Question

which reaction results in a negative δs? 2hgo(s)→2hg(l) + o₂(g) nacl(s)→na⁺(aq) + cl⁻(aq) h₂o(l)→h₂o(s)

Explanation:

To determine which reaction has a negative \(\Delta S\) (entropy change), we analyze the change in disorder (entropy) for each reaction:

Step 1: Recall Entropy Trends

Entropy (\(S\)) generally increases with:

• Phase changes: \( \text{solid}

ightarrow \text{liquid}
ightarrow \text{gas} \) (more disorder).

• Formation of more moles of gas (or particles) from fewer.
• Dissolution (solids dissolving into ions in solution, increasing disorder).

Step 2: Analyze Each Reaction

1. **\( 2\text{HgO}(s)

ightarrow 2\text{Hg}(l) + \text{O}_2(g) \)**

• Reactant: solid (\( \text{HgO} \)).
• Products: liquid (\( \text{Hg} \)) and gas (\( \text{O}_2 \)).
• Phase change: solid → liquid + gas (more disorder).
• Moles of gas: 0 → 1 (increase).
• \(\Delta S > 0\) (entropy increases).

1. **\( \text{NaCl}(s)

ightarrow \text{Na}^+(aq) + \text{Cl}^-(aq) \)**

• Reactant: solid (\( \text{NaCl} \)).
• Products: aqueous ions (\( \text{Na}^+ \), \( \text{Cl}^- \)).
• Dissolution: solid → ions in solution (more disorder).
• \(\Delta S > 0\) (entropy increases).

1. **\( \text{H}_2\text{O}(l)

ightarrow \text{H}_2\text{O}(s) \)**

• Reactant: liquid (\( \text{H}_2\text{O}(l) \)).
• Product: solid (\( \text{H}_2\text{O}(s) \)).
• Phase change: liquid → solid (less disorder, more ordered).
• \(\Delta S < 0\) (entropy decreases).

Answer:

\(\boldsymbol{\text{H}_2\text{O}(l)
ightarrow \text{H}_2\text{O}(s)}\) (the third option)