What is Real gas: Definition and 45 Discussions

Real gases are nonideal gases whose molecules occupy space and have interactions; consequently, they do not adhere to the ideal gas law.
To understand the behaviour of real gases, the following must be taken into account:

compressibility effects;
variable specific heat capacity;
van der Waals forces;
non-equilibrium thermodynamic effects;
issues with molecular dissociation and elementary reactions with variable compositionFor most applications, such a detailed analysis is unnecessary, and the ideal gas approximation can be used with reasonable accuracy. On the other hand, real-gas models have to be used near the condensation point of gases, near critical points, at very high pressures, to explain the Joule–Thomson effect and in other less usual cases. The deviation from ideality can be described by the compressibility factor Z.

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  1. G

    I Heat capacity for a real gas using the ideal gas (zero pressure) equation

    Summary:: Heat capacity for real gas with ideal gas (zero pressure) equation I'm looking at this problem and I'm stuck. I usually question everything but this problem is confusing me. I don't know how they've made the jump from reduced properties (from generalized Cp charts(?)) to...
  2. MichPod

    I Why the statistics for a real gas are not quantum in nature?

    A gas of bosons or fermion particles follows a particular quantum statistics. Then why a molecular gas (say, H2) follows a classical distribution statistics? Is it not the case that the molecules should be indistinguishable one from another and be either bosons or fermions? What is exactly the...
  3. Kaushik

    Van der Waals constant 'b' for a real gas

    Doesn't volume of ideal gas include volume of the molecules? What I was taught in school is that, when two molecules collide, no other molecule can come around it (inside the green part in the figure) and that volume is excluded volume? Is it correct? If yes, why?
  4. W

    Equation of state for a real gas

    My work was as follows: The first law states ##dU = TdS - PdV##, and thus $$p =- (\partial U/\partial V)| _S$$ $$U = -RT \ln(V-b) + f(S)$$ To determine ##f(S)##, I reasoned that in the ideal gas limit of ##b = 0##, ##U## should take the form of the ideal gas' molar internal energy ##\frac{3}{2}...
  5. P

    A Free expansion of a real gas vs Joule-Thomson effect

    I am writing a document on the Joule-Thomson effect. But in my research for it I have come across something that I am not completely clear on. In my document I explained the free expansion of an ideal gas and am trying to transition/segue into the Joule-Thomson effect. However now I am having...
  6. L

    Final temperature real gas behaving ideally

    Homework Statement Please consider a mixture of oxygen (1 mole), nitrogen (4 mole), and carbon dioxide (3 mole). The mixture was heated in a well - insulated vessel with 753 kJ. Determine the final temperature if the mixture is composed of real gases behaving ideally as described by Eq. 3.48...
  7. ScottAllenRauch

    Fluid Flow Loss Analysis Using Real Gas Properties

    I am trying to model fluid flow and losses through a component (e.g., pipe) using REAL GAS properties since 1) I have access to RefProp from NIST, and 2) I am dealing with refrigerants, which are far from ideal. I have seen nowhere an analysis of fluid flow losses (e.g., drag, friction...
  8. M

    I Is it Reasonable to Assume (3/2)*P*V as the Internal Energy of a Real Gas?

    I am looking over the kinetic theory of gases. It is most commonly described as U = (3/2)*N*k*T = (3/2)*mass*R*T for a monatomic gas, assuming the gas is ideal. This is based on the derivation, where ultimately (3/2)*P*V = N*K = total kinetic energy of particles. My question, for a real gas...
  9. arpon

    Adiabatic process of real gas

    Homework Statement Show that for a gas obeying the van der Waals equation ##\left(P+\frac{a}{v^2}\right)(v-b)=RT##, with ##c_v## a function of ##T## only, an equation for an adiabatic process is $$T(v-b)^{R/c_v}=constant$$ Homework Equations ##TdS=c_vdT+T\left(\frac{\partial P}{\partial...
  10. K

    Real Gas Problem Homework Help

    Homework Statement The task is the following: Homework EquationsThe Attempt at a Solution I calculated the derivatives for Vm (crit) and set them = 0, but somehow I can't solve the equation to obtain Vm (crit) on one side alone. This is how far i managed to do it: Can someone help me?
  11. S

    How was the van der Waals equation derived?

    the equation (P+a(n/V)^2)(V-nb)=nRT was derived in this manner: The pressure of a real gas is affected by intermolecular forces and so the a(n/V)^2 term must be added to the measured pressure to obtain the ideal pressure where Pmeasured+a(n/V)^2=Pideal On the other hand when they explained...
  12. VoteSaxon

    Van der Waals and boiling point

    Homework Statement The bulb of a constant volume gas thermometer is immersed in an ice/water/water vapour mixture at equilibrium and the recorded pressure is 0.400 atm. It is then immersed in a boiling liquid and the pressure is 0.844 atm. Sufficient gas is then removed from the bulb such that...
  13. S

    Compressibility factor and van der Waals equation for temp

    The pressure exerted on the walls of the container by a real gas is less compared to an ideal gas. This is due to the attractive forces of the gas pulling the molecules back towards the rest of the gas molecules. However, there is also a relationship whereby at lower temperatures, the z is even...
  14. S

