Q.13
Trouton’s rule is obeyed by
Options:
(A) hydrogen
(B) methanol
(C) benzene
(D) acetic acid
Trouton’s Rule states that for many non-polar liquids, the molar entropy of vaporization ΔSvap = ΔHvap/Tb at boiling point is approximately constant at 85-88 J/mol·K. This empirical rule applies best to liquids without strong hydrogen bonding or associations.
The correct answer is (C) benzene, a non-polar aromatic hydrocarbon that closely follows the rule with ΔSvap ≈ 89 J/mol·K.
Why (C) Benzene Obeys Trouton’s Rule
Benzene’s weak London dispersion forces allow similar entropy increase during vaporization as other non-polar liquids like toluene or chloroform. Its value matches the standard range, making it a textbook example in physical chemistry for non-associated liquids boiling above ~150 K.
Explanation of All Options
Options deviate based on molecular interactions:
- (A) Hydrogen
Fails due to low boiling point (~20 K) and quantum effects; small molecules have low rotational entropy, violating the rule’s assumptions for normal liquids. - (B) Methanol
Positive deviation from hydrogen bonding; structured liquid phase reduces ΔSvap relative to expectation, exceeding 88 J/mol·K. - (C) Benzene
Obeys perfectly as a non-polar liquid with minimal associations; ΔSvap near 89 J/mol·K confirms compliance. - (D) Acetic acid
Negative deviation; forms dimers even in gas phase, creating order that lowers entropy gain during vaporization.
Quick Comparison Table
| Option | Substance | Key Reason for Obeying/Deviating | ΔSvap Status |
|---|---|---|---|
| A | Hydrogen | Low Tb, quantum effects | Does not obey |
| B | Methanol | H-bonding (positive deviation) | Does not obey |
| C | Benzene | Non-polar, weak forces | Obeys |
| D | Acetic acid | Gas-phase dimers (negative) | Does not obey |
Relate to biotech: Similar entropy rules apply in solvent selection for protein folding studies. Master by calculating ΔSvap for CCl4 (obeys) vs. ethanol (deviates).
