Question

When \(50 \mathrm{mL}\) of liquid water at \(25^{\circ} \mathrm{C}\) is added to \(50 \mathrm{mL}\) of ethanol (ethyl alcohol), also at \(25^{\circ} \mathrm{C},\) the combined volume of the mixture is considerably less than \(100 \mathrm{mL}\). Give a possible explanation.

Short answer

The observed decrease in the total volume of the water-ethanol mixture can be attributed to hydrogen bonding between water and ethanol molecules. This interaction causes the molecules to pack more tightly together, occupying less space than if they were separate.

Step-by-step solution

Understand the properties of water and ethanol

Water and ethanol are both liquids with unique molecular structures. Water molecules are polar due to the presence of hydrogen bonding, while ethanol is a slightly polar molecule with a hydroxyl group (-OH) and a hydrocarbon chain.

Discuss the molecular interactions between water and ethanol

When water and ethanol are mixed, the polar nature of both molecules allows them to interact with each other, forming hydrogen bonds between the water's oxygen and hydrogen atoms and the hydroxyl group of ethanol. These hydrogen bonds result in a partially negatively charged oxygen in water and partially positively charged hydrogen in ethanol. The polar nature of ethanol allows it to interact with water molecules without disrupting their hydrogen bonding significantly.

Explain volume reduction due to molecular interactions

The interaction between water and ethanol molecules results in an overall decrease in the volume of the mixture. This can be attributed to the hydrogen bonding between water and ethanol causing the molecules to pack more tightly together and, therefore, occupying less space than if they were separate. The intermolecular forces between the molecules lead to a reduction in the overall volume of the liquid mixture compared to the simple sum of their individual volumes.

Provide the explanation for the observed phenomenon

The possible explanation for the observed phenomenon in which the combined volume of the liquid water and ethanol mixture is less than 100 mL is due to the molecular interactions and hydrogen bonding between water and ethanol molecules. The hydrogen bonding causes the molecules to pack more closely together and occupy less space, resulting in a decrease in the total volume of the mixture.

Key concepts

Understanding Hydrogen Bonding

At the heart of the interaction between water and ethanol lies a peculiar type of force known as hydrogen bonding. This force is a type of dipole-dipole attraction, which occurs when the positive end of a polar molecule (the hydrogen side) closely approaches the negative end (such as oxygen or nitrogen) of another polar molecule.

Water, with its H2O structure, is a prime example of a molecule where hydrogen bonds are pivotal to its properties. Each water molecule can form up to four hydrogen bonds with its neighbors, which explains many of its unique characteristics, such as high surface tension, boiling point, and solid-liquid phase behavior. Water’s proclivity for hydrogen bonding makes it an exceptional solvent as well.

In the mixture with ethanol, the hydrogen atoms in water's H2O can form bonds with the oxygen in ethanol's -OH group. It's like each molecule reaches out an arm to embrace the other, resulting in a complex network of intermolecular bonds. These new bonds pull the molecules closer together, a bond far stronger than the average van der Waals forces present in non-polar substances. The close-knit nature of these bonds explains why we observe such surprising phenomena in mixtures of water and ethanol.

Volume Contraction in Mixtures

Ever wonder why mixing two liquids doesn't always result in a volume that's the simple sum of the two? This is exemplified in the curious case of water and ethanol. When these two substances are combined, we witness a phenomenon called volume contraction.

Why does this happen? When water and ethanol molecules come together, they find a more efficient way to pack into a space, thanks to those cozy hydrogen bonds we talked about earlier. Think of it as a perfectly organized suitcase versus one stuffed haphazardly; the neat one will have less 'empty' space. This is what occurs at a molecular level in our mixture.

During this molecular tango, ethanol's -OH group bonds with water's hydrogen and oxygen atoms, pulling each molecule closer than they would be if they were simply neighboring non-bonded molecules. Because they occupy less space, the sum of the volumes after mixing is less than what you'd expect, demonstrating volume contraction.

Properties of Water and Ethanol

Water and ethanol, two common liquids, exhibit a multitude of interesting and contrasting properties. Starting with water, a polar molecule par excellence, it owes much of its behavior to those stalwart hydrogen bonds that give it high heat capacity, surface tension, and boiling point. It's essential for life as we know it, acting not just as a thirst-quencher but as a universal solvent in biological systems.

On the flip side, ethanol is an organic compound famous for its role in alcoholic beverages. While it's also polar due to the presence of a hydroxyl (–OH) group, ethanol is less polar than water because of its hydrocarbon chain. This affects its solubility in water and also makes it a good solvent for substances that water can't dissolve. When these two meet, their contrasting properties come into play, but they're still capable of mingling due to the common ground of polarity.

In our mixture scenario, ethanol's hydrocarbon 'tail' doesn't disrupt the water's hydrogen bonding as drastically as non-polar molecules would. As a matter of fact, it complements the water by adding to the hydrogen bonding network with its -OH group, further exhibiting the intricate dance of molecular interactions.