What is the distinguishing feature of fractional distillation compared to simple distillation?
A It is used to isolate volatile compounds from nonvolatile ones.
B It separates volatile compounds if boiling points differ by at least 40-50 °C.
C It separates each volatile compound from a miscible mixture in pure form if boiling point difference is less than ~40 °C.
D It is based exclusively on Raoult’s law of mole fractions.
The correct answer and explanation is:
The correct answer is:
C. It separates each volatile compound from a miscible mixture in pure form if boiling point difference is less than ~40 °C.
Explanation:
Fractional distillation and simple distillation are both techniques used to separate mixtures based on differences in boiling points, but they differ primarily in the precision and effectiveness of the separation, especially when the boiling points of components are close.
- Simple distillation works well when the boiling points of components differ significantly (usually more than 40-50 °C). This method involves heating the mixture so that the component with the lower boiling point vaporizes first, then condensing the vapor to collect it separately. Because the separation is based on a single vaporization-condensation cycle, it is not very effective for components with closer boiling points.
- Fractional distillation is designed to separate mixtures where the boiling points differ by less than about 40 °C. This technique uses a fractionating column between the boiling flask and the condenser. The column provides a large surface area, allowing multiple vaporization-condensation cycles (theoretical plates) to occur as vapor rises through the column. Each cycle enriches the vapor in the more volatile component, resulting in a more efficient separation.
Because of this repeated vaporization-condensation, fractional distillation can separate each volatile compound from miscible mixtures in a nearly pure form even if their boiling points are close. This makes it essential for separating mixtures like ethanol and water, which differ by about 20 °C in boiling points.
Why the other options are incorrect:
- A: Simple distillation can also isolate volatile compounds from nonvolatile ones. This is not unique to fractional distillation.
- B: This describes simple distillation better, as it effectively separates components with at least 40-50 °C boiling point difference, not fractional distillation.
- D: Raoult’s law (which relates to vapor pressure and mole fractions in ideal mixtures) underpins the principles of distillation but is not exclusive to fractional distillation, nor is fractional distillation exclusively based on it.
In summary, fractional distillation’s distinguishing feature is its ability to separate volatile compounds in mixtures with small boiling point differences through repeated vaporization-condensation cycles within the fractionating column, achieving high purity.