Justifying Properties with Intermolecular Forces

Step one: state the structural feature, such as an O-H group or a long carbon chain. Step two: name the intermolecular force that feature produces and compare its strength with the other substance. Step three: state the property consequence. Skipping any step leaves the argument incomplete.

For a comparison, name both substances and a shared condition. “Ethanol has hydrogen bonding” is an observation about one liquid, not yet an explanation of why it boils above dimethyl ether. Add that ether cannot donate conventional hydrogen bonds to itself, that their sizes are comparable, and that separating ethanol molecules requires more energy. Avoid saying an oxygen atom itself “has hydrogen bonding”; the interaction exists between a donor and an acceptor partner.

Stronger cohesive attractions generally raise boiling point, heat of vaporization, viscosity, and surface tension, and lower vapor pressure, when other factors are reasonably comparable. Melting point is less predictable from IMF strength alone because crystal packing and symmetry also matter. Volatile means a substance evaporates readily and has relatively high vapor pressure.

Vapor pressure is an equilibrium property in a closed container containing both liquid and vapor. Particles continually evaporate and condense; at equilibrium the rates match. At the same temperature, stronger cohesive attractions generally favor a smaller vapor population and lower vapor pressure. Heating raises vapor pressure by increasing the population able to escape. Compare liquids at the same temperature, since temperature can otherwise dominate the result.

Normal boiling point is the temperature where vapor pressure reaches 1 atm. A lower-vapor-pressure liquid generally needs more heating to reach that condition. Viscosity measures resistance to flow, while surface tension describes the energy cost per added surface area; both can increase with cohesive forces, but shape and temperature also matter. Melting point is especially sensitive to crystal packing.

Dissolving requires separating solute particles and solvent molecules, then forming new solute–solvent attractions. Both the energy changes and the entropy change matter, so “like dissolves like” is a useful first clue, not a complete explanation. Water hydrates Na⁺ and Cl⁻ ions strongly, offsetting much of the energy needed to separate the crystal; NaCl dissolution is slightly endothermic near room temperature, yet mixing increases entropy and dissolution is favorable. Hexane cannot stabilize separated ions effectively, so NaCl does not dissolve appreciably in it.

Build a dissolution argument in three energy steps: separate some solute particles, separate some solvent particles, then form new solute–solvent attractions. The first two cost energy and the third releases it. Favorable mixing can still occur when the net enthalpy change is positive because entropy also matters. A polar solute need not dissolve readily in every polar solvent; an ionic lattice can be too costly to separate, or the competing interactions can be unfavorable. “Like dissolves like” is a screening rule, not a complete thermodynamic calculation.

When size, polarity and shape all change substantially, a unique ranking may require data. A defensible explanation states which factor is being controlled and which evidence supports the final property comparison.