Distillation: Vapor–Liquid Equilibria

Vapor-liquid equilibrium describes how components distribute between vapor and liquid phases during distillation. It determines which component boils first and how the composition of the distillate changes throughout the process. Understanding this equilibrium helps predict boiling points and identify when distillation ends based on mole fractions of the mixture components.

As the more volatile component boils off, the mixture becomes enriched in the less volatile component, which requires higher temperatures to vaporize. The temperature rise reflects the changing composition of the remaining liquid. This temperature change is directly related to the vapor-liquid equilibrium of the binary mixture and helps indicate the distillation stage.

Early distillate is rich in the more volatile component because it vaporizes first. As distillation continues, the middle distillate becomes a more even mixture of both components. The final distillate is rich in the less volatile component. This compositional progression reflects the changing vapor-liquid equilibrium as the mixture composition shifts.

A vapor-liquid equilibrium diagram plots equilibrium temperature against the mole fraction of components in a binary mixture. The x-axis shows mole fraction and the y-axis shows temperature. These diagrams, available in literature for common mixtures, allow chemists to determine boiling points, predict liquid and vapor composition at any temperature, and identify when distillation will end.

Distillation requires miscible liquids with a significant difference in boiling points, typically at least 20 degrees Celsius. This substantial difference ensures that the more volatile component vaporizes preferentially, allowing effective separation. Without sufficient boiling point difference, the vapor-liquid equilibrium relationships cannot provide adequate separation between components.

The condenser is a glass tube with separate inner and outer sections. Vapor travels into the inner section where it contacts cold water flowing through the outer section. This temperature difference condenses the vapor back to liquid, called distillate, which is then collected in a graduated cylinder or test tube for analysis.

A component enters the vapor phase when the mixture reaches its bubble point temperature. The more volatile component has a lower bubble point and vaporizes first. As heating continues and composition changes, the bubble point rises, allowing the less volatile component to eventually vaporize. This behavior is governed by the states of matter and phase changes occurring in the mixture.