Content Curator
Liquefaction of gases can be described as a conversion of any gas into a liquid form or state (condensation). This method is done physically. This is a complex procedure that involves several expansions and compressions in order to achieve extremely low temperatures and high pressures. This procedure is mainly done to transport gases from one place to another. For example – Oxygen, which is by exerting pressure and reducing the temperature, gas can be converted into liquid form.
Read Also: Three States of matter
| Table of Content |
Key Terms: Gas, liquefaction, Liquefaction, Critical Pressure, Critical Temperature, Critical Volume, Carbon Dioxide, Isotherms
Liquefaction of Gases: Conditions
[Click Here for Sample Questions]
The two most important conditions to liquefy a gas are:
- High-Temperature
- Low-Pressure
Read About: Class 12 Chemistry Chapter 1 The Solid State
Critical Pressure of a Gas
[Click Here for Previous Years Questions]
Critical Pressure can be termed as the minimal pressure needed to liquefy the gas. It is denoted by the symbol Pc.
The formula for critical pressure is - Pc = a/27b2
Here, a and b are Van der Waal's Constant.
Read More:
| Related Articles | ||
|---|---|---|
| Daltons Law of Partial Pressure | What is partial Pressure? | Real Gas |
| Ideal Gases Law | Boyle's Law | Critical Pressure |
Critical Temperature of a Gas
[Click Here for Sample Questions]
Critical Pressure of a gas can be stated as the temperature which is needed to liquefy gas with a certain amount of pressure. The gas will be impossible to liquefy above that required temperature. For Example, the critical temperature of carbon dioxide is 30.98. It is denoted by the symbol Tc.
The formula for critical temperature is: Tc = 8 a/27bR
Here, a and b are Van der Waal’s Constant. R is gas constant.
Also Read: Ideal Gas Equation
Critical Volume of a Gas
[Click Here for Previous Year Questions]
Critical volume can be stated as 1 mole of the volume of gas (given in this critical pressure & temperature) is called critical volume. The critical volume of a gas is denoted by a symbol - Vc
The formula of the critical volume of a gas is Vc = 3b.
Here, b is the effective volume of molecules per mole of gas.
Also Read: Avogadro Law (Volume - Amount Relationship)
Isotherms of Carbon Dioxide Gas
[Click Here for Sample Questions]
A renowned scientist T. Andrews conducted a scientific test to find out the pressure-volume relation of Carbon dioxide at various temperatures. The isotherms which are obtained are given names on the given curve in the graph above. Isotherm is obtained at Temperatures – 0, 21, 31.1, 50.
Isotherms of Carbon Dioxide Gas
- Carbon dioxide which is at point A, needed the lowest temperature, 13.1 at low pressure
- The curve is seen with a drop of the volume of the gas with an increase in pressure
- When the gas reaches a temperature of 21.5, it still behaves like a gas at point B.
- At point B, carbon dioxide exists in the dual state (both as gas and liquid)
- Carbon dioxide starts condensing, which results in an increased pressure at point C.
- The volume of the gas is seen to be decreasing rapidly due to the reason that liquid has a relatively smaller volume than gas.
- Due to the complete liquefaction and the rise in pressure having little impact on volume, a steep curve is produced.
- Now, the gas over 30.98cannot be liquefied. So, the critical temperature is attained
- Finally, it is seen that due to the isothermal compression, all gases behave identically as the carbon dioxide
Read About: Charles Law Formula
Liquefaction of Gases: Principles
[Click Here for Previous Year Questions]
The following principles given below can be used to cool gas for its liquefaction:
- Joule-Thomson Effect: the movement of highly compressed gas from a high-pressure zone to a low-pressure zone through a throttle under adiabatic conditions, results in temperature drop below the inversion temperature of the gas.
- Gas can be compressed below its critical temperature.
- Involvement of Adiabatic Expansion: when a gas undergoes this type of expansion, it involves mechanical work. In this case, the temperature drops and a loss in some of its kinetic energy takes place.
Read More About: Daltons law of Partial Pressure
Liquefaction of Gases: Methods
[Click Here for Sample Questions]
- Linde’s Method: This procedure is based on the Joule-Thomson Effect. Firstly, any gas is compressed to about 200 atm and then allowed to enter the concentric pipes in the inner tube. The valve of the jet gets opened and gas starts to expand suddenly inside the wide chamber C. The expanded gas is then cooled and the pressure is reduced to 50atm. It is passed through the outer tube ‘O’ where the outgoing gas cools the incoming gas and it is circulated in. This process is repeated until the liquid airdrops are released from the jet to the bottom part of the chamber.
- Claude’s Method: It is a process that is based on a principle that the expansion of gas takes place adiabatically against any external pressure, say as a piston in an engine, it results in some external work. The work is done at the cost of the kinetic energy of the molecules due to which the temperature of the gas falls and it causes cooling. Finally, Liquefaction takes place with an isenthalpic expansion inside the thermal valve.
