Problem 69 - Chemistry Matter and Change

Question

Explain why water has a higher surface tension than benzene, whose molecules are nonpolar.

Step-by-Step Solution

Answer

Water has a higher surface tension than benzene because it forms strong hydrogen bonds due to its polar nature, while benzene, being nonpolar, experiences weaker dispersion forces. This strong attractive force in water leads to higher resistance to deformation at the surface, resulting in a higher surface tension. In contrast, the weaker dispersion forces in benzene require less energy to overcome, leading to a lower surface tension.

Step 1: Understand molecular structures of water and benzene

Water (H2O) is a polar molecule, which means it has a positive and a negative end due to the uneven distribution of electrons. In water, the oxygen atom is more electronegative than hydrogen atoms, resulting in a partial negative charge on the oxygen and partial positive charges on the hydrogens. Benzene (C6H6) is a nonpolar molecule, formed by a hexagonal ring of carbon atoms with alternating single and double bonds. Each carbon atom is bonded to one hydrogen atom. Since the electron distribution is relatively even, benzene is nonpolar.

Step 2: Identify the intermolecular forces

In water, the polar nature of molecules allows for strong hydrogen bonding. Hydrogen bonds are formed when a hydrogen atom, bonded to a highly electronegative atom like oxygen, forms an attractive force with another electronegative atom (such as oxygen in a neighboring water molecule). This leads to a network of interconnected hydrogen bonds which, collectively, provide a strong attractive force between water molecules. Benzene, being nonpolar, does not form hydrogen bonds. It experiences weaker intermolecular forces, specifically dispersion forces (also known as van der Waals or London forces) which are short-range, attractive forces caused by temporary fluctuations in electron distribution in molecules.

Step 3: Relate intermolecular forces to surface tension

Surface tension is a measure of the energy required to increase the surface area of a liquid by a unit area. It results from the imbalance of forces at the liquid-air interface, where molecules at the surface experience an inward net force due to being surrounded by other molecules on only one side. This creates a natural tendency for the surface to contract, resist deformation, and minimize its area. In water, the strong hydrogen bonds between molecules result in a high degree of attraction between the molecules at the surface. This leads to a higher surface tension, as it is more difficult to break or stretch these hydrogen bonds to increase the surface area. In benzene, the weaker dispersion forces between molecules mean that there is less energy required to increase the surface area. This results in a lower surface tension, as it is easier to move and separate benzene molecules from one another.

Step 4: Conclusion

Water has a higher surface tension than benzene because its polar nature allows for strong hydrogen bonding between water molecules, creating a strong attractive force and resistance to deformation at the surface. In contrast, benzene's nonpolar nature results in weaker dispersion forces, which require less energy to overcome and lead to a lower surface tension.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including attraction or repulsion. These are different from intramolecular forces, which are the forces within a molecule that hold it together, such as covalent or ionic bonds. Intermolecular forces are typically weaker than intramolecular forces, but they are crucial for determining the physical properties of substances, such as melting and boiling points, viscosity, and surface tension.

Types of Intermolecular Forces

There are several types of intermolecular forces, with varying degrees of strength. The key types include dispersion forces, dipole-dipole interactions, and hydrogen bonds. The nature of these forces depends on the molecular structure and polarity. For instance, nonpolar molecules like benzene primarily experience dispersion forces, while polar molecules like water engage in both dipole-dipole interactions and hydrogen bonding, which are much stronger.

Polarity of Water Molecules

The concept of polarity in chemistry describes how electrons are distributed around a molecule, leading to areas of partial positive and negative charges. Water molecules are polar because of the bent shape of the molecule and the high electronegativity of oxygen. Electronegativity refers to the tendency of an atom to attract a shared pair of electrons towards itself.

Electronegativity and Polarity

In a water molecule (H_{2}O), the oxygen atom has a greater electronegativity compared to the hydrogen atoms. This causes the electrons shared in the covalent bonds to be pulled closer to the oxygen, leaving it with a slight negative charge and the hydrogen atoms with slight positive charges. The polar nature of water is what allows hydrogen bonds to form between molecules, a key contributor to water's higher surface tension compared to nonpolar liquids like benzene.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs in molecules containing hydrogen atoms bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. In water molecules, the partially positive hydrogen atoms of one molecule are attracted to the partially negative oxygen atoms of surrounding molecules.

The Strength of Hydrogen Bonds

Although hydrogen bonds are weaker than covalent and ionic bonds, they are stronger than other intermolecular forces. The cumulative effect of these bonds in water leads to a cohesive network that holds the molecules together tightly, contributing to water's unique properties such as high boiling point and high surface tension. Understanding hydrogen bonding is crucial for explaining why water droplets hold together and why they bead up on certain surfaces.

Dispersion Forces

Dispersion forces, also referred to as London dispersion forces or van der Waals forces, are the weakest type of intermolecular forces. They are present in all molecules, whether polar or nonpolar, due to fluctuations in the electron distribution within molecules.

Role in Nonpolar Molecules

These temporary dipoles induce dipoles in adjacent molecules, leading to an attractive force between them. For nonpolar molecules like benzene, dispersion forces are the main type of intermolecular force. Since these forces are relatively weak, nonpolar molecules tend to have lower boiling points and surface tensions compared to polar molecules with stronger intermolecular forces such as hydrogen bonds. Understanding dispersion forces helps explain why substances like benzene have lower surface tension than water.