    Question about compressibility factor

    At low temperatures, z falls below 1 and the reason for that is because the intermolecular interactions cause the pressure exerted to be lesser than expected. PVm/RT=z and since P is less than expected z drops below 1. However, as the pressure increases z increases to be above 1 because as P...
  15. HelloCthulhu

    Electrolysis System Work and Real Gas Law

    Greetings Everyone! I read a very good thread regarding how Ideal Gas Law is used to calculate system work for the electrolysis of water: W = PΔV = (101.3 x 103 Pa)(1.5 moles)(-22.4 x 10-3 m3/mol)(298K/273K) = -3715 J https://www.physicsforums.com/threads/work-of-electrolysis-of-water.756240/...
  16. B

    Adiabatic expansion of a real gas

    Homework Statement Which of the following changes during the free adiabatic expansion of a real gas? (I) internal energy (II) temperature (III) pressure Homework Equations PV=nRT; ΔU= q + w The Attempt at a Solution For ideal gases under adiabatic conditions, we know that there is no heat...
  17. Soho

    Temperature varying in vacuum chamber during pressure adjust

    Hello everybody, I am working on some tests on materials and structures in a vacuum chamber. I noted that during the final moments of the pressure rising (when I open the valve to let the air in), temperature sensors installed in the chamber record the temperature rising more than ten degrees...
  18. MexChemE

    Enthelpy change for a real gas (Carbon dioxide)

    Homework Statement Find ΔH for the isothermal expansion of one mole of CO2 from a pressure of 1 atm to zero at 300 K. The critical point of CO2 is TC = 31 °C and PC = 73 atm. Use the equation for ΔH you previously derived from the Berthelot equation of state. (Answer provided by textbook: ΔH =...
  19. H

    Real gas equation in terms of Cv?

    Is there a form of the van der waal equation in terms of the Cv of the gas? Is it derivable?
  20. fluidistic

    Real gas, unable to reach the correct expression

    Homework Statement Consider a system of N particles contained in a volume V. The Hamiltonian of the system is ##H=\sum{i=1}^N \frac{\vec p_i}{2m}+\sum _{i<j}u(|\vec r_i - \vec r_j|)## where p_i and r_i are the momentum and position of the i-th's molecule. 1)Show that the state equation of the...
  21. B

    Calculating Temperature Change in Real Gas Isolated Chamber with Changed Volume

    Homework Statement We have a real gas in an isolated chamber. The potential between the molecules is described as ##\phi (r)=\phi _0e^{-(\frac{r}{\sigma})^{2}}##, where ##\phi _0=5\times 10^{-4}eV## and ##\sigma =5 nm##. At 300 K we have ##10^{24}## molecules per ##m^3##. Calculate the...
  22. T

    How Does Potential Energy Affect the Internal Energy of Real Gases?

    Hello everybody. The concept of internal energy of a real vs ideal gas has perplexed me. From what I understand, an ideal gas solely considers the kinetic energy of gas molecules (temperature) where as real gases consider kinetic energy of particles in addition to potential energy. So...
  23. J

    Isentropic Relations for Real Gas in Converging-Diverging nozzle

    Hello, I am looking at a problem concerning flow through a converging-diverging nozzle. The governing equations are relatively straight-forward for gasses that closely follow the ideal gas law. However I am looking at an unusual gas which is certainly not represented by the ideal gas...
  24. M

    Calculating Cp and Cv of Real Gases: A Guide for Solving Differential Equations

    Homework Statement Hello there, I was given a tasks to find Cp and Cv of nitrogen and carbon dioxide in real gas situation. Homework Equations From what i could find, the suitable equation would be: Cv = Cv° - (2(∂B/∂T) + T(∂²B/∂T²)P The Attempt at a Solution The...
  25. O

    Van der Waals gas is not real gas?

    From van der Waals , (P+a/v^2)(v-b)=RT, At critical temperature, I get (∂P/∂V)at constant temperature =0 and (∂^2P/∂V^2) at constant temperature ,T=0. then critical pressure,P = a/(27b^2)--------1 critical volume,v=3b-----------2 critical temperature=8a/(27Rb)----------3 then simultaneous...
  26. M

    Some useful form of the eq. of state of a REAL gas?

    It is written that: Another useful form of the equation of state of a real gas is: Pv= A + B/v + C/v^{2} + ... Where A, B, C, .. are functions of temperature and are called the "VIRIAL COEFFICIENTS". (what does this mean?) Theoretical derivations of the equation of state, based on...
  27. L

    Heat capaticy of isobaric process with real gas

    Hey, during last lecture of thermodynamics we did polytropic processes, and with no discuss teacher said, that isobaric process is in general polytropic. If we had ideal gas, then it is clear, bud what if the gas isn't ideal? I tried to proof it, but I stuck and don't know how to continue...
  28. H

    A real challenge: real gas isothermal delta h

    Homework Statement Using any software, create the enthalpy table for the following two cases of superheated steam: 1. Isobaric process at a chosen pressure in 100 spaces of 5 degrees celcius. 2. Isothermal process at a chosen temperature, given Vg from the book, in 1000 spaces of 0.1 bar...
  29. A

    How can I find the adiabatic equation for real gases?

