![]() Using what we learned about predicting relative bond polarities from the electronegativities of the bonded atoms, we can make educated guesses about the relative boiling points of similar molecules. Within a series of compounds of similar molar mass, the strength of the intermolecular interactions increases as the dipole moment of the molecules increases, as shown in Table 2.12.1. Thus a substance such as HCl, which is partially held together by dipole–dipole interactions, is a gas at room temperature and 1 atm pressure, whereas NaCl, which is held together by interionic interactions, is a high-melting-point solid. In contrast, the energy of the interaction of two dipoles is proportional to 1/ r 6, so doubling the distance between the dipoles decreases the strength of the interaction by 2 6, or 64-fold. Doubling the distance ( r → 2 r) decreases the attractive energy by one-half. Recall that the attractive energy between two ions is proportional to 1/ r, where r is the distance between the ions. In addition, the attractive interaction between dipoles falls off much more rapidly with increasing distance than do the ion–ion interactions. The first two are often described collectively as van der Waals forces.įigure 2 Both Attractive and Repulsive Dipole–Dipole Interactions Occur in a Liquid Sample with Many Moleculesīecause each end of a dipole possesses only a fraction of the charge of an electron, dipole–dipole interactions are substantially weaker than the interactions between two ions, each of which has a charge of at least ☑, or between a dipole and an ion, in which one of the species has at least a full positive or negative charge. In this section, we explicitly consider three kinds of intermolecular interactions: There are two additional types of electrostatic interaction that you are already familiar with: the ion–ion interactions that are responsible for ionic bonding and the ion–dipole interactions that occur when ionic substances dissolve in a polar substance such as water. (For more information on the behavior of real gases and deviations from the ideal gas law.) These interactions become important for gases only at very high pressures, where they are responsible for the observed deviations from the ideal gas law at high pressures. Because electrostatic interactions fall off rapidly with increasing distance between molecules, intermolecular interactions are most important for solids and liquids, where the molecules are close together. Like covalent and ionic bonds, intermolecular interactions are the sum of both attractive and repulsive components. Intermolecular forces are electrostatic in nature that is, they arise from the interaction between positively and negatively charged species. ![]() Similarly, solids melt when the molecules acquire enough thermal energy to overcome the intermolecular forces that lock them into place in the solid. Liquids boil when the molecules have enough thermal energy to overcome the intermolecular attractive forces that hold them together, thereby forming bubbles of vapor within the liquid. ![]() Intermolecular forces determine bulk properties such as the melting points of solids and the boiling points of liquids.
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