How Can You Determine If A Molecule Has A Higher Boiling Point?

Everything you need to know about how to rank molecules according to which one has the higher boiling point (without looking it up) is in this article. Let's start with some basics.

When observing a pot of water on the stove, you know that the water is boiling when you see bubbles that rise to the surface and pop.

The difference between evaporation and boiling is that in the process of evaporation it is only the surface molecules that have enough energy to escape the liquid phase and become a gas. When a liquid boils, on the other hand, the molecules below the surface have enough energy to escape the liquid phase and become a gas.

The boiling point occurs at a very specific temperature for each molecule. That is why it is often used to identify an unknown substance in qualitative chemistry. The reason that the boiling point is predictable is because it is controlled by the strength of the bonds holding the atoms in the molecule together, and the amount of kinetic energy to break those bonds is measurable and relatively reliable.

All molecules have kinetic energy; they are vibrating. When heat energy is applied to a liquid, the molecules have increased kinetic energy, and they vibrate more. If they vibrate enough, they bump into each other. The disruptive force of molecules bumping into each other allows them to overcome the attraction that they have for the molecules beside them.

What condition must exist for a liquid to boil? Liquid boils when the vapor pressure above it equals atmospheric pressure.

If you are comparing molecules to determine which has the higher boiling point, consider the forces that are at work within the molecule. These can be grouped into the following three factors.

The molecules within the liquid are attracted to each other. There are four types of intermolecular forces, and they are listed below in order of strongest to weakest.

1. Ionic bond Ionic bonding involves an electron being donated from one atom to another (e.g. NaCl, table salt). In the example of NaCl, the positively-charged sodium ion is held in close proximity to the negatively-charged chloride ion and the net effect is a molecule that is electrically neutral. It is this neutrality that makes the ionic bond so strong, and why it would take more energy to break that bond than a different type of bond. 

2. Hydrogen bond A hydrogen atom that is bonded to another atom by sharing its valent electron has low electronegativity (e.g. HF, hydrogen fluoride). The electron cloud around the fluorine atom is large and has high electronegativity while the electron cloud around the hydrogen atom is small and has much less electronegativity. This represents a polar covalent bond in which the electrons are shared unequally.

Not all hydrogen bonds have the same strength, it depends on the electronegativity of the atom it is bonded to. When hydrogen is bonded to fluorine, the bond is very strong, when bonded with chlorine it has moderate strength, and when bonded with another hydrogen, the molecule is non-polar and is very weak.

3. Dipole-Dipole A dipole force occurs when the positive end of a polar molecule is attracted to the negative end of another polar molecule (CH₃COCH₃, propanone).

4. Van der Waals forces Van der Waals forces account for the attraction of the shifting electron-rich portion of one molecule to the shifting electron-poor portion of another molecule (temporary states of electronegativity, e.g. He₂).

A larger molecule is more polarizable, which is an attraction that keeps the molecules together. They need more energy to escape to the gas phase, so the larger molecule has the higher boiling point. Compare sodium nitrate and rubidium nitrate in terms of molecular weight and boiling point:

Molecules that form long, straight chains have stronger attractions to the molecules around them because they can get closer. A straight-chain molecule like butane (C₄H₁₀) has a small electronegativity difference between carbon and hydrogen.

A molecule with a double-bonded oxygen, like butanone (C₄H₈O) is peaked in the middle where the oxygen is bonded to the carbon chain. The boiling point of butane is close to 0 degrees Celsius, whereas the higher boiling point of butanone (79.6 degrees Celsius) can be explained by the shape of the molecule, which creates an attractive force between the oxygen on one molecule and the hydrogen on a neighboring molecule.

The following features will have the effect of creating a higher boiling point:

• the presence of a longer chain of atoms in the molecule (more polarizable)
• functional groups that are more exposed (that is, at the end of a chain,
rather than in the middle)
• the polarity ranking of functional groups: Amide>Acid>Alcohol>Ketone or Aldehyde>Amine>Ester>Alkane

1. Compare these three compounds:
a) Ammonia (NH₃), b) hydrogen peroxide (H₂O₂) and c) water (H₂O)

NH₃ is non-polar (weak)
H₂O₂ is strongly polarized by hydrogen-bonds (very strong)
H₂O is polarized by hydrogen-bonds (strong)

You would rank these in order (strongest to weakest): H₂O₂>H₂O>NH₃

2. Compare these three compounds:
a) Lithium hydroxide (LiOH), b) hexane (C₆H₁₄) and c) iso-butane (C₄H₁₀)

LiOH is ionic (very strong)
C₆H₁₄ is a straight chain (strong)
C₄H₁₀ is branched (weak)

You would rank these in order (strongest to weakest): LiOH>C₆H₁₄>C₄H₁₀

The effect of increasing the pressure is to raise the boiling point. Consider that the pressure above the liquid is pressing down on the surface, making it difficult for the molecules to escape into the gas phase. The more pressure, the more energy is required, so the boiling point is higher at higher pressures.

At high altitudes, the atmospheric pressure is lower. The effect of this is that boiling points are lower at higher altitudes. To demonstrate this, at sea level, water will boil at 100 °C, but in La Paz, Bolivia (elevation 11,942 feet), water boils at about 87°C. Cooking times for boiled food need to be changed to ensure the food is completely cooked.

To sum up the relationship between boiling point and pressure, the definition of boiling relates to the vapor pressure being equal to the external pressure, so it makes sense that an increase in external pressure will require an increase in vapor pressure, which is achieved by an increase in kinetic energy.