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Ch.19 - Chemical Thermodynamics
Brown - Chemistry: The Central Science 15th Edition
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Capitolo 19, Problema 42c

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

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Identify the initial and final states of the system. Initially, you have gaseous CO and H_2, and the final product is liquid CH_3OH.
Consider the phase change involved. The reactants are in the gaseous state, and the product is in the liquid state. Generally, gases have higher entropy than liquids.
Recognize that the number of moles of gas decreases during the reaction. You start with two moles of gas (CO and H_2) and end with one mole of liquid (CH_3OH).
Understand that a decrease in the number of gas molecules typically leads to a decrease in entropy (ΔS_sys < 0).
Conclude that the entropy of the system decreases because the reaction results in fewer gas molecules and a phase change from gas to liquid.

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Entropy (ΔS)

Entropy, represented as ΔS, is a measure of the disorder or randomness in a system. In chemical reactions, changes in entropy can indicate whether a process is spontaneous. A positive ΔS suggests an increase in disorder, while a negative ΔS indicates a decrease in disorder.
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Entropy in Thermodynamics

Phase Changes and Entropy

Phase changes significantly affect entropy. Transitioning from gas to liquid typically results in a decrease in entropy because gases have higher disorder compared to liquids. In the given reaction, gaseous reactants forming a liquid product suggests a reduction in the system's entropy.
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Entropy in Phase Changes

Reaction Direction and Spontaneity

The direction of a chemical reaction and its spontaneity can be influenced by changes in entropy and enthalpy. For the reaction of gaseous CO and H2 to form liquid methanol, the decrease in entropy (negative ΔS) indicates that the process is less favorable in terms of disorder, which can affect the overall spontaneity of the reaction.
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Spontaneity of Processes
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