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Mixtures and their Separation - Application of Fractional Distillation-advanced

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Fractional distillation is a process used to separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K25\text{ K} (or 25∘C25^\circ\text{C}).

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The apparatus includes a fractionating column packed with glass beads. These beads provide a larger surface area for the hot vapors to cool and condense repeatedly, enhancing separation efficiency.

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The component with the lower boiling point distills over first, while the component with the higher boiling point remains in the distillation flask or at a lower level in the column.

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A major application is the separation of different gases from air. Air is first compressed and cooled by increasing pressure and decreasing temperature to get liquid air.

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In the separation of air, components are collected based on their boiling points: Nitrogen (−196∘C-196^\circ\text{C}), Argon (−186∘C-186^\circ\text{C}), and Oxygen (−183∘C-183^\circ\text{C}).

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Another critical application is the refining of petroleum (crude oil) into various fractions like petrol, diesel, kerosene, and paraffin wax.

📐Formulae

ΔTb.p.<25 K\Delta T_{b.p.} < 25\text{ K}

T(K)=T(∘C)+273.15T(K) = T(^\circ\text{C}) + 273.15

Efficiency∝Surface Area of Fractionating Column\text{Efficiency} \propto \text{Surface Area of Fractionating Column}

💡Examples

Problem 1:

Air contains Nitrogen (b.p.−196∘Cb.p. -196^\circ\text{C}), Argon (b.p.−186∘Cb.p. -186^\circ\text{C}), and Oxygen (b.p.−183∘Cb.p. -183^\circ\text{C}). If liquid air is warmed up slowly in a fractional distillation column, arrange the gases in the order they are collected at the top of the column.

Solution:

  1. Convert boiling points to Kelvin: N2:−196+273=77 KN_2: -196 + 273 = 77\text{ K} Ar:−186+273=87 KAr: -186 + 273 = 87\text{ K} O2:−183+273=90 KO_2: -183 + 273 = 90\text{ K}
  2. The gas with the lowest boiling point evaporates first and is collected at the top.
  3. Order of collection: Nitrogen →\rightarrow Argon →\rightarrow Oxygen.

Explanation:

Nitrogen has the lowest boiling point (−196∘C-196^\circ\text{C}), meaning it turns into vapor most easily as the liquid air is warmed. Oxygen has the highest boiling point among the three and remains liquid the longest.

Problem 2:

Calculate the temperature difference in Kelvin between the boiling points of Acetone (56∘C56^\circ\text{C}) and Methyl Alcohol (65∘C65^\circ\text{C}). Is fractional distillation required for this mixture?

Solution:

65∘C−56∘C9∘C\begin{array}{r} 65^\circ\text{C} \\ - 56^\circ\text{C} \\ \hline 9^\circ\text{C} \end{array} Since a difference of 1∘C1^\circ\text{C} is equal to a difference of 1 K1\text{ K}, ΔT=9 K\Delta T = 9\text{ K}.

Explanation:

Because the boiling point difference ΔT=9 K\Delta T = 9\text{ K} is significantly less than the threshold of 25 K25\text{ K}, simple distillation cannot achieve pure separation. Therefore, fractional distillation is required.