INTERMOLECULAR?

Intermolecular forces?

• Which of the following statements about interionic and intermolecular forces are true ? a) London forces are the weakest type of intermolecular forces. b) Substances for which dipole-dipole interactions are present would have lower boiling points than that of substances where only london forces are present. c) The only intermolecular forces present between F2(g) molecules are London (dispersion) forces. d) The strongest intermolecular forces present between CO2(g) molecules are dipole-dipole forces. e) Dipole-Dipole forces are only present between molecules that have a zero dipole moment

• Answer:

  OK, discussion first. Dipole - dipole interactions and polarity. When atoms covalently bond with other atoms they share electrons. All atoms have different electronegativities. The electronegetivity of an atom is its tendancy to pull electrons toward itself. If two atoms with the same electronegativity bond, then they share the electrons evenly. The bond is said to be non-polar and has no dipole moment. If two atoms with different electronegativities bond, then the atom with the highest electronegativity pulls the electrons more strongly to itself. The electrons are shared unevenly, and the electrons spend more time with te electronegative atom. The electronegative atom ends up with a slightly negative charge and the less electronegative atom ends up with a slightly positive charge. We say that the bond is polar, it has a permanent dipole. (positive and negative end) London forces In an atom or molecule the electrons are located around the atoms in an electron cloud. At any one time the electrons can be anywhere within that cloud. Sometimes, the electron density is higher on one side of the atom then on the other. This causes a momentary dipole to form, so one side of the atom has a slightly positve charge and the other side has a slightly negative charge because the electrons are not evenly distributed. The dipoles does not last for very long, because electrons move pretty fast. The momentary dipoles allow weak attractions to occur between molecules, where the positive momentary dipole of one molecule is attracted to the negative of another, and vise versa. The attractions are very weak because the dipoles that form do not last long, however they are continually forming within all the molecules in the sample. a) True They do not last long and are constantly breaking and reforming elsewhere as the electrons shift around. So their effects are quite weak b). False dipole - dipole interactions form between molecules that have a permanent dipole. (ie polar molecules that have a region of the molecule that is slightly positive and a region that is slightly negative.) The positve region of one molecule will be attracted to the negative region of another. Sometimes these attractions can be quite strong. Boiling involves adding enough energy to a liquid to provide enough energy to individual molecues to be able to break any attractions between itself and surrounding molecules and go wizzing off into the air. Since dipole-dipole interactions are stronger the London forces the energy required to break attractions between them is higher, so the boiling point is higher. c) True (I THINK) The bond in F2 is completely non-polar, because all F atoms have the same electronegativity. This means that in the F-F bond both F's will share electrons evenly, so there is no dipole at all in the bond. To me this suggests that the only type of interation possible would be from the London dispersion forces as described above. d) False (I think) CO2 is a linear molecule O=C=O. Because it is linear it does not have a dipole moment. The bonds between C and O in CO2 are polar, however, because the O's are equal angles apart they both pull electrons away from the C in equal but opposite directions, so the overall effect on the entire molecule is that it is non-polar. It has no permanent dipole moment, and so will not form dipole-dipole interactions. e) False As in b), dipole- dipole interactions form between molecules with a dipole moment.