Why are Na⁺ and Cl⁻ ions strongly solvated in water but not in hexane?
Strongly solvated is a relative term. It’s related to how stabilized the ions are. The phrasing of the question suggests that these ions are very strongly stabilized in water but not in hexane. Our job is to figure out why that is.
Sodium plus and Cl⁻ minus ions come together to form the solid sodium chloride. When sodium chloride is mixed with water, it readily dissolves, forming solvated Na⁺ ions and Cl⁻ ions. The enthalpy change for this process is given the name enthalpy of solvation. The enthalpy of solvation is the enthalpy change per mole of dissolved substance. However, be careful, the enthalpy of solvation is not what we are looking for.
What we’re actually looking at is the enthalpy of hydration. The enthalpy of hydration is the enthalpy change per mole of ions between them being in the gas phase and being dissolved in water. So, for the sodium ion, it’s the enthalpy change per mole of sodium ions between being completely naked and being completely surrounded by water.
We can call the equivalent process where hexane is the solvent Δ𝐻 hex. Our job is to work out why dissolving sodium ions and chloride ions in water releases more energy, is more exothermic, than the same process when dissolving in hexane. Na⁺ and Cl⁻ are both ions. They’re both attracted to the opposite charge. When combined with the opposite charge, they each become more stable than when they were separated. Now let’s have a look at our two solvents and see which properties might lead to a difference in the energy released when these ions dissolve.
Water, because of the two lone pairs on the oxygen atom, has a bent shape. Oxygen is much more electronegative than hydrogen, leading to the polar bonds and an overall dipole. So, the oxygen atom is Δ negative and the hydrogen atoms are Δ positive. Hexane, on the other hand, is composed of carbon and hydrogen. And it’s an alkane. The difference in electronegativity between carbon and hydrogen is much smaller than that between oxygen and hydrogen. And the overall structure of the molecule means that there’s a very very tiny overall dipole. It’s so small that the molecule is considered nonpolar.
In water, sodium plus and Cl⁻ experience strong ion-dipole interactions. They are both stabilized by the Δ negative or Δ positive charges on the water molecules. In hexane, the ions experience only weak dispersion forces and so are not significantly stabilized. So, our answer to why are Na⁺ and Cl⁻ ions strongly solvated in water but not in hexane is ion-dipole forces between ions and water are stronger than the hexane forces because hexane is nonpolar and thus interacts with ions far more weakly.