IntermolecularForces

There are multiple types of intermolecular forces with varying strengths TABLE 5.1 Interionic and Intermolecular Interactions* Type of interaction ion—ion ion—dipole dipole—dipole dipole—induced dipole London (dispersion)t hydrogen bonding Typical energy 250 15 2 0.3 2 2 20 Interacting species ions only ions and polar molecules stationary polar molecules rotating polar molecules at least one molecule must be polar all types of molecules molecules containing N, O, F; the link is a shared H atom The total interaction experienced by a species is the sum of all the interactions in which it can participate. tAlso known as the induced-dipole—induced-dipole interaction.
Interaction between two charged particles. This is largely dictated by Coulomb's law However, this would predict that a radius of zero would maximize the stability of the interaction - this clearly cannot be the case because molecules cannot exist in the same physical space. A more complex version of Coulomb's law can be derived (not done here) that includes a repulsion term. Untitled picture.emf Stabilization Energy Ion-Ion Interaction between two polar molecules - partially positive and partially negative poles align to form a favorable Coulombic interaction. Dipole-Dipole These are forces that exist in ALL molecules. Consequently, they can stabilize interactions between nonpolar molecules. This is important because nonpolar molecules have no other way of interacting (and we know that nonpolar compounds can exist in condensed phases so there must be intermolecular forces present). London Dispersion Forces (Induced Dipoles) �𝐸�𝑝�∝��𝑞�1��𝑞�2��𝑟� Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings The strength of this intermolecular force can be estimated using a variation of Coulomb's law: �𝐸�𝑝�∝��𝜇�1��𝜇�2���𝑟�3�� The important thing to note here is that the stabilization energy decreased proportional to r3. Ink Drawings Ink Drawings Ink Drawings                
These are forces that exist in ALL molecules. Consequently, they can stabilize interactions between nonpolar molecules. This is important because nonpolar molecules have no other way of interacting (and we know that nonpolar compounds can exist in condensed phases so there must be intermolecular forces present). These forces originate from the fact that all molecules can have short-lived, transient, dipoles that are created due to incredibly rapid electron motion. For a brief instant, non-polar molecules become polar and if there is something around that can stabilize the polarity, the induced dipole can become more permanent. The strength of LDF increases with molecule size. A special class of dipole-dipole interactions. When the positive pole (hydrogen) of a H-X bond (X can be N, O, or F) interacts with the negative pole of another H-X bond (the X), a very strong difference in electronegativity exits. Strong enough, in fact, that a lone pair on the negative pole can "reach out" and make a pseudo-bond with the hydrogen. This pseudo-bond is known as a hydrogen bond. H-bonds can play very major roles in the physical properties of molecules. For example, the properties of water are largely dictated by the complex array of H-bonds that exist between molecules (each water can make 4 H-bonds) Molecules can be characterized as a H-bond donor if they have an X-H bond donor (where the H is available to participate in an H-bond), an H-bond acceptor (where lone pairs on the X are available to reach out to form the H-bond with a hydrogen), or both. It's notable that H-bond acceptors are NOT required to actually contain an H-X, they only need to have X (N/O/F) in a polar bond with lone pairs available. For example, the ether below can be an H-bond acceptor even though there is not a H-O bond. H-bonds Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings                             Ink Drawings Ink Drawings Ink Drawings Ink Drawings 
This type of H-bond interaction explains why some molecules are more soluble in water than would be expected based purely on dipole-dipole interactions. Ink Drawings Ink Drawings      

 

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