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?
Ch.19 - Chemical Thermodynamics

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Capitolo 19, Problema 42b
Predict the sign of ΔSsys for each of the following processes: (b) Gaseous Cl2 dissociates in the stratosphere to form gaseous Cl atoms.
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Identify the type of process: The dissociation of Cl2 into Cl atoms is a decomposition reaction, where a single molecule breaks down into simpler substances.
Consider the molecular complexity before and after the reaction: Initially, there is one type of molecule, Cl2. After the reaction, there are individual Cl atoms.
Analyze the change in the number of particles: The dissociation of Cl2 results in an increase in the number of gaseous particles. One Cl2 molecule produces two Cl atoms.
Relate the change in particle number to entropy: An increase in the number of gas particles generally leads to an increase in entropy (\(\Delta S > 0\)) because there are more ways to arrange and distribute the energy among the particles.
Predict the sign of \(\Delta S_{\text{sys}}\): Since the number of particles increases, the system's entropy likely increases, indicating that \(\Delta S_{\text{sys}}\) is positive.

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Entropy (ΔS)
Entropy, denoted as ΔS, is a measure of the disorder or randomness in a system. In chemical processes, an increase in the number of gas molecules typically leads to an increase in entropy, as gases have higher disorder compared to solids or liquids. Understanding how changes in the state of matter affect entropy is crucial for predicting the sign of ΔS.
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Dissociation of Gases
Dissociation refers to the process where a compound breaks down into its constituent atoms or simpler molecules. In the case of gaseous Cl<sub>2</sub> dissociating into Cl atoms, the reaction increases the number of gaseous particles in the system. This increase in the number of particles generally contributes to a positive change in entropy (ΔS > 0).
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Percent Dissociation Example
System vs. Surroundings
In thermodynamics, it is essential to distinguish between the system (the part of the universe being studied) and the surroundings (everything else). The change in entropy of the system (ΔS<sub>sys</sub>) reflects the disorder within the system itself. For the dissociation of Cl<sub>2</sub>, the increase in gaseous Cl atoms signifies an increase in the system's entropy, leading to a positive ΔS<sub>sys</sub>.
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Entropy of Surroundings Formula
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