Bonding, Structure, and Resonance
The Four Intermolecular Forces and How They Affect Boiling Points
Last updated: December 14th, 2022 |
Properties like melting and boiling points are a measure of how strong the attractive forces are between individual atoms or molecules. (We call these intermolecular forces – forces between molecules, as opposed to intramolecular forces – forces within a molecule. )
It all flows from this general principle: as bonds become more polarized, the charges on the atoms become greater, which leads to greater intermolecular attractions, which leads to higher boiling points.
There are four major classes of interactions between molecules and they are all different manifestations of “opposite charges attract”
The four key intermolecular forces are as follows:
Ionic bonds > Hydrogen bonding > Van der Waals dipole-dipole interactions > Van der Waals dispersion forces.
Let’s look at them individually, from strongest to weakest.
Table of Contents
- Ionic Forces
- Hydrogen Bonding
- Van Der Waals Dipole-Dipole Interactions
- Van der Waals Dispersion Forces (“London forces”)
- Bottom Line
1. Ionic forces
Ionic forces are interactions between charged atoms or molecules (“ions”).
Positively charged ions, such as Na(+) , Li(+), and Ca(2+), are termed cations.
Negatively charged ions, such as Cl(–), Br(–), HO(–) are called anions (I always got this straight through remembering that the “N” in “Anion” stood for “Negative”).
The attractive forces between oppositely charged ions is described by Coulomb’s Law, in which the force increases with charge and decreases as the distance between these ions is increased.
The highly polarized (charged) nature of ionic molecules is reflected in their high melting points (NaCl has a melting point of 801 °C) as well as in their high water solubility (for the alkali metal salts, anyway; metals that form multiple charges like to leave residues on your bathtub)
2. Hydrogen bonding
Hydrogen bonding occurs in molecules containing the highly electronegative elements F, O, or N directly bound to hydrogen.
Since H has an electronegativity of 2.2 (compare to 0.9 for Na and 0.8 for K) these bonds are not as polarized as purely ionic bonds and possess some covalent character.
However, the bond to hydrogen will still be polarized and possess a dipole.
The dipole of one molecule can align with the dipole from another molecule, leading to an attractive interaction that we call hydrogen bonding.
Owing to rapid molecular motion in solution, these bonds are transient (short-lived) but have significant bond strengths ranging from (9 kJ/mol (2 kcal/mol) (for NH) to about 30 kJ/mol (7 kcal) and higher for HF.
As you might expect, the strength of the bond increases as the electronegativity of the group bound to hydrogen is increased.
So in a sense, HO, and NH are “sticky” – molecules containing these functional groups will tend to have higher boiling points than you would expect based on their molecular weight.
3. Van Der Waals Dipole-Dipole Interactions
Other groups beside hydrogen can be involved in polar covalent bonding with strongly electronegative atoms. For instance, each of these molecules contains a dipole:
These dipoles can interact with each other in an attractive fashion, which will also increase the boiling point.
However since the electronegativity difference between carbon (electronegativity = 2.5) and the electronegative atom (such as oxygen or nitrogen) is smaller than it is for hydrogen (electronegativity = 2.2), the polar interaction is not as strong.
So on average these forces tend to be weaker than in hydrogen bonding.
4. Van der Waals Dispersion Forces (“London forces”)
The weakest intermolecular forces of all are called dispersion forces or London forces.
These represent the attraction between instantaneous dipoles in a molecule.
Think about an atom like argon. It’s an inert gas, right? But if you cool it to –186 °C, you can actually condense it into liquid argon. The fact that it forms a liquid it means that something is holding it together. That “something” is dispersion forces.
Think about the electrons in the valence shell. On average, they’re evenly dispersed. But at any given instant, there might be a mismatch between how many electrons are on one side and how many are on the other, which can lead to an instantaneous difference in charge.
It’s a little like basketball. On average, every player is covered one-on-one, for an even distribution of players.
But at any given moment, you might have a double-team situation where the distribution of players is “lumpy” (it also means that somebody is open). In the valence shell, this “lumpiness” creates dipoles, and it’s these dipoles which are responsible for intermolecular attraction.
The polarizability is the term we use to describe how readily atoms can form these instantaneous dipoles.
