1. Intermolecular Forces (IMFs)
* Hydrogen Bonding: The strongest type of IMF, occurring between molecules containing hydrogen bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This leads to high boiling points. (e.g., water, ethanol)
* Dipole-Dipole Interactions: These occur between polar molecules due to their permanent dipoles. These forces are weaker than hydrogen bonds. (e.g., acetone, chloroform)
* London Dispersion Forces (LDFs): These are present in all substances, but they are the weakest IMFs. They arise from temporary, induced dipoles due to the movement of electrons. LDFs increase with increasing molecular size and surface area. (e.g., methane, propane)
2. Molecular Size and Shape:
* Larger molecules: Generally, larger molecules have more electrons, leading to stronger LDFs and higher boiling points.
* Shape: More compact molecules have less surface area for interaction, leading to weaker LDFs and lower boiling points compared to elongated molecules.
3. Molecular Weight:
* Higher molecular weight: Generally, substances with higher molecular weights have stronger LDFs, resulting in higher boiling points.
4. Branching:
* Branched molecules: Branching reduces surface area for interaction, making LDFs weaker and lowering boiling points.
5. External Pressure:
* Lower pressure: A lower external pressure results in a lower boiling point because molecules need less energy to overcome the pressure and escape into the gas phase.
Examples:
* Water (H2O): Has a very high boiling point (100°C) due to strong hydrogen bonding.
* Ethanol (C2H5OH): Also has a high boiling point due to hydrogen bonding.
* Methane (CH4): Has a low boiling point (-161.5 °C) because it only exhibits weak LDFs.
* Hexane (C6H14): Has a higher boiling point than methane due to its larger size and stronger LDFs.
In summary: The strength of IMFs is the primary factor determining boiling points. Larger size, higher molecular weight, and the presence of strong IMFs like hydrogen bonding generally lead to higher boiling points. Branching and weaker IMFs result in lower boiling points.
