Which of the following reactions would be expected to have the highest entropy change?

Analysis

The question asks us to identify the process that results in the highest positive entropy change ($\Delta S$). Entropy is a measure of the degree of disorder or randomness in a system.

Concept Explanation

• Entropy ($\Delta S$): In thermodynamics, entropy increases as a substance moves from a more ordered state to a more disordered state. • Solid: Particles are tightly packed in a fixed, orderly structure (lowest entropy). • Liquid: Particles are close together but can move past one another (higher entropy than solid). • Gas: Particles are far apart, moving randomly at high speeds (highest entropy).
• Entropy Change ($\Delta S$): • A transition from a more ordered state to a less ordered state results in a positive $\Delta S$ (increase in disorder). • A transition from a less ordered state to a more ordered state results in a negative $\Delta S$ (decrease in disorder).

Step-by-Step Solution

1. Evaluate the options: • A. Liquid → Gas: The substance moves from a state of moderate disorder to a state of high disorder (a large increase in entropy). • B. Solid → Liquid: The substance moves from a state of low disorder to moderate disorder (an increase in entropy, but smaller than liquid to gas). • C. Gas → Liquid: The substance moves from high disorder to moderate disorder (a decrease in entropy). • D. Gas → Solid: The substance moves from high disorder to very low disorder (a large decrease in entropy).

2. Compare the changes: • The transition from Liquid to Gas involves the greatest increase in particle freedom and randomness, as the intermolecular forces are almost entirely overcome. Therefore, this process has the largest positive entropy change.

Conclusion

Exam Tip: Always remember the "Order of Entropy": Gas > Liquid > Solid. If you are moving "up" this ladder (e.g., Solid to Gas), entropy is increasing ($\Delta S > 0$). If you are moving "down" the ladder (e.g., Gas to Solid), entropy is decreasing ($\Delta S < 0$).