Boiling/Melting Points

[StephanieIb]

Can someone tell me what are all of the things that cause molecules to have higher melting/boiling points? Or how do we determine this when given a molecule?

[AnjikaFriedman-Jha2D]

Boiling and melting points are mostly determined by polarizability, which depends on the number of electrons and the size of atom/molecule. For instance, when comparing two compounds such as CH4 and CCl4, Cl is much more polarizable because it is larger and the electrons are more shielded from the nucleus, so it has greater dispersion forces and therefore a higher boiling point. Always look at the IMF

[Joseph Hsing 2C]

I think one factor to focus on for determining boiling points is examining how strong the force of attractions that are present and how many there are between the atoms in a molecule. For example, more Van der Waals forces results in more energy needed to melt or boil a compound and thus increases the temperature points.

[Brennan McGurrr 3C]

The biggest factor when determining BP and MP are the type of intermolecular forces present. If there are two different types, the one with the stronger type will have a higher BP and MP. For example, water forms hydrogen bonds, so it has a higher BP than methane, which only has London Dispersion forces. If the type of intermolecular forces is the same, polarizability and the magnitude of the dipoles need to be taken into account.

[Hailey Qasawadish 2J]

The biggest factor in determining boiling/melting points is the strength of the intermolecular forces. The stronger the intermolecular force, the higher the boiling/melting point because it takes more energy to break the bonds if they are stronger.

[ellenulitsky Dis 1I]

When looking at boiling points or melting points, you want to be using INTERmolecular forces NOT INTRAmolecular forces. This helped me a lot when figuring out how to solve this type of problem. Hope this helps!

[Chris_Butler_1A]

Understanding that dispersion forces are what affect the boiling and melting point of molecules, I was wondering if someone could clarify how temperature interacts with these dispersion forces. Is it that with a rising temperature, electrons become more excited and thus are more likely to affect neighboring atoms?

[Ansh Patel 2I]

Hi! Intermolecular forces are what determine the boiling or melting points of molecules; the stronger the forces, the higher the boiling/melting point, and vice versa.

[Alejandro Gonzalez 2G]

I think looking at intermolecular forces/interactions like London or Van der Waals forces help understand the differences in boiling points between two compounds, as those with stronger forces will need more energy to break the interactions, resulting in a higher boiling point.

[rhettfarmer-3H]

For melting and boiling points lavelle talked about polarizability being the main factor. So, the common element that add to polarizability is the size of the molecule. So when you compare two elements look at the size. when I mean size atomic radius is a key trend or number of electrons.

[Chris_Butler_1A]

For melting and boiling points lavelle talked about polarizability being the main factor. So, the common element that add to polarizability is the size of the molecule. So when you compare two elements look at the size. when I mean size atomic radius is a key trend or number of electrons.

So if I am understanding correctly, an element like Titanium with higher polarizability would have a higher melting/boiling point than say Fluorine with lower polarizability?

[Brian Nguyen 2I]

Higher boiling and melting points are indicated by if there are stronger intermolecular forces present. For example, if u have a molecule with dipole-dipole forces and London dispersion forces and a molecule with only London dispersion forces, then the molecule that has dipole-dipole forces has a higher boiling point and melting point since there are stronger intermolecular forces present.

[Adalia 3E]

One way to think about it is that going from solid -> liquid -> gas requires energy to separate the molecules. Molecules with greater intermolecular forces would require more energy (heat) to separate, and thus have a higher boiling/vaporizing point.

[Heidi Buri 2I]

Hi! There are many reasons that molecules can have higher melting points. It has to do with qualities of the molecules. Usually, if molecules have higher dispersion forces or hydrogen bonding, the molecules have higher melting points. Also, ionic compounds have higher melting points due to the electrostatic forces between the ions.

[Neel Sharma 3F]

Whenever a question discusses melting and boiling points, it is calling your attention to the intermolecular forces at play. Stronger intermolecular forces such as between ions in a crystal, H-bonds, or large London Dispersion forces are all reasons for high melting and boiling points. Therefore comparing intermolecular forces is a good way to tell which molecules will have higher boiling and melting points. Also on a somewhat related note, vapor pressure has an inverse relationship to boiling point. Therefore, molecules with weak intermolecular forces will have low boiling points and high vapor pressure and vice versa. Hope this helps!

[Andre Fabian 1F]

Hi there!

We can determine relative melting and boiling points by looking at bond strength within a molecule.
For example, a molecule with a single bond is much weaker than the same molecule harboring a double bond. As such, (given that as the bond order increases, the length of the bond decreases, and the strength of the bond increases) the molecule with the single bond would have lower melting and boiling points, given that it would take less energy to break the weaker bond.

Hope this helps!