What intermolecular forces are present between to molecules NaCl and water?

Answer 1

There are no molecules of NaCl. NaCl is an "ionic" solid (The bonds have 70% ionic character.) It exists as a network of alternating Na and Cl atoms. When NaCl dissolves in water it ionizes, producing Na+ and Cl- ions, each surrounded by a sphere of water molecules due to the attraction between the charged ion and the polar water molecules.

Intermolecular forces, collectively known as van der Waals's forces are attractions between discrete molecules. There are several types of van der Waals forces: London dispersion forces, Debye forces, Keesom forces, and hydrogen bonding. The various kinds of intermolecular forces have a bearing on properties like melting point, boiling point and surface tension to name three.

London dispersion forces, which are present between all molecules, are the electrostatic attraction between temporary dipoles. A normally nonpolar molecule can become a temporary dipole due to electron fluctuations withing the molecule.

The strength of London dispersion forces are sometimes (erroneously) attributed to variations on molecular weight. The connection between the strength of London dispersion forces and molecular weight is coincidental. The strength of London dispersion forces are a function of the polarizability of the molecule, which in turn, depends on the number of electrons and the surface area of the molecule. The greater the number of electrons, the greater the polarizability and the stronger the London dispersion forces.

Dipole-dipole attraction (Keesom forces) exist between molecules which exhibit a permanent dipole moment, that is, molecules which are polar. The positive end of one molecule is electrostatically attracted to the negative end of an adjacent molecule.

Debye forces exist between a polar molecule and a nonpolar molecule by inducing a dipole in the normally nonpolar molecule. As a polar molecule approaches a nonpolar molecule, the charged end of polar molecule induces the opposite charge in the nonpolar molecule by either attracting or repelling the electrons in the nonpolar molecule, thus causing electrostatic attractin between the polar and the nonpolar molecule.

London dispersion forces and Keesom forces exist between like molecules, as well as unlike molecules. Debye forces only exist between unlike molecules. Of the three, Keesom forces are the strongest, followed by Debye forces and then London dispersion forces. That being said, London dispersion forces can be strong enough to hold large molecular weight alkanes together in the solid state.

Hydrogen bonding is unique among molecular interactions due to the formation of a weak covalent bond between adjacent molecules. Hydrogen bonding occurs when hydrogen is internally bonded to N, O or F, and weakly covalently bonded to a N, O or F atom in an adjacent molecule. The hydrogen atom essentially bridges the two molecules. Hydrogen bonding is generally considered the strongest of the intermolecular interactions.