    Hey! Maybe anyone knows, how to get real gas adiabatic equation. It's not a big deal, when you need to get ideal gas equation, but I haven't got any ideas about real gas. I'm wondering maybe it's something with Van der Waals equation, but in my opinion it can't be like this ...
  30. C

    Is Entropy Change of a Real Gas Dependent on Path in Reversible Processes?

    For a real gas (non-ideal gas) in a reversible process, the way to calculate \Delta{S} should also be independent to path simply because entropy is a state function. However, I got strange solution while taking different path. Here is the condition: 1. Equation of state (EOS) for the real gas...
  31. C

    Alternative Real Gas Equation of State

    1. A friend suggests to you an alternative to the Virial equation of state: P=(RT/Vm)-(B/Vm2)+(C/Vm3) a.) Show that this equation is useful by proving that it demonstrates critical behavior. b.) Find the critical constants Pc, Vm,c, and Tcin terms of B, C and R. c.) Calculate the...
  32. A

    Real Gas PV Curve: Volume Explained

    http://faculty.wwu.edu/vawter/PhysicsNet/Topics/Thermal/gifs/vdWaalEOS02.gif After the gas has been liquified, the gas volume should have disappeared. Why the liquid part of the curve still possesses volume?
  33. R

    Real gas compressibility isotherms(graphs)

    Homework Statement Sketch the following compressibility isotherms(fancy word for graphs) (compressibility factor, PV/nRT versus pressure :) for oxygen at a temperature just above the critical temperature ::) for oxygen at a temperature well above the critical temperature eg. 200 degrees...
  34. C

    Adiabatic Expansion of a real gas

    I am having trouble with the very last part of this question. I will write the whole question just to make sure that I haven't screwed up the first part too :) When a certain freon used in refrigeration was expanded adiabatically from an initial pressure of 32 atm and 0degrees C to a final...
  35. kandelabr

    Real gas tables: isentropic work

    Hello. I am writing a Matlab program for calculating properties and processes of dry air using real gas tables. The only tables I could find were from Perry's Chemical Engineer's Handbook (7th edition, 1999). Mostly I make use of cubic interpolation and in some critical cases numerical...
  36. kandelabr

    What are the equations of state for air at high pressures and temperatures?

    Hello, I am looking for a real gas equation of state for air, if possible a MATLAB script. I am working on pressures up to 500 bar and to 500°C. The thing is that I not only need a p-v-T relation, I also need to calculate work done by compressed air and heat that is involved in...
  37. L

    Calculate the number of moles in a real gas

    Homework Statement I would like to know how to calculate how much moles of gas I have in the following in a cylinder with a certain volume and pressure. The gas in the cylinder is a mixture of air, with added oxygen and helium, the mixture is 18% Oxygen, 36,6% Nitrogen, 45% Helium and 0,4%...
  38. 1

    Real Gas vs Ideal Gas: Compressibility Ratio

    The compressibility ratio of a gas is PV/nRT. What happen to the compressibility ratio if the attration force between the gas molecules increase? The answer says it decreases because the more molecules interact with each other, so the pressure they exert on the container decreases, thus...
  39. kandelabr

    Does Throttling Compressed Air Cause Liquefaction?

    Homework Statement I need to know what happens if compressed air from a vessel at room temperature is throttled from 450 bar to 2 bar. Homework Equations I know Joule-Thomson effect (http://http://en.wikipedia.org/wiki/Joule-Thomson_effect" ) deals with this, but I'd need it explained a...
  40. Y

    MC: Chemistry Ideal Gas vs Real Gas

    Homework Statement 24. At constant temperature, the behavior of a sample of a real gas more closely approximates that of an ideal gas as its volume is increased because the: a. Collisions with the walls of the container become less frequent b. Average molecular speed decreases c. Average...
  41. A

    Can Real Gases Have a Lower Pb/Pm Ratio Than Ideal Gases?

    Ok, let's say we have at our disposal a thermometer of constant volume gas. Inside it, we'll put a real gas, like Helium or Molecular Hydrogen. Ok, the gas compartment is placed inside a mixture of liquid water and ice, so we guarantee the gas inside is at 273.15 K. We measure its pressure...
  42. S

    Real vs Ideal Gas: Pressure & Temperature Effects

    If a sample of air had high pressure and high temperature, would it be a real or ideal gas?
  43. C

    Beattie-Bridgeman Real Gas Model

    I need to calculate the compressability of several gases as they are being added to a storage cylinder. I would like to use the Beattie-Bridgeman equation as described here. I need to be able to compute the Z factor (compressability) for a given gas at a given pressure. I am a bit new to...
  44. P

    Difference between an ideal gas and a real gas

    Hey Could somebody please explain what the difference between an ideal gas and a real gas are? Or post a suitable link from which I can understand the concept surrounding this issue. Thanks Pavadrin
  45. N

    Real Gas Laws: Exploring Alternatives to PV=nRT

    the ideal gas law PV=nRT only can used on the ideal gas, right? if the we want consider the real gas... what equation should we used??