Question

The second law of thermodynamics states that for any spontaneous process:

• A. Entropy of the system decreases
• B. Entropy of the universe increases
• C. Entropy of the universe remains constant
• D. Enthalpy of the system increases

Hint:

Always distinguish between the system and the universe.
The entropy of a system can increase, decrease, or remain constant, but the entropy of the universe can never decrease in any real process.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the fundamental statement of the Second Law of Thermodynamics regarding the entropy changes during a spontaneous (natural/irreversible) process.
This is a core concept of classical thermodynamics.

Step 2: Key Formula or Approach:
The universe is defined as the combination of the system under study and its surroundings:
\[ \Delta S_{\text{universe}} = \Delta S_{\text{system}} + \Delta S_{\text{surroundings}} \]
The Second Law states that all natural processes tend to move in a direction that increases disorder.

Step 3: Detailed Explanation:

• Mathematical Statement of Second Law:
For any real, spontaneous, and irreversible process:
\[ dS_{\text{universe}} \gt 0 \]
For an idealized, completely reversible process:
\[ dS_{\text{universe}} = 0 \]
Combining these, we get Clausius inequality:
\[ dS_{\text{universe}} \ge 0 \]

• System vs Universe: The entropy of a system can decrease during a process (e.g., water freezing into ice), but this requires heat transfer to the surroundings, which increases the surroundings' entropy by an even larger amount.
As a result, the total entropy of the universe must increase for any spontaneous process.


Step 4: Final Answer:
Therefore, the Second Law of Thermodynamics asserts that for any spontaneous process, the entropy of the universe increases.