Claude’s Method
Read About: Phase changes
Things to Remember
- The relationship between pressure, volume and temperature were discovered by a scientist named Thomas Andrews.
- Gases are liquefied due to transportation purposes.
- It was discovered as per the research that under, the gas cannot be liquefied under high pressure and the high-temperature isotherms are equal to the ideal gas.
- All gases in the process of liquefaction behave quite similar to carbon dioxide. Such behaviour at some constant pressure & temperature is called isothermal compression.
- Gases need compression and cooling for the liquefaction process.
- The highest temperature of a gas in which the liquefaction of gas takes place is called critical temperature.
- The more the critical temperature, the easier the liquefaction of a gas.
Read More:
Previous Years Questions
- A bubble of air is underwater at temperature…...[NEET 2011]
- If molecular mass of gas B is 36, the molecular mass of gas A will be….[NEET 2012]
- 50 mL of hydrogen diffuses out through a small hole of a vessel….[NEET 1994]
- The temperature of the gas is raised from...[NEET 1994]
- A certain gas takes three times as long to effuse out as helium...[NEET 2012]
- A gas at 350 K and 15 bar has molar volume 20 percent smaller than...[NEET 2019]
- A gas such as carbon monoxide would be most likely to obey the ideal gas...[NEET 2015]
- A gaseous mixture was prepared by taking equal moles of...[NEET 2011]
- A liquid can exist only...[NEET 1994]
- The van der Waals equation of state reduces itself to the ideal gas equation….[NEET 1980]
- Given van der Waals constant for….[NEET 2018]
- A solid with high electrical and thermal conductivity from the following is….[NEET 1994]
- van der Waals' real gas, act as an ideal gas, at which condition….[NEET 2002]
- Absolute zero is defined as the temperature...[NEET 1990]
- In van der Waals equation of state for a non-ideal gas….[NEET 1990]
- Root mean square velocity of a gas molecule is proportional to….[NEET 1990]
- An ideal gas cannot be liquefied because….[NEET 1992]
- If the temperature is constant, what volume will the oxygen occupy at 760 mm pressure?...[NEET 1990]
- By what factor does the average velocity of a gaseous molecule increase when the temperature (in kelvin) is doubled?..[NEET 2011]
- Correct gas equation is...[NEET 1989]
Sample Questions
Ques. What do you mean by liquefaction? (1 mark)
Ans. Liquefaction can be termed as the conversion of gaseous matter into liquid form.
Like, Oxygen which is a gas that can be converted into liquid form by exerting required pressure and lowering the temperature.
Ques. What is the role of pressure in the Liquefaction of a gas? (2 mark)
Ans. Pressure has a critical role to play in the Liquefaction of a gas. To a gas, when sufficient pressure is applied, the space between the gaseous particles of a gas shrinks and starts compressing. This is how the gas starts to liquefy.
Ques. If the Critical temperature of He is 5.3 K and N2 is 126K. State the gas that will liquefy first. (1 mark)
Ans. In the case of N2, the intermolecular forces are much higher. The critical temperature will be higher too which will lead to a faster change in state. Hence, N2 will liquefy first.
Ques. What steps are involved in the process of gas liquefaction? (1 mark)
A) The process involves two major steps given below:
- Drop in Temperature
- Increase in the pressure
Ques. Where are the liquefied gases used? (2 mark)
Ans. The liquefaction of gases is don’t to liquefy gas for various fields like scientific, medical or industrial, etc.
- Bio-sample Preservation like freezing of semen.
- Liquid form of ammonia used in ice plants for cooling purposes
- Liquid forms of Oxygen and Hydrogen used in Rocket Propellant
- Used as home fuel like LPG for cooking, etc.
Ques. State the importance of the liquefaction of gases. (2 mark)
Ans. The importance of the liquefaction of gases is:
- The gases can be conveniently stored in liquid form
- The gases will be easier to transport in liquid form
Ques. What is the importance of critical temperature in liquefaction of gases? (1 mark)
Ans. Critical Temperature is very important for the process of liquefaction of gases. The gases will not get liquefied above their critical temperature, no matter how much pressure is exerted.
Ques. What are permanent gases? (1 mark)
Ans. Some of the gases like He, O2, H2 are present which are impossible to be liquefied at room temperature. These gases are known as the permanent gases
Ques. Explain how the pv / RT function can be used to show that gases do not behave ideally at high pressures. (1 mark)
Ans. The pv / RT ratio is equal to the number of moles of the ideal gas in the sample. This number should be stable for all the conditions of pressure, volume, and temperature. If the value of this ratio changes with increasing pressure, the gas sample will not behave ideally.
Ques. What are Van der Waals forces? (1 mark)
Ans. Van der Waals forces are weak intermolecular forces that depend upon the space between atoms. These forces are generated by interactions between uncharged molecules.
Do Check Out:
Comments