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Joule Thomson Effect is defined as the phenomenon of change in temperature of a fluid (real gas or liquid) when it is passed from a high pressure to a low pressure region. The fluid is usually kept in an insulated valve so that no heat is exchanged out to the environment. It is also defined as a thermodynamic process that helps in expansion of the fluid at constant enthalpy. The positive and negative value of the Joule Thomson coefficient denotes whether the fluid warms or cools down upon expansion.
Also Read: Joule’s Law
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Joule Thomson Experiment
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To illustrate the experiment a gas packet is placed opposite to the direction of flow of restriction in an insulated valve. Refer to the diagram below:
Joule Thomson Experiment
- Thomson (Lord Kelvin) worked alongside Joule to carry a series of experiments to explain the Joule Thomson effect.
- The experiment was to throttle a gas from a high pressure side to the low pressure side which are denoted as side A and C respectively in the above diagram.
- The interposing plug B in between was used so that gas would flow slowly from side A to side C.
- The pressure on side A is P1 whereas the pressure on side C is P2.
- The pressure on both sides is maintained using a piston as shown in the diagram above.
- The entire setup is insulated so that there is no exchange of heat to the outside or we can say it is an adiabatic process or Q = 0.
- As the gas flows from side A to side C the volume of gas decreases by V1/mole of gas at side A and increases at V2/mole of gas at side B.
- Therefore the work done on the gas by the piston in the left side is P1V1 whereas on the other side is P2V2.
Thus according to the first law of Thermodynamics, the change in internal energy is shown as:
E2-E1 = -w = -(P2V2-P1V1) ----- (1)
or
E2 + P2V2 = E1 + P1V1------- (2)
Therefore,
H2 = H1 ------ (3)
Equation (3) denotes that the enthalpy of gas remains constant during the Joule Thomson expansion.
Also Read:
| Important Topics Related to Joule Thomson Effect | ||
|---|---|---|
| Thermal Conductivity | Unit of Conductivity | Difference Between Heat and Temperature |
| Hess’s Law | Standard Enthalpy of Formation | First Law of Thermodynamics |
| Second Law of Thermodynamics | Isobaric Process | Isothermal process |
Joule Thomson Coefficient
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The Joule Thomson Coefficient can be defined as the differential change in temperature with respect to differential change in pressure at constant enthalpy. The coefficient is as denoted below:
\(\mu_{JT} = (\frac{\partial T}{\partial P})_H \)- As it can be seen from above the Joule Thomson Coefficient is denoted by μJT.
- A positive value of the Joule Thomson coefficient signifies that the real gas has a cooling effect during Joule Thomson expansion, unlike Hydrogen or Helium.
- The Joule Thomson coefficient for Hydrogen is negative at room temperature.
The Joule Thomson coefficient for other gases is as depicted below:
Joule Thomson Coefficient for Gases
Also Read: Gibbs Free Energy
Temperature Inversion Curve
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Before understanding the temperature inversion curve, let us understand the inversion temperature.
Inversion Temperature
All non-ideal gases will experience a temperature decrease below the inversion temperature and a temperature increase above the same. The principle is for all gases that expand at constant enthalpy. The inversion temperature is nothing but a critical temperature at which the fluids do not experience the Joule Thomson effect. It is only above or below the inversion temperature a significant change in temperature can be seen.
- For Hydrogen, the inversion temperature is around -78 degree Celsius (where equals zero).
- As seen from the graph below, the grey area inside the curve denotes positive, where the hydrogen has a cooling effect on expansion.
- The grey portion of the curve can also be said as the cooling region and the area outside as the heating region.
- It must be noted that the inversion temperature highly depends on the nature of the gas.
The figure above shows the relationship between inversion temperature and JT coefficient more clearly.
Inversion Temperature Vs Joule Thomson Coefficient
Also Read: Thermochemistry
Applications of Joule Thomson Effect
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Some of the applications of Joule Thomson Effect include:
- Liquefaction of gases
- Refrigeration
- Industrial and cryogenic applications
- Liquification of helium
Also Read: Kelvin Planck Statement
Important QuestionsQues. Give the basic principle of Joule Thomson Effect? Soln. Joule Thomson Effect is based on heat transfer. At normal temperature and pressure, all the real gases undergo expansion and this is called as liquification of gases. However, hydrogen and helium are an exception to this. Ques. What is Joule Coefficient for an ideal gas? Ans. The Joule Coefficient for an ideal gas is zero. This is because the enthalpy of gases is dependent on temperature. Ques. What is the maximum inversion temperature for hydrogen and helium? Ans. The maximum inversion temperature for hydrogen is 200K and for helium it is 24 K. Ques. Why is Joule Thomson effect not applicable in the case of hydrogen gas? Ans. At extremely high temperatures, hydrogen behaves closely like an ideal gas and hence Joule Thomson effect is applicable for hydrogen at lower temperatures only. Ques. What factors govern the change in temperature experienced by gas during expansion? Ans. The factors which govern the change in temperature are:
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Things to Remember
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- There is no heat exchange to the environment when carrying the Joule Thomson experiment.
- The main advantage of the effect is liquefaction of fluids.
- JT coefficient can simply be defined as partial differentiation of temperature with respect to pressure.
- Gases like hydrogen and helium warms when expanded during Joule Thomson Experiment unlike other gases that have a cooling effect.
