5.9: The implications of bond polarity

Melting and boiling points are important physical properties of small molecules. They apply to pure samples composed of large numbers of molecules. They are irrelevant to larger molecules, which generally change their structure (denature) at temperature extremes. Let us start at a temperature at which the sample is liquid. The molecules are moving with respect to one another, there are transient interactions between the molecules, who are constantly switching neighbors. As we increase the temperature of the system, the energetics of collisions are such that all interactions between neighboring molecules are eventually broken, and the molecules move away from one another. When they to collide with one another, they (generally) do not adhere; the bond that might form between them is not strong enough to resist the kinetic energy delivered by collisions with other molecules. The molecules are in a gaseous state and the transition from liquid to gas is the boiling point. Similarly, starting with a liquid, when we reduce the system temperature, the interactions between molecules become longer lasting until a temperature is reached at which the energy transferred through collisions is no longer sufficient to disrupt the interactions between molecules.^[228] As more and more molecules interact, the positions of neighboring molecules become more highly constrained - the liquid transitions into a solid. Because neighboring molecules in a liquid can move with respect to one another, liquids flow and assume the shape of their containers. Solids, in contrast, maintain their shape – neighboring molecules stay put. The temperature at which a liquid changes to a solid is known as its melting point. These temperatures mark what are known as phase transitions: solid to liquid and liquid to gas.

At the macroscopic level, we see the dramatic effects of bond polarity on melting and boiling points by comparing molecules of similar size with and without polar bonds and the ability to form H-bonds (↓). For example, CH₄(methane) or Ne (neon) do not contain polar bonds and so do not form intra-molecular H-bonds. In contrast NH₃ (ammonia), H₂O (water), and FH (hydrogen fluoride) have three, two and one polar bonds, respectively, and so can take part in one or more intra-molecular H-bonds. All five compounds have the same number of electrons, ten. When we look at their melting and boiling point temperatures, we see how the presence of polar bonds influences these properties. In particular, water stands out as dramatically different from the rest, with significantly higher (> 70ºC) melting and boiling points than its neighbors.

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