Question

For which one of the following reactions will there be a positive \(\Delta S\)?

• A. \(H_2O(g) \rightarrow H_2O(l)\)
• B. \(H_2 + I_2 \rightarrow 2HI\)
• C. \(CaCO_3(s) \rightarrow CaO(s) + CO_2(g)\)
• D. \(N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)\)

Hint:

Entropy increases when gases are produced or number of gas moles increases. Solid \(\rightarrow\) gas reactions usually give positive \(\Delta S\).

Answer 1

The Correct Option is C

Step 1: Understand entropy change.
Entropy increases when disorder increases.
Formation of gas molecules generally increases entropy greatly.
Step 2: Check each reaction.
(A) Gas \(\rightarrow\) liquid reduces randomness, so \(\Delta S<0\).
(B) Gas + gas \(\rightarrow\) gas, moles remain same, change small (often near zero).
(C) Solid \(\rightarrow\) solid + gas increases disorder due to gas formation, so \(\Delta S>0\).
(D) 4 moles gas \(\rightarrow\) 2 moles gas reduces randomness, so \(\Delta S<0\).
Step 3: Therefore reaction with gas production is correct.
Final Answer: \[ \boxed{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)} \]