(a) In a chemical reaction, two gases combine to form a solid. What do you expect for the sign of ΔS?

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Identify the phase changes in the reaction. In this case, two gases combine to form a solid.

Recall that entropy (ΔS) is a measure of the randomness or disorder of a system. Gases have higher entropy compared to solids because gas particles are more spread out and move more freely.

Understand that when gases transform into a solid, the system becomes more ordered. This decrease in randomness leads to a decrease in entropy.

Conclude that the sign of ΔS for this reaction, where gases form a solid, would be negative because the entropy of the system decreases.

Summarize that the expected sign of ΔS for the reaction is negative, indicating a decrease in entropy as the system changes from a more disordered state (gases) to a more ordered state (solid).

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Key Concepts

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

Entropy (ΔS)

Entropy, denoted as ΔS, is a measure of the disorder or randomness in a system. In chemical reactions, changes in entropy can indicate whether the products are more or less disordered than the reactants. A positive ΔS indicates an increase in disorder, while a negative ΔS indicates a decrease.

Phase Changes and Entropy

The phase of a substance significantly affects its entropy. Gases have higher entropy due to their greater freedom of movement and disorder compared to solids. When gases react to form a solid, the overall disorder of the system decreases, leading to a negative change in entropy (ΔS < 0).

Gibbs Free Energy and Spontaneity

Gibbs Free Energy (ΔG) combines enthalpy and entropy to determine the spontaneity of a reaction. A reaction is spontaneous if ΔG is negative, which can occur even if ΔS is negative, provided that the enthalpy change (ΔH) is sufficiently negative. Understanding the relationship between ΔS and ΔG is crucial for predicting reaction behavior.