Talk:Ideal gas

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The b term is an excluded volume term. It's there because gas molecules have a certain size, and two cannot coexist at the same place. Van Der Waals gas is very important because its theory is also the base for the description of liquid mixesUser:ThorinMuglindir

Bold textAnybody know about the deviation of real gas with ideal gas in PV/RT graph? for ideal gas, pv/rt=1. for most of the real gas, (a)p<500atm, pv/rt<1;(b)p greater 500atm, pv/rt greater 1. for (a), it's intermolecular attraction factor; can some one explain the factor of (b) molecular volumn factor? i don't understand....

Could please someone clarify whether a noble gas such as helium which I understand is normally monoatomic could be considered as an Ideal gas. As written in this article, an Ideal gas is defined as molecules... L.L.

Although this is inconsistent with common usage, a molecule is technically one or more atoms. That is, even if an ideal gas is defined as molecules, that would include the noble gases. Brianjd

See molecule.

Contents 1 Real Gas 2 The entropy of an ideal gas 3 Assumptions 4 nR is amount of gas 5 molecules 6 Difference between thermally perfect and calorically perfect gas

[edit] Real Gas

Why is there no article real gas? --Lode 16:44, 19 Jul 2004 (UTC)

There is no need for an article real gas. A real gas is just a gas.

Real gas redirects here - I have added a definition in this article. Brianjd 08:45, 2004 Dec 12 (UTC)

i dont think real gas should redirect here. It should redirect to gas. ~~

I agree - I changed it. PAR 00:23, 23 November 2006 (UTC)

[edit] The entropy of an ideal gas

Well done, PAR. Just a few suggestions.

• We have once told that kN=nR, so don't repeat it

- done

• Use the dimensionless entropy S/kN

- I can't find a natural point for introducing this which doesn't clutter things except the final equation, when it is clear that everything is intensive, so thats the only one I changed.

• Don't restrict the analysis to monatomic gases, but use the symbol rather than the constant value 3/2 for the dimensionless heat capacity at constant volume.

- done

• An intermediate result would be helpful after the integral

- That integration was messed up - I fixed it, hopefully its clear now.

• The value of the Sackur-Tetrode equation is unclear: "The simple formula breaks down at low temp. So introduce the S-T equation. That breaks down too". So what is the point ? Bo Jacoby 12:47, 9 November 2005 (UTC)

- Tried to make that a little clearer. PAR 16:46, 9 November 2005 (UTC)

[edit] Assumptions

Shouldn't there be a list of the assumptions made when dealing with ideal gases?

I.e. no intermolecular attractions etc...

The widipedia entry "gas" seems to list properties that I think are Ideal gas properties not real gas properties, I think that this needs clarification between the two.

An ideal gas is defined by its behavior, not by the statistical mechanics that explain its behavior. PV=NRT defines an ideal gas, no further assumptions are needed.

I think he/she/it meant "what assumptions are we taking when we treat a real gas as an ideal gas". These are now there.

[edit] nR is amount of gas

Hi PAR. If n is the amount of gas measured in mol, and the gas constant R is measured in J·K⁻¹·mol⁻¹, then nR is the amount of gas measured in J·K⁻¹. Bo Jacoby 17:35, 8 December 2006 (UTC)

Hi, I'll give a quick comment here, without reading the rest of the discussion (I'm giving myself a WP-pullout week due to irritations at WP:ELAC). With that said, R is just a constant, e.g. 1, 2, 3, etc., and n is a measure of the exact number atoms or molecular units in the ideal gas system, e.g. if n = 3.2, then there would be 1.93E24 atoms (or molecular units) in the gas phase system. As to nR, there is no special significance here; it is simply the value of the pressure times the volume divided by the temperature for any ideal gas at that value of n (as shown below). --Sadi Carnot 12:25, 13 December 2006 (UTC)

But Bo, I don't understand the significance of R. If I multiply n by c, the speed of light, is nc the amount of gas in mol-m/sec ? PAR 20:24, 8 December 2006 (UTC)

If some fixed amount of gas has the pressure P, measured in pascal, and volume V, measured in cubic meter, and temperature T, measured in kelvin, then the product PV is measured in joule, and the expression PV/T, measured in joule per kelvin, happens to be asymptotically constant for sufficiently low pressure and correspondingly large volume. This constant value is an expression for the amount of gas, because it is proportional to the volume at constant pressure and temperature. Chemists, however, divide the mass (kg) of some substance by the molecular weight (kg/mol) to get the amount of substance measured in mol. The gas constant R is simply the conversion factor between these two units of measurement: mol and J·K⁻¹. If the mol was never invented, then molecular weight would be measured in kg·J⁻¹·K, and the gas constant would disappear from all the formulas. A third unit of measurement is the molecule. The conversion factor between molecule and J·K⁻¹ is the boltzmann constant, and the conversion factor between mol and molecule is avogadro's number. Please don't introduce a fourth unit of measurement, mol·m·s⁻¹. Bo Jacoby 06:09, 9 December 2006 (UTC)

