Question

Vapor pressures of large molecules. Why do large molecules, such as polymers, proteins, and DNA, have very small vapor pressures?

Short answer

Large molecules have very small vapor pressures due to strong intermolecular forces and high energy requirements to transition to the gas phase.

Step-by-step solution

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It indicates how easily molecules escape from the liquid or solid phase into the gas phase.

Molecular Size and Escaping Tendency

Large molecules have a lower tendency to escape into the gas phase due to their size and the numerous intermolecular forces acting among them. These intermolecular forces include van der Waals forces, hydrogen bonds, and sometimes even covalent bonds in polymers.

Intermolecular Forces in Large Molecules

Large molecules like polymers, proteins, and DNA have extensive intermolecular forces that hold the molecules together more tightly in the liquid or solid state. This reduces their volatility and results in lower vapor pressures.

Energy Required for Phase Transition

A higher amount of energy is required to overcome the intermolecular forces in large molecules to transition from liquid or solid to gas. This higher energy requirement translates to lower vapor pressure because not many molecules gain enough kinetic energy to escape into the gas phase.

Final Analysis

The combination of strong intermolecular forces and large molecular size means large molecules have insufficient energy to escape readily into the vapor phase, resulting in very small vapor pressures.

Key concepts

intermolecular forces

Intermolecular forces play a critical role in determining the behavior of molecules, especially large ones like proteins and DNA. These forces are the attractions between molecules, which can be quite strong in large molecules. Some common types of intermolecular forces include:

• Van der Waals forces: These are weak attractions that occur between all types of molecules due to temporary dipoles.
• Hydrogen bonds: These are stronger than van der Waals forces and occur when a hydrogen atom is attracted to a highly electronegative atom like oxygen or nitrogen.
• Covalent bonds: Some large molecules, like polymers, have covalent bonds that can form between chains, increasing their stability.

Because these forces hold the molecules tightly together, large molecules are less likely to transition into the gas phase, resulting in lower vapor pressures. For example, the numerous hydrogen bonds in proteins and DNA create a stable structure that resists evaporation.

molecular size and volatility

Molecular size significantly influences a molecule's volatility, which is its tendency to vaporize. Large molecules generally have a lower volatility compared to small ones due to several reasons:

• Surface area: Larger molecules have more surface area for intermolecular forces to act upon, making it more difficult for them to escape into the gas phase.
• Mass: The greater the mass of the molecule, the more energy is needed to give the molecules enough kinetic energy to overcome these forces.
• Complexity: Large molecules, such as polymers, often have complex structures that increase the strength and number of intermolecular forces holding them together.

Because of these factors, larger molecules have smaller vapor pressures. In other words, they do not easily evaporate because the energy required to break their intermolecular bonds and transition to the gas phase is higher.

phase transition energy

The phase transition energy is the energy required to change a substance from one phase to another, such as from a liquid to a gas. This energy is crucial in understanding vapor pressure for large molecules:

• High energy requirement: Large molecules need more energy to transition from liquid or solid to the gas phase due to their strong intermolecular forces.
• Energy distribution: For a molecule to evaporate, it needs enough kinetic energy to overcome the intermolecular forces. In large molecules, not many molecules gain sufficient energy to escape into the vapor phase.

In essence, the stronger the intermolecular forces, the higher the phase transition energy required. This results in a lower number of molecules being able to vaporize, contributing to the small vapor pressures observed in large molecules.