(a) In a chemical reaction two gases combine to form a...

(a) In a chemical reaction two gases combine to form a solid. What do you expect for the sign of $\Delta S ?$ (b) How does the entropy of the system change in the processes described in Exercise $19.12 ?$

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Key Concepts

Entropy (Disorder)

Entropy is a fundamental thermodynamic variable that quantifies the amount of disorder or randomness in a system. A system where particles can occupy many microstates (such as a gas) has a high entropy, while a system constrained to fewer microstates (such as a solid) has a lower entropy.

Phase Changes and Entropy

Phase changes involve significant alterations in the entropy of a system. When substances change from one phase to another—for instance, from a gas to a solid—the molecular freedom decreases dramatically, leading to a decrease in entropy.

Entropy Change in Chemical Reactions

Chemical reactions can involve changes in entropy depending on the states and number of reactants and products. Reactions in which multiple high-entropy species (like gases) combine to form a low-entropy product (like a solid) typically exhibit a negative change in system entropy.

Second Law of Thermodynamics

The second law of thermodynamics establishes that the total entropy of an isolated system can never decrease over time. This law explains why spontaneous processes tend toward states of higher overall disorder, even if the entropy of a specific part of the system, such as the products of a reaction, decreases.

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