Question Video: Separating the Components of Air Chemistry

The components of air can be separated in a fractionating column, as shown. a) Fraction A is collected at the top of the column. Which range of temperatures must be maintained at the top of the column? [A] −195 to −190°C [B] −190 to −185°C [C] −185 to −180°C [D] −205 to −200°C [E] −200 to −195°C b) Fraction B is collected at the bottom of the column. Which range of temperatures must be maintained at the bottom of the column? [A] −195 to −190°C [B] −190 to −185°C [C] −185 to −180°C [D] −205 to −200°C [E] −200 to −195°C c) Identify the major component of fraction A. [A] Gaseous argon [B] Liquid nitrogen [C] Liquid oxygen [D] Gaseous nitrogen [E] Gaseous oxygen d) Identify the major component of fraction B. [A] Gaseous argon [B] Liquid nitrogen [C] Liquid oxygen [D] Gaseous nitrogen [E] Gaseous oxygen

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Video Transcript

The components of air can be separated in a fractionating column, as shown. (a) Fraction A is collected at the top of the column. Which range of temperatures must be maintained at the top of the column? (A) Negative 195 to negative 190 degrees Celsius, (B) negative 190 to negative 185 degrees Celsius, (C) negative 185 to negative 180 degrees Celsius, (D) negative 205 to negative 200 degrees Celsius, or (E) negative 200 to negative 195 degrees Celsius.

The question asks about separating the components of air. To separate the components of air, air must first be filtered to remove dust and particulates. It is then compressed under high pressure and cooled in stages. In the first stage, air is cooled to about six degrees Celsius, at which point almost all the water has condensed and this is removed by a filter. The air is now dry and water free. It is further cooled to negative 79 degrees Celsius, at which point carbon dioxide gas solidifies to solid carbon dioxide. The solid carbon dioxide is then easily removed.

The last stage in cooling is to cool the remaining components, which are nitrogen and oxygen, to negative 200 degrees Celsius, at which point they are both in the liquid form. In reality, there is a tiny fraction of argon also present in the mixture, but this fraction is less than one percent. And here, we will assume that the main fractions are nitrogen and oxygen.

The mixture of liquid nitrogen and liquid oxygen are then fed into a fractionating column, where they are separated by a process called fractional distillation. This process is similar to the process for separating the components of crude oil. The mixture is fed into the bottom of the column, where it is warmer. At the top of the column, it is cooler. The negative 200 degrees Celsius liquid mixture of nitrogen and oxygen is then warmed slowly.

When the temperature reaches negative 196 degrees Celsius, which is nitrogen’s boiling point, the nitrogen boils off and leaves the top of the column as fraction A. The temperature at the top of the column is maintained just above nitrogen’s boiling point, or just slightly warmer than nitrogen’s boiling point. It cannot be much warmer than this. Otherwise, oxygen might also boil off. Oxygen’s boiling point is negative 183 degrees Celsius. And so, the temperature range at the top of the column must be cooler than oxygen’s boiling point. So, the range of temperatures that must be maintained at the top of the column is negative 195 to negative 190 degrees Celsius.

Question (b) fraction B is collected at the bottom of the column. Which range of temperatures must be maintained at the bottom of the column? The answer options are the same as for part (a).

When all the nitrogen gas has left as fraction A at the top of the column, what is left is liquid oxygen. This is fraction B. We saw that oxygen’s boiling point is negative 183 degrees Celsius. And to prevent the oxygen from boiling off, the temperature at the bottom of the column is maintained between negative 190 and negative 185 degrees Celsius, which is cooler than oxygen’s boiling point.

Ensuring that the oxygen remains as a liquid, a valve or tap is then opened at the bottom of the fractionating column, and the oxygen in liquid form is easily collected. So, which range of temperatures must be maintained at the bottom of the column? The answer is negative 190 to negative 185 degrees Celsius, which is just cooler than oxygen’s boiling point.

Question (c) identify the major component of fraction A. (A) gaseous argon, (B) liquid nitrogen, (C) liquid oxygen, (D) gaseous nitrogen, or (E) gaseous oxygen.

We have seen that the major component of fraction A is gaseous nitrogen because nitrogen boiled off when the temperature in the column reached about negative 196 degrees Celsius. So, the major component of fraction A is gaseous nitrogen.

Question (d) identify the major component of fraction B. And the answer options are the same as for part (c).

We have seen that the major component of fraction B is liquid oxygen. Because often nitrogen comes off as a gas, the only component left is oxygen. It’s kept at a temperature cooler than its boiling point, in other words, cooler than negative 183 degrees Celsius, and easily drained out the bottom of the column by opening a tap or a valve. So, the major component of fraction B is liquid oxygen.

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