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Ch.18 - Free Energy and Thermodynamics
Tro - Chemistry: A Molecular Approach 4th Edition
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Capitolo 18, Problema 35b

Without doing any calculations, determine the sign of ΔSsys for each chemical reaction. b. CH2=CH2( g) + H2( g) → CH3CH3( g)

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Identify the number of moles of gas molecules on both sides of the reaction. On the left side, there is one mole of CH2=CH2 and one mole of H2, totaling two moles of gas. On the right side, there is one mole of CH3CH3.
Recognize that the entropy of a system, ΔS, is related to the number of ways particles can be arranged and the number of energy states they can occupy. More moles of gas generally mean higher entropy because there are more particles to arrange and more energy states available.
Compare the total moles of gas before and after the reaction. The reaction goes from two moles of gas on the reactants side to one mole of gas on the products side.
Conclude that the decrease in the number of gas moles leads to fewer available microstates for the system, thus reducing the system's entropy.
Determine the sign of ΔSsys based on the change in entropy. Since the entropy decreases, ΔSsys is negative.

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Entropy (ΔS)

Entropy, denoted as ΔS, is a measure of the disorder or randomness in a system. In chemical reactions, an increase in the number of gas molecules or a transition from a more ordered state to a less ordered state typically results in a positive ΔS, indicating greater disorder. Conversely, a decrease in the number of gas molecules or a transition to a more ordered state results in a negative ΔS.
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Entropy in Thermodynamics

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. Understanding the stoichiometry helps in predicting changes in the number of molecules during a reaction. In the given reaction, analyzing the number of gaseous reactants and products is crucial for determining the sign of ΔSsys.
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Stoichiometry Concept

Phase and State Changes

The phase and state of matter play a significant role in entropy changes. Gaseous states generally have higher entropy than liquids or solids due to their greater freedom of movement. In the reaction provided, recognizing that all species are gases allows for a clearer assessment of how the reaction affects the overall disorder of the system.
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Entropy in Phase Changes
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