurations and have more ways to distribute energy among their atoms.
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Phase of Matter

The phase of matter significantly influences entropy, with gases generally having higher entropies than liquids and solids. This is due to the greater freedom of movement and higher kinetic energy of gas molecules, which allows for a larger number of microstates compared to more ordered phases.
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Related Practice
Textbook Question

Predict the sign of ΔSsys for each of the following processes: (b) Gaseous Cl2 dissociates in the stratosphere to form gaseous Cl atoms.

Textbook Question

Predict the sign of ΔSsys for each of the following processes: (c) Gaseous CO reacts with gaseous H2 to form liquid methanol, CH3OH.

Textbook Question

Predict the sign of ΔSsys for each of the following processes: (d) Calcium phosphate precipitates upon mixing Ca(NO3)2(aq) and (NH4)3PO4(aq).

Textbook Question

Cyclopropane and propylene are isomers that both have the formula C3H6. Based on the molecular structures shown, which of these isomers would you expect to have the higher standard molar entropy at 25 °C?