Question

Which liquid is expected to have the larger surface tension at a given temperature, \(\mathrm{CCl}_{4}\) or \(\mathrm{H}_{2} \mathrm{O}\) ? Explain your answer.

Short answer

\(\mathrm{H}_{2} \mathrm{O}\) is expected to have a larger surface tension at a given temperature than \(\mathrm{CCl}_{4}\) because it exhibits stronger intermolecular forces in the form of hydrogen bonding.

Step-by-step solution

Understanding Surface Tension

Surface tension is a measure of the elastic force in the liquid’s surface. It is caused due to the imbalance of intermolecular forces experienced by the molecules on the surface of a liquid compared to the forces experienced by molecules within the liquid. Molecules with stronger intermolecular forces generally have larger surface tension.

Compare Intermolecular Forces

Compare the intermolecular forces present in each liquid. - \(\mathrm{CCl}_{4}\) has van der Waals forces (induced dipole-induced dipole interactions) because it is non-polar.- \(\mathrm{H}_{2} \mathrm{O}\) has strong hydrogen bonds, which are much stronger than van der Waals forces because water is a polar molecule with a significant dipole moment.

Predicting Surface Tension

Since \(\mathrm{H}_{2} \mathrm{O}\) has stronger intermolecular forces (hydrogen bonds) compared to \(\mathrm{CCl}_{4}\) (van der Waals forces), \(\mathrm{H}_{2} \mathrm{O}\) is expected to have larger surface tension at a given temperature.

Key concepts

Understanding Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). These forces vary in strength but are generally weaker than the intramolecular forces that hold compounds together. They are vitally important as they determine many of the physical properties of substances, including boiling points, melting points, and surface tension. For example, the stronger the intermolecular forces, the higher the boiling point, as more energy is required to break these forces apart.

There are several types of intermolecular forces, each with different strengths and characteristics. Types of intermolecular forces include ionic bonds, dipole-dipole interactions, hydrogen bonds, and van der Waals forces, among others. Understanding these forces allows us to explain why different substances behave in unique ways under similar conditions.

Dipole-Dipole Interactions

• Occur between polar molecules with permanent dipoles.
• These molecules align so that the positive end of one molecule is near the negative end of another molecule.

London Dispersion Forces (Van der Waals Forces)

• Found in all molecules, whether polar or non-polar.
• Arise from the movement of electrons which create a temporary dipole in a molecule.

Hydrogen Bonding

Hydrogen bonding is a special subtype of polar interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom—usually nitrogen (N), oxygen (O), or fluorine (F)—and is in close proximity to another electronegative atom. This bond is notably stronger than other dipole-dipole interactions and contributes profoundly to the physicochemical properties of compounds.

Hydrogen bonds can influence many properties, such as the high surface tension, viscosity, and boiling point of water compared to other molecules of similar size. This type of bonding is also crucial in biological systems; for instance, it plays a key role in the structure of DNA and proteins.

Properties of Hydrogen Bonds

• Directional and strong compared to other dipole-dipole interactions.
• Not as strong as covalent or ionic bonds, but significantly influences structure and behavior of compounds.

Let's not forget that the presence of hydrogen bonding in water contributes to its ability to dissolve many substances, leading to its designation as a universal solvent.

Van der Waals Forces

Van der Waals forces encompass a variety of intermolecular forces, including dipole-dipole interactions, dipole-induced dipole interactions, and London dispersion forces. These forces are named after Dutch scientist Johannes Diderik van der Waals, acknowledging his work on an equation of state for gases that accounts for the non-ideal behavior due to these forces.

London dispersion forces, a type of van der Waals force, are particularly important because they are present in all molecular interactions. Although they are the weakest of the intermolecular forces, they become significant in large, bulky molecules and can be powerful accumulatively.

Characteristics of Van der Waals Forces:

• Affected by the size of the electron cloud: larger atoms or molecules, with more electrons, exhibit stronger dispersion forces.
• Temporary and arise due to random fluctuations in the electron distribution within molecules or atoms.

Recognizing the effects of van der Waals forces is essential in understanding the behavior of non-polar substances like gases, as well as the properties of organic compounds and many plastics.