Question

The viscosity of water at \(20^{\circ} \mathrm{C} 1.00 \times 10^{-3}\left(\mathrm{~N} \cdot \mathrm{s} / \mathrm{m}^{2}\right)\) is higher than dimethyl sulfide \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~S}\right] 2.8 \times 10^{-4}\left(\mathrm{~N} \cdot \mathrm{s} / \mathrm{m}^{2}\right) .\) Explain the dif- ference in viscosity based on chemical structure.

Short answer

Water has stronger intermolecular forces (hydrogen bonds) than dimethyl sulfide, accounting for its higher viscosity.

Step-by-step solution

Understand Definitions

Viscosity is the measure of a fluid's resistance to deformation at a given rate. Essentially, it's how 'thick' or 'sticky' the fluid is.

Examine Water's Structure

Water (H₂O) is a polar molecule with hydrogen bonds between molecules. These hydrogen bonds lead to strong intermolecular forces, which contribute to water's relatively high viscosity.

Examine Dimethyl Sulfide's Structure

Dimethyl sulfide (\((CH_3)_2S\)) is a nonpolar molecule with weak van der Waals forces due to less significant intermolecular forces compared to water. This results in a lower viscosity.

Compare Intermolecular Forces

The main difference in viscosity arises from the type and strength of intermolecular forces: hydrogen bonding in water vs. van der Waals forces in dimethyl sulfide.

Conclusion

Water's higher viscosity than dimethyl sulfide is due to its polar nature and the presence of hydrogen bonds, which create strong intermolecular attractions.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, interacts with a lone pair of electrons on another electronegative atom. This interaction creates a strong bond between molecules. In the case of water (H₂O), each water molecule is capable of forming hydrogen bonds with neighboring molecules. This results in a highly cohesive liquid structure.

Water molecules form these bonds because the oxygen atom is much more electronegative than hydrogen. This means it pulls shared electrons between hydrogen and oxygen more toward itself, resulting in a partial positive charge on hydrogen and a partial negative charge on oxygen. This partial charge allows hydrogen bonds to occur.

• Water's hydrogen bonding contributes to its high boiling point.
• It also gives water its properties of surface tension and capillarity.
• These bonds make water an excellent solvent.

Compared to other intermolecular forces, hydrogen bonding is significantly stronger, accounting for water's high viscosity relative to compounds like dimethyl sulfide.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles (atoms, molecules, or ions). These forces are responsible for the physical properties of substances, such as boiling point, viscosity, and surface tension.

There are several types of intermolecular forces:

• **London Dispersion Forces**: These are weak forces resulting from temporary shifts in electron density, which create temporary dipoles in molecules. They are present in all molecules, significant in nonpolar molecules like dimethyl sulfide.
• **Dipole-Dipole Forces**: These occur between molecules with permanent dipoles, found in polar substances.
• **Hydrogen Bonds**: A special, stronger type of dipole-dipole interaction, as discussed earlier, found in molecules like water.

In the comparison between water and dimethyl sulfide, water's hydrogen bonds (a type of dipole-dipole interaction) make it "stickier" and more resistant to flow than dimethyl sulfide, which primarily features weaker London dispersion forces.

This variance in intermolecular forces is why water has a higher viscosity than dimethyl sulfide.

Chemical Structure

The chemical structure of a substance plays a crucial role in its properties, such as viscosity. The arrangement and types of atoms within a molecule determine how it interacts with other molecules.

**Water's Chemical Structure**: Water has a simple molecular structure of one oxygen atom bonded to two hydrogen atoms (H₂O). Its bent shape and polar nature allow for hydrogen bonding, leading to strong intermolecular attractions that give water a high viscosity.

**Dimethyl Sulfide's Chemical Structure**: Dimethyl sulfide \((CH_3)_2S\) consists of a sulfur atom bonded to two methyl groups. Its structure is more compact and symmetrical, making it nonpolar. Since there are no effective intermolecular attractions like hydrogen bonds, dimethyl sulfide has much weaker van der Waals forces at play, contributing to its lower viscosity.

• The shape and electron distribution within these molecules largely affect their physical properties.
• Polarity and available bonding interactions heavily influence viscosity.

Ultimately, the distinct chemical structures of water and dimethyl sulfide result in very different intermolecular interactions and, therefore, differences in their viscosity.