Liquid Phase – Intermolecular Forces

Intermolecular forces are the forces that exist between molecules in a covalently bonded substance. These forces are what need to be broken apart in order for covalent substances to change phases.4 When ionic substances change phase, bonds between the individual ions are actually broken. When covalent substances change phase, the bonds between the individual atoms remain in place, it is just the forces that hold the molecules to other molecules that break apart. The three intermolecular forces are: dipole-dipole forces, hydrogen bonds and London dispersion forces.

Dipole–dipole forces occur between neutral, polar molecules.4 The positive end of one polar molecule is attracted to the negative end of another polar molecule. Molecules with greater polarity will have greater dipole–dipole attraction, so molecules with larger dipole moments tend to have higher melting and boiling points.4 Dipole–dipole attractions are relatively weak, however, and these substances melt and boil at very low temperatures. Dipole–dipole interactions are present in the solid and liquid phases but become negligible in the gas phase because of the significantly increased distance between gas particles.

Hydrogen bonds are similar to dipole–dipole attractions.4 In a hydrogen bond, the positively charged hydrogen end of a molecule is attracted to the negatively charged end of another molecule containing an extremely electronegative element (fluorine, oxygen, or nitrogen). Hydrogen bonds are much stronger than normal dipole–dipole forces because when a hydrogen atom gives up its lone electron to a bond, its positively charged nucleus is left basically unshielded. Substances that have hydrogen bonds, such as water and ammonia, have higher melting and boiling points than substances that are held together only by other types of intermolecular forces. Water is less dense as a solid than as a liquid because its hydrogen bonds force the molecules in ice to form a crystal structure, which keeps them farther apart than they are in the liquid form.

The bonding electrons in nonpolar covalent bonds may appear to be shared equally between two atoms, but at any point in time, they will be located randomly throughout the orbital. In a given moment, the electron density may be unequally distributed between the two atoms. This results in a rapid polarization and counterpolarization of the electron cloud and the formation of short-lived dipole moments. Subsequently, these dipoles interact with the electron clouds of neighboring compounds, inducing the formation of more dipoles. The momentarily negative end of one molecule will cause the closest region in any neighboring molecule to become temporarily positive itself. This causes the other end of the neighboring molecule to become temporarily negative, which in turn induces other molecules to become temporarily polarized, and the cycle begins again. The attractive interactions of these short-lived and rapidly shifting dipoles are known as London dispersion forces, a type of van der Waals force. London dispersion forces occur between all molecules.4 These very weak attractions occur because of the random motions of electrons on atoms within molecules. At a given moment, a nonpolar molecule might have more electrons on one side than on the other, giving it an instantaneous polarity. For that fleeting instant, the molecule will act as a very weak dipole. Since London dispersion forces depend on the random motions of electrons, molecules with more electrons will experience greater London dispersion forces. So among substances that experience only London dispersion forces, the one with more electrons will generally have higher melting and boiling points. They do not extend over long distances and are, therefore, significant only when molecules are in close proximity. The strength of the London force also depends on the degree and ease by which the molecules can be polarized, that is, how easily the electrons can be shifted around. Large molecules with electrons that are far from the nucleus are easily polarizable and thus possess greater dispersion forces.4 While dispersion forces are the weakest of the intermolecular attractions, when there are millions of these interactions there is an amazing power of adhesion. This is demonstrated by geckos’ feet as the animal’s ability to climb smooth, vertical, and even inverted surfaces is due to dispersion forces. If it weren’t for them, the noble gases would not liquefy at any temperature because no other intermolecular forces exist between the noble gas atoms. The low temperatures at which noble gases liquefy are indicative of the very small magnitude of the dispersion forces between the atoms.

 

References

1) Authors, C. (2004). Caltech Library Service. Retrieved from

http://authors.library.caltech.edu/25050/12/Chapter_11.pdf

2) University of California, S. B. (2015). Retrieved from

http://web.chem.ucsb.edu/~devries/chem1C/handouts/zumdahl_chemprin_6e_csm_ch14.pdf

3) Romero, A. (2007). Retrieved from

http://www.cabrillo.edu/~aromero/CHEM_1B/1B_Handouts/Isomers%20Handout.pdf

4) Lufaso, M. (2016). Retrieved from University of North Florida:

https://www.unf.edu/~michael.lufaso/chem2046/2046chapter11_2slide.pdf

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