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Ch.19 - Chemical Thermodynamics
All textbooksBrown 14th EditionCh.19 - Chemical ThermodynamicsProblem 112a
Chapter 19, Problem 112a

When most elastomeric polymers (e.g., a rubber band) are stretched, the molecules become more ordered, as illustrated here:
Suppose you stretch a rubber band. (a) Do you expect the entropy of the system to increase or decrease?

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1
Understand the concept of entropy, which is a measure of the disorder or randomness in a system.
Recognize that when the rubber band is unstretched, the polymer molecules are in a tangled, disordered state.
Consider that stretching the rubber band aligns the polymer molecules, making them more ordered compared to their initial state.
Recall that an increase in order corresponds to a decrease in entropy according to the second law of thermodynamics.
Conclude whether the entropy of the rubber band increases or decreases based on the change in molecular order when stretched.

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Key Concepts

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

Entropy

Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it quantifies the number of possible arrangements of particles in a system. A higher entropy indicates a more disordered state, while lower entropy suggests a more ordered state. Understanding how entropy changes during physical processes is crucial for predicting the behavior of materials, such as elastomers.
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Molecular Order in Polymers

Polymers, like rubber, consist of long chains of repeating units. When stretched, the molecular chains align more closely, leading to increased order within the material. This alignment reduces the freedom of movement of the polymer chains, which can affect the overall entropy of the system. Recognizing how stretching influences molecular arrangement is key to understanding the thermodynamic properties of elastomers.
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Thermodynamic Principles

Thermodynamic principles govern the behavior of energy and matter in systems. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. When analyzing the stretching of a rubber band, it is essential to consider how energy input (stretching) affects the system's entropy and whether the overall process leads to an increase or decrease in entropy.
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Related Practice
Textbook Question

The reaction SO2(g) + 2 H2S(g) ⇌ 3 S(s) + 2 H2O(g) is the basis of a suggested method for removal of SO2 from power-plant stack gases. The standard free energy of each substance is given in Appendix C. (b) In principle, is this reaction a feasible method of removing SO2?

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Textbook Question

The reaction SO2(g) + 2 H2S(g) ⇌ 3 S(s) + 2 H2O(g) is the basis of a suggested method for removal of SO2 from power-plant stack gases. The standard free energy of each substance is given in Appendix C. (c) If PSO2 = PH2S and the vapor pressure of water is 25 torr, calculate the equilibrium SO2 pressure in the system at 298 K.

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Textbook Question

The reaction SO2(g) + 2 H2S(g) ⇌ 3 S(s) + 2 H2O(g) is the basis of a suggested method for removal of SO2 from power-plant stack gases. The standard free energy of each substance is given in Appendix C. (d) Would you expect the process to be more or less effective at higher temperatures?

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Textbook Question
Hydrogen gas has the potential for use as a clean fuel in reaction with oxygen. The relevant reaction is 2 H21g2 + O21g2 ¡ 2 H2O1l2 Consider two possible ways of utilizing this reaction as an electrical energy source: (i) Hydrogen and oxygen gases are combusted and used to drive a generator, much as coal is currently used in the electric power industry; (ii) hydrogen and oxygen gases are used to generate electricity directly by using fuel cells that operate at 85 °C. (a) Use data in Appendix C to calculate ∆H° and ∆S° for the reaction. We will assume that these values do not change appreciably with temperature.
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