The value of \(\Delta H_{\text {soln }}\) for the formation of an ethanolhexane solution is positive. Explain this in general terms that involve intermolecular forces of attraction.

Intermolecular forces are the attractive forces between molecules, which hold substances together and determine many of their physical properties. They vary in strength and include hydrogen bonding, dipole-dipole interactions, and van der Waals forces. In the context of solutions, they play a crucial role.

For example, when you dissolve a substance in a solvent, these forces must be overcome to separate the molecules of the solute. This is followed by the formation of new intermolecular forces between the solute and solvent molecules. If the new interactions are weaker than the original ones, energy is absorbed in the process, resulting in a positive change in enthalpy ( Delta H_{ text{soln}} ).

Significance in Solutions

In the case of an ethanol-hexane solution, ethanol's strong hydrogen bonds must be overcome by inputting energy, which can result in a positive Delta H_{ text{soln}} as observed.

An endothermic process occurs when a system absorbs heat from its surroundings. This concept is often encountered in chemistry, especially in reactions and phase changes such as melting or evaporating. When a reaction or process is endothermic, the energy of the system increases, and the temperature of the surroundings decreases.

Considering the ethanol-hexane solution, the positive Delta H_{ text{soln}} indicates an endothermic process. The system (ethanol-hexane mixture) requires more energy to break the intermolecular forces within the pure substances than is released when the new solution forms, leading to a net absorption of heat from the surroundings.

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, and is attracted to another electronegative atom in a different molecule. This interaction is significantly stronger than regular van der Waals forces but weaker than covalent or ionic bonds.

In our ethanol-hexane example, ethanol molecules feature hydrogen bonds due to the hydroxyl (–OH) group present in their structure. These bonds are strong and require a significant amount of energy to break, which contributes to the endothermic nature of the solution process when mixed with hexane.

van der Waals forces are relatively weak interactions compared to other intermolecular forces, but they are ubiquitous. These forces include London dispersion forces and dipole-dipole interactions that occur in all molecules, whether polar or nonpolar. London dispersion forces are especially important in nonpolar molecules such as hexane.

While ethanol relies on hydrogen bonding, hexane's molecules interact primarily through van der Waals forces. The disparity in interaction strengths means that when the two liquids are mixed, the strong hydrogen bonds in ethanol are not fully compensated by the weaker van der Waals forces in the resulting solution, further explaining the positive Delta H_{ text{soln}} observed.