Intermolecular forces from heats of vaporisation: Given ΔHvap (kJ per mol) for CS2 = 26.8, C2H5OH (ethanol) = 38.6, and H2O = 40.6, arrange the liquids in decreasing order of intermolecular forces.

Introduction / Context:
The heat of vaporisation reflects the energy needed to overcome intermolecular forces from liquid to vapour. Larger ΔHvap typically indicates stronger intermolecular attractions. This principle is widely used to rationalise boiling points and solvent properties.

Given Data / Assumptions:

• ΔHvap(H2O) = 40.6 kJ per mol, ΔHvap(EtOH) = 38.6 kJ per mol, ΔHvap(CS2) = 26.8 kJ per mol.
• Similar temperature reference assumed for comparison.
• No association changes between liquids beyond typical behaviour.

Concept / Approach:
Stronger intermolecular forces require more energy to vaporise a liquid. Water exhibits strong hydrogen bonding, ethanol has hydrogen bonding but weaker than water, while carbon disulfide is nonpolar with weaker dispersion forces, giving the smallest ΔHvap among the three.

Step-by-Step Solution:

Verification / Alternative check:
Boiling points correlate: H2O boils at 100 °C, ethanol near 78 °C, CS2 near 46 °C at 1 atm—consistent with the order of intermolecular forces.

Why Other Options Are Wrong:

• Any order placing CS2 above ethanol or water contradicts the given ΔHvap values and known polarity considerations.

Common Pitfalls:
Confusing polarity with hydrogen bonding strength, and ignoring that nonpolar liquids usually have lower ΔHvap unless very large molar masses are involved.

Final Answer:
H2O > C2H5OH > CS2