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Ch.19 - Chemical Thermodynamics
Chapter 19, Problem 50

Three of the forms of elemental carbon are graphite, diamond,and buckminsterfullerene. The entropies at 298 Kfor graphite and diamond are listed in Appendix C.(b) What would you expect for the S° value of buckminsterfullerene(Figure 12.49, p. 509) relative to the valuesfor graphite and diamond? Explain.

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Identify the molecular structures of graphite, diamond, and buckminsterfullerene. Graphite consists of layers of carbon atoms arranged in a hexagonal lattice, diamond is a three-dimensional tetrahedral network of carbon atoms, and buckminsterfullerene (C60) is a spherical molecule composed of 60 carbon atoms arranged in a pattern of hexagons and pentagons.
Understand that entropy (S°) is a measure of the randomness or disorder of a system. The more ways the atoms can be arranged without changing the overall structure, the higher the entropy.
Compare the structural rigidity and complexity of the three forms. Diamond has a very rigid and tightly bonded structure, leading to lower entropy. Graphite has more freedom in the layers' sliding over each other, providing slightly higher entropy.
Consider the molecular freedom of buckminsterfullerene. The spherical shape and molecular structure allow for rotation and other molecular motions that are not possible in the rigid lattice of diamond or the planar layers of graphite.
Conclude that the entropy of buckminsterfullerene would likely be higher than that of diamond due to its greater molecular freedom, but potentially similar or slightly higher than graphite depending on the extent of internal molecular motion allowed in its structure.

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Key Concepts

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

Entropy (S°)

Entropy is a measure of the disorder or randomness in a system. It quantifies the number of possible microstates that correspond to a given macrostate. In thermodynamics, higher entropy values indicate greater disorder and more available energy states, which is crucial for understanding the stability and reactivity of substances.
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Allotropes of Carbon

Allotropes are different structural forms of the same element, in this case, carbon. Graphite, diamond, and buckminsterfullerene (C60) are distinct allotropes with unique arrangements of carbon atoms, leading to different physical and chemical properties, including variations in entropy due to differences in molecular structure and bonding.
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Comparative Entropy of Allotropes

When comparing the entropies of different allotropes, one must consider their molecular complexity and structural arrangements. Graphite, with its layered structure, typically has higher entropy than diamond, which has a rigid three-dimensional lattice. Buckminsterfullerene, being a spherical molecule with a unique structure, is expected to have an entropy value that reflects its complexity, likely falling between those of graphite and diamond.
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