Previous Year Questions
- Calculate the heat liberated. [KCET 2004]
- What is the maximum work done? [VITEEE 2006]
- When ideal gas expands in vacuum, the work done by the gas is? [VITEEE 2006]
- Calculate the volume at C for a cyclic process. [JIPMER 2019]
- In the equation PVϒ , ϒ is unity. This process is? [BITSAT 2010]
- Calculate the entropy change of the body. [BITSAT 2012]
- Calculate the pressure relation. [BITSAT 2006]
- Which is close to an ideal black body. [NEET 2002]
- In an adiabatic compression, the decreased volume is associated with? [BITSAT 2008]
- Calculate the increase in internal energy. [BHU UET 2009]
- In a reversible process ΔSsystem + ΔSsurrounding is? [JKCET 2004]
- Kelvin Equation is related to? [UPSEE 2019]
- Enthalpy of a compound is equal to its? [VITEEE 2010]
- Which reaction has a positive ΔS? [VITEEE 2010]
- How much water is decomposed by 130 kJ of heat? [MHT CET 2010]
- Calculate the amount of heat absorbed by the gas in the given process. [JEE MAIN 2017]
- The mass and energy are both conserved in an isolated system, this is? [MHT CET 2019]
- Name the process in which Boyle’s Law is applicable? {JEE Advanced 2019]
- If a thermodynamical process is changed from one state to another, which quantity remains the same? [BITSAT 2017, 2013]
- Calculate the heat of atomisation? [BITSAT 2013]
Also Read: Zeroth Law of Thermodynamics
Sample Questions
Ques: Is Joule Thomson coefficient positive or negative? (2 Marks)
Ans: Both. The Joule Thomson coefficient can be negative or positive depending on the temperature of the gas. The JT coefficient is positive when the temperature of the gas is below the inversion temperature and negative when the temperature is above the inversion temperature.
Ques: What is the basic principle of the Joule Thomson effect? (2 Marks)
Ans: The Joule Thomson effect follows the basic principle of transfer of heat. All real gases at ordinary temperature and pressure tend to expand thus leading to liquefaction of gases. The Joule Thomson effect however is not applicable for ideal gases. Real gases have a cooling effect unlike gases like hydrogen and helium. The Joule Thomson effect is simply a thermodynamic process. Furthermore, this process happens when the expansion of fluid takes place from high to low pressure at constant enthalpy.
Ques: Is the Joule Thomson effect applicable to Hydrogen or Helium? (3 Marks)
Ans: Yes, according to the Joule Thomson inversion curve temperature both the gases have very low temperature at 1 atmospheric pressure. Hydrogen has a negative Joule Thomson effect. At room temperature, all gases except hydrogen, helium, and neon cool upon expansion by the Joule–Thomson process when being throttled through an orifice.
These three gases experience the same effect but only at lower temperatures. As compared to other gases that cool down due to Joule Thomson expansion, hydrogen and helium exhibit heating effects. We can simply state that hydrogen and helium both the gases get warm due to Joule Thomson expansion.
Ques: How is Joule Thomson effect different from cooling? (3 Marks)
Ans: Unlike cooling, The Joule Thomson effects the fluids need not necessarily have to cool down. The fluid temperature can either decrease or increase. Also, JT effect has industrial, cryogenic, refrigeration applications.
At ordinary temperatures and pressures, all real gases except hydrogen and helium cool upon such expansion; this phenomenon often is utilized in liquefying gases. The cooling occurs because work must be done to overcome the long-range attraction between the gas molecules as they move farther apart. Hydrogen and helium will cool upon expansion only if their initial temperatures are very low because the long-range forces in these gases are unusually weak.
Ques: Is Joule Thomson effect reversible? (1 Mark)
Ans: No, the Joule Thomson effect cannot be reversed. There is a change in temperature of a gas or a liquid without a change in enthalpy in the Joule Thomson effect. Thus, the process is inherently irreversible.
Ques. What is the foundation of Thermodynamics? (1 Mark)
Ans. The foundation of thermodynamics is the law of conservation of energy and the fact the heat flows from a hot body to a cold body.
Ques. State zeroth law of thermodynamics? (1 Mark)
Ans. Acc. to this, when the thermodynamic system A and B are separately in thermal equilibrium with a third thermodynamic system C, then the system A and B are in thermal equilibrium with each other also.
Ques. Why is the energy of thermal radiation less than that of visible light? (1 Mark)
Ans. The energy of an electromagnetic ware is given by :- E = hf
h = Planck’s constant; f = frequency of wave. Since the frequency of thermal radiation is less than that of visible light, the energy associated with thermal radiation is less than associated with visible light.
Ques. Which has a higher specific heat ; water or sand? (1 Mark)
Ans. Water has higher specific heat than sand as
\(\bigtriangleup = \frac{Q}{mc}\), where T = Temperature, Q = Heat, m = Mass, C = Specific heat; Since water temperature increases less slowly than sand hence the result.Ques. If a body is heated from 270 C to 9270C then what will be the ratio of energies of radiation emitted? (3 Marks)
Ans. Since, By Stefan’s law:→
E = Energy radiated
T = Temperature.
E1, T1 Initial energy and temperature
E2, T2 Final energy and temperature.
T1 = 270C = 27+273 = 300K
T2 = 9270C = 927+273K = 1200K.
E = constant T4
So, E1 = constant T14
Equating equation 1) &2)
or E1 : E2 = 1 : 256
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