Question

For ideal solutions, the volumes are additive. This means that if $5\mathrm{mL}$ of $\mathrm{A}$ and $5\mathrm{mL}$ of $\mathrm{B}$ form an ideal solution, the volume of the solution is $10\mathrm{mL}$. Provide a molecular interpretation for this observation. When 500 $\mathrm{mL}$ of ethanol $\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\right)$ is mixed with $500\mathrm{mL}$ of water, the final volume is less than 1000 mL. Why?

Short answer

Ethanol and water form strong hydrogen bonds, reducing the total volume of the mixture.

Step-by-step solution

Understanding Ideal Solutions

In an ideal solution, the volume of the solution is simply the sum of the volumes of the individual components. This occurs because the molecular interactions between different components are similar to the interactions within each pure component.

Introducing Non-ideal Solutions

In real solutions, volumes may not be strictly additive due to molecular interactions. For instance, there could be stronger or weaker forces between the components once mixed, compared to the pure substances.

Examining Ethanol-Water Mixture

When ethanol and water are mixed, the resulting solution often has a decreased total volume due to the strong hydrogen bonding between ethanol and water molecules, causing them to pack more closely together than in their separate forms.

Molecular Interpretation

The hydroxyl group of ethanol can form hydrogen bonds with water more effectively than with another ethanol molecule or water with itself. This strong attraction causes the molecules to come together more tightly, reducing the total volume.

Key concepts

Molecular Interactions

Molecular interactions refer to the forces that occur between molecules. In ideal solutions, these interactions between different molecules are similar to those between like molecules. This balance ensures that the volumes of solutions are simply the sum of the parts. In non-ideal solutions, however, these interactions can vary considerably.
When liquids are mixed, such as ethanol and water, the strength and type of molecular interactions can change. This often leads to different solution properties. For example:

• Van der Waals forces: These relatively weak forces may exist between non-polar molecules.
• Dipole-dipole interactions: These occur between molecules with permanent dipoles.
• Hydrogen bonding: A strong form of dipole-dipole interaction seen in molecules like water.

Understanding molecular interactions helps explain why solutions do not always behave as ideal ones.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction between molecules, where a hydrogen atom is directly bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. This bond considerably influences the behavior of solutions.
In the example of an ethanol-water mixture, hydrogen bonds form between the hydroxyl group (-OH) of ethanol and water molecules. This type of bonding is much stronger than typical interactions, causing the molecules to draw closer together.
Effects of hydrogen bonding include:

• Increased boiling and melting points of substances.
• Shrinkage of volume in solutions, as seen in ethanol-water mixtures.
• Unique molecular arrangements that influence physical properties.

Recognizing the role of hydrogen bonds is crucial in predicting the behavior of non-ideal solutions.

Non-ideal Solutions

Non-ideal solutions are those in which the properties can’t be predicted by simple addition of component properties, due to significant molecular interactions that differ from those in ideal solutions.
In non-ideal solutions, these interactions lead to variations in physical properties, like volume or enthalpy.
For example, in the ethanol-water mixture, the deviations are largely due to distinct hydrogen bonding capabilities. Because of this, the final volume doesn't simply match the sum of individual volumes.
Key characteristics of non-ideal solutions include:

• Deviation from Raoult's Law in vapor pressures.
• Altered mixing enthalpies and volumes.
• Local structural changes within the solvent mixture.

Understanding non-ideal solutions requires considering the unique molecular interactions present.

Volume Additivity

Volume additivity refers to the property of some ideal solutions where the total volume is simply the sum of their component volumes. This occurs when molecular interactions remain consistent after mixing.
Ideal behavior assumes uniform interaction energies with no significant expansion or contraction.
Characteristics of volume additivity in ideal solutions:

• No significant changes in density after mixing.
• Predictable volumetric properties useful in industrial processes.
• Absence of strong specific interactions, such as extensive hydrogen bonds.

However, in many real-world cases, such as with ethanol and water, volume additivity breaks down due to pronounced molecular interactions.

Ethanol-Water Mixture

The ethanol-water mixture is a common example of a non-ideal solution, predominantly due to significant hydrogen bonding.
When equal volumes of ethanol and water are mixed, the total volume is less than expected. This occurs because:

• The hydroxyl groups enhance hydrogen bonding, drawing molecules closer.
• Intermolecular spaces decrease, resulting in volume contraction.
• A balance of hydrophilic and hydrophobic interactions influences structure.

This unique behavior of ethanol and water exemplifies how complex molecular interactions dictate non-ideal solution properties.