Intermolecular Forces

When water boils, what is happening to the water molecules? ▫They are not breaking into oxygen and hydrogen atoms; rather they are separating from each other to form gaseous water. The addition of heat energy to a molecular substance initially separates the molecules from each other and changes their state.

Intermolecular Forces The bonds that are being broken are not covalent bonds, they are intermolecular forces. The strength of intermolecular forces is determined by the strength of the electrostatic attraction between molecules that, in turn, is dependent on whether the molecules are polar or non- polar, big or small.

van der Waals’ Forces While many molecules such as water and hydrogen chloride have a permanent dipole, there are many other substances whose molecules are non-polar. These non-polar molecules must be attracted to each other by some electrostatic forces because many of them are liquids or solids at room temperature.

van der Waals’ Forces If there was no force at all between non-polar molecules, they would all be gases at temperatures well below 0 o C. However, characteristics suggest that the bonding between these molecules is weak.

van der Waals’ Forces To appreciate the source of the weak attractive forces acting between these molecules, we must consider the orientation of all electrons, both bonding and non-bonding, at a particular instant rather than over a period of time.

van der Waals’ Forces Take the simplest molecule, hydrogen, and consider the orientation of the two bonding electrons. While it is possible that these electrons will be symmetrically oriented around the two hydrogen nuclei, it is far more likely that they will be found at one side of the molecule. When this happens a temporary or instantaneous dipole is formed.

van der Waals’ Forces

Once that temporary dipole has been generated, it will influence the orientation of electrons within molecules close to it. Electrons in neighboring molecules will be repelled by the negative end of the temporary dipole, creating a new induced dipole.

van der Waals’ Forces A moment later, the electrons attain a different orientation, and a new set of interacting dipoles will be generated. Over time, the orientation in three-dimensional space of the electrons of a specific molecule will average out to produce no permanent dipole.

van der Waals’ Forces However, the weak interactions generated by billions of temporary dipoles will have resulted in a weak overall attractive force. These weak bonding forces are known collectively as van der Waals’ forces or dispersion forces or London forces. They exist in all molecules, irrespective of any other intermolecular forces that may also be present.

van der Waals’ Forces The larger a molecule, and so the greater the number of electrons present, the more pronounced will be these van der Waals’ forces.

Permanent Dipole Attraction Polar molecules have permanent dipole. One “end” of the molecule is always positive in relation to the other “end” of the molecule. ▫HCl molecule has a permanent dipole because the chlorine atom is more electronegative than the hydrogen atom, and so the bonding electrons will more often be located nearer to the chlorine than the hydrogen.

Permanent Dipole Attraction ▫As electrons have a negative charge, the chlorine end of the molecule will tend to carry more negative charge than the slightly positive hydrogen end. As these molecules have a positive part and a negative part, they can attract other polar molecules by electrostatic attraction.

Permanent Dipole Attraction The strength of this attraction depends on the size of the dipole, and this is determined by the difference in the electronegativities of the atoms in the molecule.

Hydrogen Bonding The most electronegative elements are all found in the top right-hand corner of the periodic table: ▫Fluorine (4.0) ▫Oxygen (3.5) ▫Nitrogen (3.0) The combination of any of these three elements with hydrogen (2.1) produces a particularly strong dipole and results in highly polar molecules such as water, ammonia and hydrogen fluoride.

Hydrogen Bonding The special name of hydrogen bonding is given to the intermolecular forces that occur between molecules that contain H- F, H-O and H-N bonds. In hydrogen bonding, the positive (H) end of a dipole is strongly attracted the the negative (F, O or N) end of the dipole of another molecule.

Hydrogen Bonding

Water Water exhibits hydrogen bonding between its molecules and it is these particularly strong intermolecular bonds that explain some of the unusual properties of water (relatively high melting and boiling points and low density when frozen).