17- Intermolecular Forces

17- Intermolecular Forces - Intermolecular Forces B. A....

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
B. A. Rowland 53750/53760 IMF I Intermolecular forces are those forces which act between the molecules in a given sample (as opposed to intra molecular forces, which are the forces which hold the atoms together in an individual molecule). Intermolecular forces are weaker than intramolecular (covalent and ionic bonds). Intermolecular forces are responsible for the solid and liquid phases of matter—if there were no intermolecular forces these states of matter could not exist. A phase change (going from a solid to a liquid, or a gas to a solid, for example) is just a manifestation of the rearrangement of the molecules in the sample as you increase/decrease the pressure and temperature. To show that intermolecular forces are indeed weaker than intramolecular forces, we can use water as an example. To break the O-H bond in water (intramolecular forces), you would require 934 kJ/mol, whereas to vaporize water (intermolecular forces), you would need 40.7 kJ/mol. IMF II There are two causes for IMFs in molecules: ions and dipoles. An ion is a completely ionized substance—it carries a full charge (or a multiple thereof): Na + , Cl - , NH 4 + , OH - would be examples. Dipoles, on the other hand, are due to asymmetrical arrangements of the electron cloud in the molecule. Dipoles are characterized by the partial charges they leave in certain regions of the molecule. Ionic interactions will always be stronger than dipole interactions. We can classify two types of dipoles that arise in matter: permanent dipoles (moderately strong) and instantaneous dipoles (fairly weak). As you can see, in order to classify IMFs we need to be able to assign dipole moments to molecules. Permanent Dipole Moments In this section I will give you an algorithm that will help you to determine whether a given molecule would exhibit a permanent dipole moment. If a molecule is determined to not have a dipole moment, you would call that molecule non-polar . If the molecule has a dipole moment, we term that molecule polar . 1. Draw the Lewis dot structure for the molecule. If the molecule contains only carbon and hydrogen then it will have NO dipole moment. 2. Determine the electronic (EG) and molecular (MG) geometries. 3. Determine if there are polar covalent bonds (DEN > 0). If there are no polar covalent bonds then there is NO dipole moment (you are done). 4. If the MG = EG AND you have polar covalent bonds AND all the terminal atoms are the same then there is NO dipole. 5. If the MG = EG AND you have polar covalent bonds AND the terminal atoms
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 4

17- Intermolecular Forces - Intermolecular Forces B. A....

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online