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Difficulty: HardSimple and Fractional Distillation

During the fractional distillation of a miscible mixture of volatile liquids, a thermal gradient is established along the fractionating column. As vapor ascends from the bottom to the top of the column, which of the following correctly describes the changes in temperature and vapor composition?

  1. The temperature decreases, and the vapor becomes increasingly enriched in the component with the lower boiling point.Answer
  2. B
    The temperature decreases, and the vapor becomes increasingly enriched in the component with the higher boiling point.
  3. C
    The temperature increases, and the vapor becomes increasingly enriched in the component with the lower boiling point.
  4. D
    The temperature increases, and the vapor becomes increasingly enriched in the component with the higher boiling point.

Answer

The temperature decreases from bottom to top along the column, while the ascending vapor becomes increasingly enriched in the component with the lower boiling point (more volatile liquid).
In a fractionating column, the base nearest the boiling liquid is hottest and the top near the thermometer and condenser outlet is coolest. As mixed vapors ascend, continuous condensation and re-evaporation occur along the packing material (glass beads). The component with the higher boiling point condenses preferentially on the cooler surfaces and returns down the column as reflux, whereas the component with the lower boiling point remains in the vapor state and concentrates at the top to exit into the condenser.

Step-by-Step Solution

1
Analyze the temperature profile along the fractionating column.
The heat source is at the bottom (distillation flask), so the temperature is highest at the base and decreases continuously toward the top of the column.
Heat dissipates as vapor moves further away from the boiling liquid flask.
2
Determine the condensation and evaporation behavior of the mixture components.
The component with the higher boiling point has a lower vapor pressure (is less volatile) and condenses easily on the cool packing surfaces, dripping back down into the flask as liquid reflux.
Higher boiling point substances require higher thermal energy to stay in the gaseous phase.
3
Evaluate the composition of the vapor reaching the top of the column.
Repeated condensation-evaporation cycles along the glass beads enrich the ascending vapor in the component with the lower boiling point (more volatile component), which distills over first into the condenser.
The more volatile component evaporates more readily at lower temperatures established at the upper region of the column.

Key Concept

Temperature Gradient and Vapor Enrichment in Fractional Distillation
Estimated Time:1m 30s
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Topics

Simple and Fractional Distillation

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