Polarizability increases with atomic size. That’s why the boiling point of argon (–186 °C) is so much higher than the boiling point of helium (–272 °C). By the same analogy, the boiling point of iodine, (I-I, 184 °C) is much higher than the boiling point of fluorine (F-F, –188°C).
For hydrocarbons and other non-polar molecules which lack strong dipoles, these dispersion forces are really the only attractive forces between molecules.
Since the dipoles are weak and transient, they depend on contact between molecules – which means that the forces increase with surface area.
A small molecule like methane has very weak intermolecular forces, and has a low boiling point.
However, as molecular weight increases, boiling point also goes up.
That’s because the surface over which these forces can operate has increased. Therefore, dispersion forces increase with increasing molecular weight. Individually, each interaction isn’t worth much, but if collectively, these forces can be extremely significant. How can a gecko lizard walk on walls? Look at its feet.
[Determining trends for hydrocarbons can get a little bit tricky depending on the exact structure – symmetry also plays a role in boiling points and melting points. We talked about this in detail previously. (See Article – Branching And Melting Points)
5. Bottom Line
- Boiling points are a measure of intermolecular forces.
- The intermolecular forces increase with increasing polarization of bonds.
- The strength of intermolecular forces (and therefore impact on boiling points) is ionic > hydrogen bonding > dipole dipole > dispersion
- Boiling point increases with molecular weight, and with surface area.
Reminder – don’t forget the free boiling point study guide (Contains all the key points discussed in this post)
29 thoughts on “The Four Intermolecular Forces and How They Affect Boiling Points”
why does HCl have diplole dipole interactions, and not hydrogen bonding?
like HF does.
HCl has *some* hydrogen bonding, it’s just not particularly strong.
All molecules with hydrogen have ‘hydrogen bonding’, but it is to such a very weak degree that it doesn’t really matter.
However, when hydrogen bonds with elements that are extremely electronegative (primarily F, O, and N) they hold on VERY tightly and the hydrogen bonding that occurs during them is extremely significant.
why does the boiling point of helium lower than the boiling point of hydrogen?
Because helium contains two electrons which are both in the 1S orbital making them EXTREMELY close to the nucleus. Helium is actually a very small atom much smaller than hydrogen since the electrons are pulled closer… it also does not want to gain or lose any so it will do what it can to keep its electrons. As we learned smaller atoms have lower boiling points.
H must be bonded to either, F, O, or N in order to exhibit Hydrogen bonding.
Thank you thank you thank you!
Loved the basketball analogy ;)
I always remember CATion is Pawsitive. Thank you for the boiling point help
Thanks for this page!
The explanation was amazing!!!!
Keep the great job!
If liquids exhibit high polar behavior does the surface tension increase?
Also tell me alcohols, esters, ethers and aromatic hydrocarbons have any relation between boiling point, dispersion, surface tension or wettability (this is specifically for liquid inks)
Very helpful for my upcoming lab-report. Just like to point a few things out that differs from this article to that I was taught in school:
1. Hydrogen has a polarity of 2.1
2. Bonds with an electronegativity of 0.4 OR less is not polar.
I don’t know which one is right, but it doesn’t seem to matter anyway.
Keep up the good job.
Just a bit clarity:
1) “Electronegativity” is a measurement of how strongly an atom wishes to hold onto its valence electrons. There are at least 5 different versions of this value, each calculated a slightly different way, though most chemists refer to (Linus) Pauling’s electronegativity, as he came up with the first method of calculation. Incidentally, his method only measures electronegativity differences (see below), so the electronegativity of hydrogen was SET at 2.20, and every other atom’s electronegativity is relative to that value.
