This is a measure of how easy it is for the electrons to move around in the molecule in response to an external electric field - from a neighbouring dipole, for example. The second one is a measure of the polarisability of the various molecules. The first column of data shows the dipole moments of the various molecules. Hiemenz and Raj Rajagopalan, published by CRC Press, third edition 1997Ī Google Books search came up with this table: Principles of Colloid and Surface Chemistry by Paul C. HCl has polar molecules, and yet most of the intermolecular forces are due to dispersion forces. The hydrogen chloride figures are particularly surprising. moleculeĭipole-dipole forces Debye forces dispersion forces Because of the way it is bonded, carbon monoxide has only got a very small permanent dipole. The other two are simple polar molecules, carbon monoxide and hydrogen chloride. Three of these are non-polar molecules where the contribution due to the dispersion forces is unsurprisingly 100%. That is then followed by a table showing percentage contributions of the three types of van der Waals forces for a few simple molecules. In the section about van der Waals bonds, they sayĪbout dipole-dipole forces: "The contribution made to the total bonding by this type of force is usually small."Ībout Debye forces: "The contribution to the total bonding from this effect is also very small."Ībout dispersion forces: "Dispersion forces usually make by far the biggest contribution to van der Waals bonding" It was only ever accessible to very bright A level students. This textbook was published in 1982 and aimed at A level and introductory degree students. Modern Physical Chemistry by Liptrot, Thompson and Walker, published by Bell and Hyman I need to mention them now, because they crop up in two important sources below. These arise from a permanent dipole on one molecule inducing a temporary dipole in a second one. The third kind of interactions (known as Debye forces) are normally overlooked at this level as being relatively minor compared with the first two. These are dipole-dipole interactions (also known as Keesom forces) and dispersion forces (also known as London forces). There are three main types of van der Waals forces, two of which I have discussed at length on the previous page. This current page is essentially a discussion document to try to get a bit closer to the truth. If you are an A level (or equivalent) student, the information on that page is what you will need for exam purposes. Note: If you have come straight to this page from a search engine, it would make sense to read my page about van der Waals forces before you go on. Having spent a couple of weeks researching and thinking about this, I still stand by my original statement, and the rest of this page looks at some of the evidence I have found to support it. Several sources actually put values on these strengths. A student challenged me about this, pointing out that many web sources and books say that dispersion forces are the weakest form of intermolecular attraction. Towards the bottom of the last page, I described dipole-dipole attractions as being "fairly minor compared with dispersion forces". THE STRENGTHS OF VAN DER WAALS DISPERSION FORCES A discussion of the strength of van der Waals dispersion forces
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |