Intermolecular Forces and Melting/Boiling Points

The melting and boiling points of a substance reflect the strength of the attractions between its particles: the stronger the intermolecular forces, the more energy is needed to separate the particles, and the higher the melting and boiling points. In brief: for substances of similar molar mass, boiling points rise in the order nonpolar < polar < hydrogen-bonded, as in butane (−0.5 °C), acetone (56 °C) and propan-1-ol (97 °C).

Melting and boiling points are among the first properties that chemists measure to characterize a substance, and they depend on the type of particles and the forces between them. This article shows how to rank substances by their intermolecular forces, compares molecular, ionic, metallic and network solids, and gives a step-by-step method with examples.

How do intermolecular forces control phase changes?

When a solid melts or a liquid boils, the particles gain enough energy to overcome the attractions that hold them in place. In a molecular substance only the weak intermolecular forces are overcome, and the covalent bonds within each molecule are left intact. When water boils, H₂O molecules separate from each other, but they are still H₂O molecules in the gas phase. For this reason, molecular substances typically have low melting and boiling points. In an ionic, metallic or network covalent solid, strong bonds throughout the structure must be broken, so the points are much higher.

The boiling point is generally more sensitive to the strength of the forces than the melting point, since boiling separates the particles completely, while melting only allows them to move past each other.

How do you rank the strength of the forces?

1. Ion–ion forces in ionic compounds are the strongest.
2. Hydrogen bonding is the strongest intermolecular force between molecules.
3. Dipole–dipole forces are weaker, and increase with the dipole moment.
4. London dispersion forces are present in all substances, and increase with the size and number of electrons. In large molecules they can outweigh dipole–dipole forces.

Substances of similar molar mass

The melting points do not follow the same order as the boiling points because they also depend on how well the molecules pack in the crystal, and not only on the strength of the forces.

Different types of solids

Metals vary widely: mercury is a liquid at room temperature, while tungsten melts at 3422 °C.

What other factors influence the points?

• Molar mass and size: larger molecules have stronger dispersion forces. The boiling points of the alkanes rise from −162 °C for methane to 126 °C for octane.
• Shape: branched molecules have a smaller surface and lower boiling points than straight-chain isomers: pentane boils at 36 °C, neopentane at 9.5 °C.
• Symmetry and packing: symmetrical molecules fit well into a crystal and have unusually high melting points. Para-xylene melts at 13 °C, while ortho-xylene melts at −25 °C, though their boiling points are similar (138 °C and 144 °C).
• Pressure: boiling points depend strongly on pressure, which is why water boils at a lower temperature at altitude. Boiling points are quoted at 1 atm unless stated otherwise.

How do you predict which substance has the higher point?

1. Decide on the type of particle: ions, atoms in a metal, network covalent or molecules.
2. For molecules, check for N–H, O–H or F–H groups (hydrogen bonding).
3. If there is none, check for polarity.
4. If the forces are of the same kind, compare molar masses and shapes.
5. Make a prediction, and state the reason in terms of forces.

Worked examples

Example 1: ethanol or dimethyl ether

Both have the formula C₂H₆O. Ethanol has an O–H group and forms hydrogen bonds, while dimethyl ether does not, so ethanol boils higher (78.4 °C against −24.8 °C).

Example 2: Br₂ or ICl

Br₂ (159.8 g/mol) is nonpolar, while ICl (162.4 g/mol) is polar, and they have a similar mass. ICl is expected to have the higher boiling point. This is correct: ICl boils at 97 °C and Br₂ at 59 °C.

Example 3: ranking four substances

Rank Ar, NaCl, CH₃OH and CO₂ (sublimes) by melting point. Ar (−189 °C) and CO₂ (−78 °C at the sublimation point) are nonpolar, and CO₂ is larger; methanol (−98 °C) has hydrogen bonds; NaCl (801 °C) is ionic. One possible order is Ar < CH₃OH < CO₂ < NaCl; note that the CO₂ value is not a true melting point at 1 atm, and that predictions of melting points from forces alone are less reliable than for boiling points.

Which mistakes are common?

• Thinking that covalent bonds break when a molecular substance boils: only the intermolecular forces are overcome.
• Using the strength of covalent bonds to explain boiling points: HF has a very strong H–F bond, but its boiling point depends on hydrogen bonding.
• Ignoring molar mass: large nonpolar molecules can boil higher than small polar ones.
• Treating melting and boiling points as equally predictable: crystal packing complicates melting.
• Forgetting the pressure: a boiling point is defined for a given pressure.