Problem 14 - Chemistry The Molecular Nature of Matter

Question

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.

Step-by-Step Solution

Answer

The positive (Delta H_{cext{soln}} ) for ethanol-hexane solution formation indicates that the process is endothermic, requiring more energy to overcome ethanol's strong hydrogen bonds than is released by the weaker intermolecular interactions in the resulting mixture.

Step 1: Understanding the Concept of Enthalpy Change

The enthalpy change of solution, c ( Delta H_{cext{soln}} ), refers to the heat absorbed or released when a solution is formed. A positive (Delta H_{cext{soln}} ) implies that the process is endothermic, meaning that the system absorbs heat from the surroundings.

Step 2: Interpreting Positive (Delta H_{cext{soln}} ) in Terms of Intermolecular Forces

A positive value for the enthalpy change indicates that the force of attraction between the solute and solvent molecules is weaker than the forces of attraction within the pure solute and within the pure solvent. In this case, more energy is needed to separate ethanol and hexane molecules than is released when they interact to form the solution.

Step 3: Applying Concepts to Ethanol-Hexane Solution

Ethanol has strong hydrogen bonding due to its hydroxyl group, while hexane has weaker van der Waals forces. Mixing them requires energy to overcome the ethanol's hydrogen bonds, which is not fully compensated by the energy released from the weaker intermolecular interactions in the resulting solution. Hence, the overall process of forming an ethanol-hexane solution is endothermic, leading to a positive (Delta H_{cext{soln}} ).

Key Concepts

Intermolecular Forces

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.

Endothermic Process

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

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

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.