Midterm 2 Question

[KDeguzman_Dis3K]

On Midterm 2, there was one question based on intermolecular forces and how it relates to boiling/melting points (I think it was the last question). Could someone please explain how you can tell which substance would have a higher or lower boiling/melting point? I had trouble with this midterm question.

[Ishpreet Kaur 3C]

The stronger the intermolecular forces, the more difficult those forces are to break apart, which would increase the boiling/melting points. For example, a molecule with dipole-dipole, hydrogen bonding, and London interactions would have a higher boiling/melting point than a molecule with just London interactions.

You can figure out which interactions occur for a molecule by determining it's VSEPR structure. If it's polar, you know dipole-dipole interactions take place. If there's a hydrogen bonded to a NOF molecule, then you know hydrogen bonding interactions take place. All molecules have London forces present as electron density continually fluctuates.

[KatieFrancisco1F]

Molecules with stronger bonds are going to have higher boiling points because it takes more energy to make it boil (same with melting) as opposed to weaker bonds that break easier. You would need to draw out the molecules to determine which bonds are present. Shape of the molecule can also play a role in the strength of the molecule. Rounder molecules will be easier to boil as their intermolecular forces are weaker than that of molecules with a more stretched out shape.

[14b_student 2E]

The stronger the bond, the higher the melting point.

[Caitlin Beale 3E]

If you remember the order of the strength of intermolecular forces: Hydrogen bonds> dipole> london dispersion forces. You can use this guide to figure out which one will have the highest boiling point because changing from the liquid to the gas phase is essentially a breaking of intermolecular forces. Therefore, the stronger the force between the molecules, the higher the boiling / melting point.

[Amy Huynh 1B]

For these questions we would need to know the relative strengths of the intermolecular forces. For example, if one compound was able to form H-bonds and LDF while the other compound was only able to form LDFs, the compound that is able to form H-bonds would have a higher melting/boiling point because it takes more energy for H-bonds to be broken.

[madeleinewright]

Stronger intermolecular forces between molecules will result in a higher melting/boiling point. For example, molecules with hydrogen bonding or dipole-dipole bonding will have a higher melting/boiling point than molecules with just London dispersion forces.

[erud]

You can look back to intramolecular forces, and the orders of bond strength(london dispersion, hydrogen bonds), to know that the stronger the bond, the higher the melting point.

[Julia Todorov 2F]

the stronger the intermolecular forces, the higher the boiling point. for example if you had two non polar molecules with only london dispersion forces, the one with more molar mass (or just bigger) would have the stronger london dispersion force, and in turn a higher boiling point

[mmcqueen]

The basics that you need to know if the intramolecular forces, and the orders of bond strength. For example, london dispersion is the weakest IMF force while hydrogen bonds is the strongest. Then, you can interpret that the stronger the bond, the higher the melting and boiling point of the element.

[Furkan Acar 3C]

You simply needed to figure out which of the given had the strongest bond. Once you did that, you can pretty much just assume that the one with the stronger bond has a higher boiling point. Hope this helps. Also, hope everyone did good on the midterm :)

[Brianna Mercado 3D]

stronger bond=stronger melting point

[Divya Mehta 2K]

Sure! So stronger intermolecular forces result in a higher melting point (meaning it is harder to melt/boil as the molecules are held together with more strength, so they require more energy/more heat to break). There are interactions such as ion-ion, dipole-dipole, hydrogen-bonding, etc which can result in higher melting/boiling points

[Milli Zunich 3I]

The ones with stronger bonds have a higher melting point.

[Maham Kazmi 2J]

Stronger bonds essentially mean that the melting point is higher. This is because more energy/heat is needed to break the intermolecular forces allowing the solid to transition to a liquid. In terms of bond strength, triple bonds are the strongest while single bonds are the weakest.

Relating the question to recent lectures: there are two pi bonds and one sigma bond in a triple bond whereas the single bond is only a sigma bond.

[trevina_brown_2A]

Higher melting points can be an indicator of which ones have stronger bonds

[Narin Maisha 2H]

You can tell a boiling point by determining the structure. You will be able to find the bond strength out through this and the stronger the bond the higher the boiling point. This means that the opposite is also true, the weaker the bond, the lower the boiling point.

[Jillian Sarquiz- 2B]

Molecules that have stronger bonds have higher boiling points since it is hard to break those bonds.

[AJackman]

Possible order of operations for figuring out a problem like this:
1. Draw Lewis structure
2. Determine all the IMFs present (sometimes identifying the VESPR structure is helpful, as it can tell you whether or not the molecule is polar)
3. Find the molecule with the strongest IMF
SO: Stronger IMF/bond ---> greater electronegativity ----> requires more energy to break ------> higher boiling point & melting point

[AJackman]

Also, where can we find practice problems of this type? I also had some trouble with them on the midterm.

[Esther Kim]

The stronger the bond the higher the melting point

[Valerie M Dis 2E]

The stronger the bond is, the higher the melting point is. It takes more energy (heat) to break the bonds when the bonds are stronger.

[Wenhan Li_3d]

The stronger the bond, the higher the melting point.