11.4: Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, and solubilities (miscibilities) of substances. Molar mass, molecular shape, and polarity affect the strength of different intermolecular forces, which influence the magnitude of physical properties across a family of molecules.

Temporary attractive forces like dispersion are present in all molecules, whether they are polar or nonpolar. They cause gases to condense (liquefy) and liquids to freeze (solidify) under very low temperature (or high pressure) conditions. Dispersion forces arise from temporary dipoles caused by the asymmetrical distribution of electrons around the atom's nucleus. Atoms (or molecules) with a greater number of electrons (higher molar mass) display stronger dispersion forces than lighter atoms (or molecules). The melting point and boiling point trend of halogens demonstrate this effect. Moving down the group, from fluorine to iodine, melting points and boiling points increase with increasing atomic size (or mass). This increase may be rationalized by considering how the strength of dispersion forces is affected by the electronic structure of the atoms or molecules in the substance. In a larger atom, the valence electrons are, on average, farther from the nuclei than in a smaller atom. Thus, they are less tightly held and can more easily form the temporary dipoles that produce the attraction. The measure of how easy or difficult it is for another electrostatic charge (for example, a nearby ion or polar molecule) to distort a molecule’s charge distribution (its electron cloud) is known as polarizability.

A molecule that has a charge cloud that is easily distorted is said to be very polarizable and will have large dispersion forces; one with a charge cloud that is difficult to distort is not very polarizable and will have small dispersion forces. The shapes of molecules also affect the magnitudes of the dispersion forces between them. For example, boiling points for the isomers n-pentane, isopentane, and neopentane are 36 °C, 27 °C, and 9.5 °C, respectively. Even though these compounds are composed of molecules with the same chemical formula, C₅H₁₂, the difference in boiling points suggests that dispersion forces in the liquid phase are different, being greatest for n-pentane and least for neopentane. The elongated shape of n-pentane provides a greater surface area available for contact between molecules, resulting in correspondingly stronger dispersion forces. The more compact shape of isopentane offers a smaller surface area available for intermolecular contact and, therefore, weaker dispersion forces. Neopentane molecules are the most compact of the three, offering the least available surface area for intermolecular contact and, hence, the weakest dispersion forces.

Polar substances exhibit dipole–dipole attractions. The effect of this attraction is apparent when comparing the properties of polar HCl molecules to nonpolar F₂ molecules. Both HCl and F₂ consist of the same number of atoms and have approximately the same molecular mass. At a temperature of 150 K, molecules of both substances would have the same average KE. However, the dipole–dipole attractions between HCl molecules are sufficient to cause them to “stick together” to form a liquid, whereas the relatively weaker dispersion forces between nonpolar F₂ molecules are not, and so this substance is gaseous at this temperature. The higher normal boiling point of HCl (188 K) compared to F₂ (85 K) is a reflection of the greater strength of dipole–dipole attractions between HCl molecules, compared to the attractions between nonpolar F₂ molecules.

A special type of dipole–dipole force—hydrogen bonds—have a pronounced effect on the properties of condensed phases (liquids and solids). For example, consider the trends in boiling points for the binary hydrides of group 15 (NH₃, PH₃, AsH₃, and SbH₃), group 16 hydrides (H₂O, H₂S, H₂Se, and H₂Te), and group 17 hydrides (HF, HCl, HBr, and HI). On progressing down the groups, the polarities of the molecules decrease slightly, whereas the sizes of the molecules increase substantially. The effect of increasingly stronger dispersion forces dominates that of increasingly weaker dipole–dipole attractions, and the boiling points are observed to increase steadily. Using this trend, the predicted boiling points for the lightest hydride for each group would be about −120 °C (for NH₃), −80 °C (for H₂O), and −110 °C (for HF). However, the measured boiling points for these compounds are about −33.34 °C (for NH₃), 100 °C (for H₂O), and 19.5 °C (for HF), all of which are dramatically higher than the predicted trends. The stark contrast between our naïve predictions and reality provides compelling evidence for the strength of hydrogen bonding.

Effect of Polarity on Miscibility

Liquids that can be homogeneously mixed in any proportion are said to be miscible. Miscible liquids have similar polarities. Consider, for example, methanol (CH₃OH) and water (H₂O), two liquids that are polar and capable of hydrogen bonding. On mixing, methanol and water will interact through intermolecular hydrogen bonds and mix; thus, they are miscible. Likewise, nonpolar liquids like hexane (C₆H₁₄) and bromine (Br₂) are miscible with each other through dispersion forces. The chemical axiom “like dissolves like” is useful to predict the miscibility of compounds. Two liquids that do not mix to an appreciable extent are called immiscible. For example, nonpolar hexane is immiscible in polar water. Relatively weak attractive forces between the hexane and water do not adequately overcome the stronger hydrogen bonding forces between water molecules.

This text is adapted from Openstax, Chemistry 2e, Section 10.1: Intermolecular forces. and Section 11.3: Solubility.

Why do molecules with increasing molar mass show a rising trend in boiling points? The difference can be attributed to their intermolecular forces. 

All substances have dispersion forces — the weakest intermolecular force. The strength of dispersion forces depends on how easily the atoms can be polarized.

Atoms with higher masses have more electrons and larger electron clouds with electrons dispersing more freely, leading to increased dispersion forces. The stronger the forces, the more energy is required to break the interactions between neighboring atoms, resulting in higher boiling points.

This also explains why an increase in the molar mass in the alkane series translates to a corresponding increase in boiling points and melting points.

However, molar mass alone does not determine the strength of the dispersion forces. 

Despite having the same masses, n-pentane and neopentane have different boiling points as a result of their differing shapes. 

The straight n-pentane provides a larger surface area to interact with the neighboring molecules, while the spherical neopentane has a smaller surface area and consequently reduced dispersion forces. This leads to a lower boiling point. 

Intermolecular forces are also crucial when mixing liquids. Liquids of similar type and magnitude of intermolecular forces are entirely soluble in all proportions, or miscible.

Consider ethanol and water, both of which exhibit hydrogen bonding interactions. Upon mixing, ethanol molecules can form hydrogen bonds with water molecules, and the two liquids are completely miscible.

In contrast, liquids with different types and magnitudes of intermolecular forces are insoluble or immiscible. Consider the polar water molecules with dispersion and hydrogen bonding forces and the nonpolar hexane molecules with only dispersion forces.

On mixing, hexane forms the top layer and water forms the bottom layer. The weaker dispersion forces in hexane cannot compete with the stronger hydrogen bonds in water. Hence, hexane and water are immiscible