How Can You Determine If a Molecule Has a Higher Boiling Point?

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The driving force between all molecular bonds is an attraction between opposing charges. Some molecules have much stronger bonds while others have much weaker bonds. It is in fact the strength of these bonds that determine a molecule's boiling point. In particular, there are four types of bonds, including, in order of strength: Ionic bonds, hydrogen bonds, van der Waals dipole bonds, and van der Waals dispersion bonds. Thus, to determine if one molecule has a higher boiling point than another, you need only identify their bonds and then compare them based on the list above.

  1. Consider the Type of Atom

  2. Identify the type of atoms being bonded. If nonmetal atoms are being bonded to metal atoms, then the molecule is ionic and thus has ionic bonds. Given the molecules PF3 and CF4, for instance, phosphorous (P) is a nonmetal while Fluorine (F) is a metalloid. Meanwhile, Carbon (C) is a nonmetal while Fluorine (F) is a metalloid. Neither molecule features a combination of non-metal and metal atoms, so neither molecule has ionic bonds.

  3. Determine Polarity

  4. Determine whether either of the molecules are polar by drawing them according to the Lewis dot diagram method and then checking to see if they have a symmetrical or asymmetrical shape. Given the example, CF4 has a symmetrical tetrahedral shape while PF3 has an asymmetrical trigonal pyramidal shape. Since CF4 is symmetrical, it therefore has dispersion bonds.

  5. Check the Electronegativity

  6. Check the electronegativity of each atom in all asymmetrical molecules by using a period table of elements. Given the example, phosphorus (P) has an electronegativity of 2.1, while Fluorine (F) has an electronegativity of 4.0. Therefore, PF3 has dipole bonds.

  7. Look for Hydrogen

  8. Confirm whether the molecules contain hydrogen atoms and, if they do, whether those hydrogen atoms are attached to fluoride, oxygen or nitrogen atoms. If an atom contains a mixture of hydrogen and one of the other three highly elegronegative elements, then the molecule has hydrogen bonds. Given the example, neither PF3 or CF3 has hydrogen atoms, so they both do not contain hydrogen bonds.

  9. Score Each Molecule

  10. Designate each molecule with scores based on their bonds. Give it 4 points for ionic bonds, 3 points for hydrogen bonds, 2 points for dipole bonds, and 1 point for dispersion bonds. Given the example, PF4 has dipole bonds, so it gets 2 points. Meanwhile, CF4 has dispersion bonds, so it gets 1 point. Since PF4 has more points than CF4, it has a higher boiling point.


About the Author

Vivek Saxena has been a full-time freelance writer since 2004, contributing to several online publications. Prior to becoming a writer, Saxena studied computer technology at Purdue University.