2) “Polarity” is a term that reflects how DIFFERENT the electronegativities are of two bonded atoms.
a) That is, polarity is always relative to the electronegativity difference between TWO atoms, and it not related to any one atom.
b) bonds with an electronegativity DIFFERENCE greater than 0 (that is, any bond that is not between two identical atoms, which is considered to be a pure covalent bond) is technically a polar bond, but the convention is that the difference should be greater than 0.5 to be considered realistically polar.
c) once that difference is greater than some threshold (different chemists have identified the cut-off at 1.7, 1.8, 2.0, or 2.2 that I know of), the electrons that compose the bond have been completely (or nearly completely) captured by the more electronegative atom. This results in one atom having a full negative charge (an anion) and one atom having a full positive charge (a cation). They are no longer sharing the electrons, but the electrostatic attraction of two oppositely charged ions, called the ionic bond, is quite strong; frequently of higher binding energy than typical covalent bonds (non-polar or polar).
d) the reality is that even with the two atoms that having the highest (F) and lowest electronegativities (Fr), the difference in electronegativity still results in a bond with only about 92% ionic character. That is, nearly every bond that we refer to as “ionic” actually still has a little bit of covalent (i.e., shared electron) character.
I hope that this helps. I have no idea how long ago you posted your question.
Thank you for this long and eloquent response, Marc.
The best trick to remember the difference between cations and anions that everyone will always remember is this: CATions are always PAWWWSSITIVE :D myy gen chem professor taught me that and it stuck forever.
Why does N2 have a lower boiling point than CO although they are isoelectronic?
Why does CH3OH have lower boiling point that NH4? They both have hydrogen bonds and nh4 is smaller.
Count the number of hydrogen bonds. More hydrogen bonds means stronger IM force
Because MeOH has the “OH” functional group, which can participate in hydrogen bonding. Ammonium can also participate in hydrogen bonding. Also mass between carbon and nitrogen affect boiling points. Yes, MeOH has a higher mass total than ammonium, but the fact that you are dealing with an alcohol versus an ion affects mp.
CH3OH has a larger molar mass than NH4. The larger the molar mass (in some cases), the stronger the IMFs.
My guess is because the Boiling Point depends on the Intermolecular Forces (IMF). IMF is determined from three things: Dispersion Forces, Dipole-to-Dipole Forces, and Hydrogen Bonding Forces. Yes, CH3OH has more electrons (is larger) so it has more Dispersion Forces. Yes, CH3OH has more Dipole-to-Dipole Forces because it is polar where NH4 is not. HOWEVER, I think Hydrogen Bond Forces is where it changes. The value that Hydrogen Bond Forces carries in the total IMF is more significant than the Dispersion and Dipole forces. CH3OH can only have hydrogen bonds on the ONE Hydrogen atom covalently bonded to the oxygen. That’s it. But, hydrogen bonds can form on all FOUR hydrogen atoms.
The polarizability is the term we use to describe how readily atoms can form these instantaneous dipoles. Polarizability increases with atomic size. That’s why the boiling point of argon (–186 °C) is so much higher than the boiling point of helium (–272 °C). By the same analogy, the boiling point of iodine, (I-I, 184 °C) is much higher than the boiling point of fluorine (F-F, –188°C).
By NH4, I assume you mean NH4+. Compounds that contain NH4+ have ionic bonds, and thus should have higher boiling points than compounds without ionic bonds, like CH3OH.
How are the following substances ranked, from weakest intermolecular force, to the strongest attractions. Heptane, Hexanoyl, Pentanoic acid, and Propyl ethanoate.
Heptane weakest. Pentanoic acid strongest. Hexanoyl… do you mean hexanol?
Can you please comment on the directional or non directional nature of the following interactions:
3. Ion-Dipole &
4. Ion-Induced Dipole
Your articles are of great help! Thank You!
Does the atmosphere also affect the boiling point? example, if I replace the atmosphere in a chamber with carbon dioxide or neon, would that change the boiling point?
Depends on the atmospheric pressure. Boiling occurs when vapor pressure is equivalent to atmospheric pressure. If the pressure of the atmosphere is still atmospheric pressure (760 torr / 1 bar / 101.25 kPa) then the boiling point should remain the same.
For gases heavier than air, however, it will require fewer moles of gas to achieve that pressure. For instance if you had a two chambers, one with argon and one with air, each with equivalent molar amounts of gas, then the pressure in the argon chamber would be higher and therefore the bp of the liquid in the argon chamber would be higher due to the fact that one mole of argon weighs more than one mole of air. Does that make sense?
what about ion-dipole interactions? these also exist right? where would it be in your ionic>hydrogen bonding>dipole-dipole>dispersion hierarchy?