100 Nano-Stories: Van der Waal Forces!
Episode #61: Intermolecular Forces Explained!
Preface! ✨
It’s your favorite material science & nanotechnology enthusiast! Yesterday, I gave a brief overview of the fundamentals of alcohols in organic chemistry!
This article is essential to understand what we will be discussing in this one!
- Essentially, any molecule that is composed of only oxygen and hydrogen (water) will have polar bonds and be covalent. This will result in hydrogen bonds that are strong and will result in a higher boiling point.
- Essentially, any molecule that is composed of only carbon or hydrogen will have non-polar bonds and still have covalent bonds. This will result in the electrical charges of the molecule being evenly distributed, and the bonds are weak. This will result in a low boiling point.
- Any alcohol will have a low boiling point because it has a molecule made of only carbon and hydrogen.
But what about alcohols with really large carbon chains or London Dispersion Forces? Do those count?
Van der Waal Forces! 💡
Definition! 🔑
Van der Waal Forces/London Dispersion Forces is an attraction of intermolecular forces between two molecules. Another way to put it is forces that make the molecule stick together.
Van der Waal forces are found in all molecules, but this type of intermolecular force is very weak. They are only strong between non-polar molecules.
Common non-polar molecules are any molecules composed of only carbon and hydrogen (alcohols, alkanes, alkenes, alkynes, hydrocarbons, etc.). The reason why is because to be non-polar, you need an electronegativity below 0.5, and when that happens, the electrical charges/electrons of the molecule are evenly distributed.
Analysis! 🔑
But even though the molecule has its electrical charges evenly distributed, the electrons are constantly moving. And in some cases, one side of the carbon/hydrogen atom will have more electrons than the other side, which will result in the molecule becoming polar for brief moments. The molecule becomes a dipole.
When this happens, we have a polarized atom. Polarization means the probability of the electron cloud of an atom/molecule being changed/deformed due to the random movement of electrons.
Dipole means the atom has one side of positive electric charge and one side of negative electric charge in a molecule (see photo above).
So How Does This Relate To Hydrocarbons? 🔑
Let’s take two non-polar molecules as an example! Both the electron clouds have changed because of the Van der Waal Forces! The attraction between the temporary dipole of the molecules!
However, sometimes a second non-polar molecule will interact with the temporary polarized molecule. This will cause the electron cloud of the second non-polar molecule to become an induced dipole!
Induced Dipole means that another (polarized) molecule had induced/changed the electrical charges of the molecule to change it into a dipole.
But if you notice, the “attraction” between these two forces contains the positive and negative sides of the molecule, similar to a magnet! This is what the Van der Waal Forces is!
As I mentioned earlier, it is present in all intermolecular interactions, but they are weak because there are other bonds found in polar/ionic bonds/hydrogen bonding, etc.
But in non-polar molecules, the Van der Waal forces are dominant ones, and because of the strong bonds between non-polar molecules, they have the ability to increase the boiling point of other certain non-polar molecules!
Key Identification Points! 🔑
To identify which non-polar molecules have the higher boiling point, there are two keys:
- The more electrons, the higher chance of polarizability, therefore, more chance of Van der Waal Forces occurring in the molecule!
- The heavier the molecular weight of a molecule, the higher probability of Van der Waal Forces Occurring!
- There is proportionality between the number of electrons and a heavier molecular weight! This will allow the boiling point of the non-polar molecules to rise!
Examples: Electrons! 🔑
Think of ethane and butane!
We could count all the electrons in ethane and butane.
Carbon atom → 6 electrons.
Hydrogen atom → 1 electron.
In Ethane, there are 2 carbons and 6 hydrogens, so 2*6 =12 electrons in both carbon atoms. 6 hydrogen atoms equals 6 electrons, so 12 + 6 = 18 electrons in ethane.
In Butane, there are 4 carbons and 10 hydrogens, so 4*6 =24 electrons in the four carbon atoms. 10 hydrogen atoms equals 10 electrons, so 24 + 10 = 34 electrons in butane.
The more electrons mean that there is a higher chance of polarization and a higher chance for Van der Waal forces and a higher boiling point! So Butane will have a higher boiling point!
Examples: Molecular Weight! 🔑
Molar Mass of Carbon → 12.
Molar Mass of Hydrogen → 1.
In Ethane, there are 2 carbons and 6 hydrogens. So 12*2 = 24, and 6 hydrogens is 6, so 24 + 6 = 30 in molar mass for ethane.
In Butane, there are 4 carbons and 10 hydrogens. So 12*4 = 48, and 10 hydrogens is 10, so 48 + 10 = 58 in molar mass for butane.
The organic compound with the higher molar mass will result in a higher boiling point!
The boiling point of ethane is -89°C (-128.2°F), and the boiling point of butane is -1°C (30.2°F).
Closing Thoughts! 💭
That’s it! Now we understand how non-polar molecules can become incredibly strong and have such a high-boiling point, despite not having hydrogen bonding!
See you tomorrow for more mathematical equations on aerogel transparency and porosity! ✌🏽
Vocabulary! 📓
Polar molecules → The electrical charges of the molecule are not evenly distributed. The OH group is a polar molecule commonly found on the surface of hydrophilic aerogels. Polar molecules are hydrophilic.
Non-Polar molecules → The electrical charges of the molecule are evenly distributed. No positive or negative charges are formed in the molecule. All alkanes, alkenes, and alkynes are non-polar molecules because they are composed of carbon and hydrogen.
Electronegativity → The chance that a pair of elections will be attached to a certain atom. If the difference between the electronegativity is greater or equal to 0.5, the molecule is polar. If the difference in electronegativity in the molecule is less than 0.5, the molecule is non-polar.
Van der Waal Forces/London Dispersion Forces → An attraction of intermolecular forces between two molecules, and are found in all molecular forces, but they are the strongest in non-polar molecules.
Polarization → The probability of the electron cloud of an atom/molecule being changed/deformed due to the random movement of electrons.
Dipole → The atom/molecule has one side of positive electric charge and one side of negative electric charge in a molecule.
Induced Dipole → Another (polarized) molecule had induced/changed the electrical charges of the molecule to change it into a dipole.
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