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Difficulty: MediumNitrogen Gas, Nitrogen Cycle, and Oxides of Nitrogen

During the industrial preparation of nitrogen gas by fractional distillation of liquid air, nitrogen, argon, and oxygen boil off at different temperatures: −196 ∘C-196\ ^\circ\text{C}, −186 ∘C-186\ ^\circ\text{C}, and −183 ∘C-183\ ^\circ\text{C}, respectively. Which gas distills over first at the top of the fractionating column, and why?

  1. Nitrogen gas, because its lower boiling point of −196 ∘C-196\ ^\circ\text{C} makes it the most volatile component.Answer
  2. B
    Oxygen gas, because its boiling point of −183 ∘C-183\ ^\circ\text{C} is higher, allowing it to vaporize first upon heating.
  3. C
    Argon gas, because monoatomic noble gases have no intermolecular forces and evaporate before diatomic molecules.
  4. D
    Oxygen gas, because its higher molar mass of 32 g/mol32\ \text{g/mol} causes it to diffuse to the top faster than nitrogen.

Answer

Nitrogen gas distills over first because its lower boiling point of −196 ∘C-196\ ^\circ\text{C} makes it the most volatile component in liquid air.
In the industrial liquefaction and fractional distillation of air, nitrogen has a boiling point of −196 ∘C-196\ ^\circ\text{C} (77 K77\ \text{K}), which is lower than that of argon (−186 ∘C-186\ ^\circ\text{C}) and oxygen (−183 ∘C-183\ ^\circ\text{C}). Because nitrogen is the most volatile component (has the lowest boiling point), it vaporizes first when the liquefied air is slowly warmed, allowing it to collect at the top of the fractionating column as a gaseous fraction.

Step-by-Step Solution

1
Compare the boiling points of the components of liquid air
Nitrogen boils at −196 ∘C-196\ ^\circ\text{C} (77 K77\ \text{K}), argon at −186 ∘C-186\ ^\circ\text{C} (87 K87\ \text{K}), and oxygen at −183 ∘C-183\ ^\circ\text{C} (90 K90\ \text{K}).
The component with the lowest boiling point requires the least thermal energy to change from liquid to gas state.
2
Determine which component vaporizes first as temperature rises during fractional distillation
As liquid air warms from −200 ∘C-200\ ^\circ\text{C}, nitrogen reaches its boiling point first at −196 ∘C-196\ ^\circ\text{C}.
Lower boiling point corresponds to higher vapor pressure/volatility at a given temperature.
3
Identify the gas collected at the top of the fractionating column
Nitrogen gas vaporizes first and is collected as distillate from the top of the column.
More volatile vapors ascend to the top of fractional distillation columns while less volatile components remain liquid longer at the base.

Key Concept

Industrial Isolation of Nitrogen Gas via Fractional Distillation of Liquid Air
Estimated Time:1m 0s
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Topics

Nitrogen Gas, Nitrogen Cycle, and Oxides of Nitrogen

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