Question

30\. 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 possible explanation for the considerably less combined volume of the water and ethanol mixture is the formation of hydrogen bonds between the water and ethanol molecules. These hydrogen bonds allow the molecules to pack together more closely, resulting in a decrease in the total volume of the liquid mixture compared to the sum of their individual volumes.

Step-by-step solution

Understanding the molecular structure of water and ethanol

Water molecules have a bent shape with one oxygen atom bonded to two hydrogen atoms. The oxygen atom is more electronegative, meaning it attracts the electrons more, leading to a partial negative charge on the oxygen end and a partial positive charge on the hydrogen ends. This creates a polar molecule. Ethanol molecules have a similar structure, with an oxygen atom bonded to a hydrogen atom (forming an -OH group) and a carbon chain. The oxygen and hydrogen atom within the -OH group have the same electronegativity difference, which also makes ethanol a polar molecule.

Understanding intermolecular forces between water and ethanol molecules

The interaction between water and ethanol molecules can be primarily attributed to hydrogen bonding, which is a strong type of intermolecular force between polar molecules. In the case of water and ethanol, the partially positively charged hydrogen atoms of water form hydrogen bonds with the partially negatively charged oxygen atoms of ethanol. Similarly, the partially positively charged hydrogen atoms of ethanol form hydrogen bonds with the partially negatively charged oxygen atoms of water.

Visualizing the volume change upon mixing

When water and ethanol are mixed, the hydrogen bonds formed between their molecules allow them to fit together more compactly than they would separately. This means that the combined volume of the mixture is less than the sum of their individual volumes because the molecules essentially "fill in" the empty spaces between each other due to hydrogen bonding.

Explanation for the observed phenomenon

The possible explanation for the considerably less combined volume of the water and ethanol mixture is the formation of hydrogen bonds between the water and ethanol molecules. These hydrogen bonds allow the molecules to pack together more closely, resulting in a decrease in the total volume of the liquid mixture compared to the sum of their individual volumes.

Key concepts

Hydrogen Bonding

Imagine holding hands with your friends in a crowded room; you pull each other close, taking up less space than if you were all standing apart. This is similar to what's happening on a tiny scale with hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, experiences an attraction to another electronegative atom from a different molecule.

Due to their positive and negative charges, these atoms come together and form a special bond—this isn't as strong as a covalent bond but significantly stronger than other types of van der Waals forces. In a water-ethanol mixture, hydrogen bonding is responsible for the attraction between the partially positively charged hydrogens of one molecule and the partially negatively charged oxygen of another molecule, which leads to a tighter packing and a reduction in overall volume.

Molecular Structure

Imagine building blocks that can connect in specific ways to create different structures. In the molecular world, the 'blocks' are atoms, and the way they connect defines the molecule's structure. The molecular structure of a compound, including the spatial arrangement of its atoms and the types of bonds holding the atoms together, significantly influences the compound's properties.

For the water (H₂O) molecule, the structure is bent, resembling a V-shape, while ethanol (C₂H₅OH) has both a straight-chain part and an -OH group, similar to water's hydroxyl group. This complementary structure allows water and ethanol molecules to effectively attract each other, resulting in a complex arrangement that reduces the volume when mixed.

Intermolecular Forces

Imagine a group of magnets—if you've ever played with them, you've noticed how they can either stick together or repel each other. Intermolecular forces work in a similar manner, being the 'glue' or 'repulsion' between molecules, influencing their physical states, and determining how substances interact. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds among others.

For water and ethanol, hydrogen bonds are the star players, but other intermolecular forces like dipole-dipole interactions also contribute to how these molecules interact. When you mix the two, these forces rearrange in such a way that the molecules 'snuggle up,' decreasing the space they occupy compared to when they're alone. This principle helps explain the volume contraction observed in the water-ethanol mixture.

Polarity of Molecules

Have you ever seen oil try to mix with water? It just doesn't work because they're fundamentally different at the molecular level. This difference is due to polarity—a molecule's uneven distribution of electrical charge. When a molecule has areas with partial positive and partial negative charges, it's polar, and it typically mixes well with other polar molecules.

Both water and ethanol are polar molecules, which is why they mix so readily. Their polarity enables the formation of hydrogen bonds, and it's also why pure water and pure ethanol have a lower volume when mixed together—like a group dance where everyone knows the moves and fits into a smaller area. It's the polarity that drives that closeness, making the mixture more compact than the separate liquids.