I introduced the "nc" example to show why "nR" is not a good measure of the amount of gas. You say of "nR" - "This constant value is an expression for the amount of gas, because it is proportional to the volume at constant pressure and temperature." This is also true of "nc", so by your own reasoning, "nc" is a good measure of the amount of gas. The units are wrong. "Amount" has nothing to do with energy, so a measure of the amount of gas should not have "Joules" in it. "Amount" has nothing to do with time, so a measure of the amount of gas should not have "sec" in it. Both "nR" and "nc" are invalid because of this. The number of molecules (N) is a valid measure of amount, the mass (m) is a valid measure, the number of molecules divided by the number of molecules in a mole (mol) is a valid measure. PAR 13:59, 9 December 2006 (UTC)

Yes, nc could be used if no better measure existed, but n is a little better, and nR is much better. The mass of the gas is unnecessary and should be erased from the theory by Occam's razor. So the mol is unnecessary, (and so is the speed of light in this context). The simplest formula for an amount of gas measured from P,V and T is PV/T, and the SI unit for this quantity is J·K⁻¹. You may multiply by constants to get other units, but that only complicates matters. (The standard cubic foot of gas refers to non-SI units of volume, pressure and temperature). The ideal gas law says that amount does relate to energy, because the amount of an ideal gas is PV/T, which is energy divided by absolute temperature. Bo Jacoby 23:34, 9 December 2006 (UTC)

Well, lets leave it out just because it is not commonly used. PAR 23:39, 9 December 2006 (UTC)

OK. It's a pity, though, because the WP article is not helpful for understanding. Even you did not understand the meaning of the gas constant. Why not leave it to people who do understand? Bo Jacoby 23:47, 9 December 2006 (UTC)

I fear this may be a issue of different terminologies used in different disciplines, but in my book the amount of substance has no vagueness and choice of dimension left to convention, but is a physical quantity with the SI unit mole and the common non-SI unit "number of molecules", related by N_A. --Pjacobi 16:10, 11 December 2006 (UTC)

So, Bo, what about it? Do you have some reference for this usage? PAR 15:16, 12 December 2006 (UTC)

Your own writing says that Nk=PV/T where N is the number of molecules and k is measured in J·K⁻¹·molecule⁻¹, such that Nk is measured in J·K⁻¹. What more do you want? I am merely clarifying what you wrote without quite understanding. Bo Jacoby 23:55, 12 December 2006 (UTC)

[edit] molecules

I changed the word 'particles' into 'molecules' because these are the free particles of a gas. We are not talking about the number of quarks and electrons of the gas, but about the number of molecules. User Sadi Carnot reverted my edit. Please explain here on the talk page. Also you claim that "N is not the number of molecules, the Nk = nR edit is false and OR; + other grammatical and factual errors." I disagree. Please discuss first and edit later. Bo Jacoby 16:49, 14 December 2006 (UTC)

As user Sadi Carnot does not answer, I am going to revert his deletion. Bo Jacoby 13:11, 15 December 2006 (UTC)

Why don't you just leave it. Three people disagree with you. PAR 13:15, 15 December 2006 (UTC)

Why can't these people explain their point of view? I hope we agree to create high quality WP articles. That Nk=nR follows from the original statement of the gas law. Agreed? That the gas particles are called molecules follow from the article on molecule: "The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition". Agreed? Bo Jacoby 13:27, 15 December 2006 (UTC)

It's not a "provable" point, its terminology. Do you have a reference for this usage? Some article or book which uses this concept? Otherwise, it looks like a made-up terminology. I think that is what people are objecting to. PAR 13:48, 15 December 2006 (UTC)

Hi PAR. Earlier, physicists were accustomed to use many units of measurements and to convert by means of conversion factors, but now the units are standardized and the conversion factors rarely occur in physical equations any more, and so a modern physicist doesn't recognize a conversion factor when he sees one. You yourself did not recognize R as a conversion factor in PV=nRT, even if it is the only constant amongst four variables. Traditionally there are many units of measurements for pressure, (pa, bar, at, atm, mmHg, psi) and for volume, (liters, cubic feet, barrels, gallons, pint, ounces ...) and absolute temperature (kelvin, rankine, ...), but there are only two units of amount of substance around, the mol and the molecule. So the gas law is written PV/T=nR=Nk, converting the number of moles or the number of molecules into the appropriate unit. The modern approach is to chose units such that the conversion factors disappear. Then the gas law is PV/T=n, where n is the amount of gas measured in the appropriate unit. This is easily understood by an old physicist, but not for a young one, and so the WP article must explain rather than take it for granted. As by now neither the reader nor the author understood the ideal gas law! Bo Jacoby 15:14, 15 December 2006 (UTC)

Again, do you have a reference? PAR 15:37, 15 December 2006 (UTC)

I probably have the same standard references as you do. I understand it. You don't. Bo Jacoby 16:21, 15 December 2006 (UTC)

[edit] Difference between thermally perfect and calorically perfect gas

The article does not mention the difference between a thermally perfect and a calorically perfect gas.

A thermally perfect gas is defined as one which obeys the ideal gas equation: , whereas a calorically perfect gas is one which has constant specific heat (i.e. C_p and C_v are constant). When a gas is both thermally and calorically perfect, it is referred to as a perfect gas or an ideal gas. Also a gas can be thermally perfect and calorically imperfect, however, the vice versa is not true.

The difference is important when dealing with large temperature ranges. In such cases one can assume the gas to be just thermally perfect but calorically imperfect. This allows the use of equation of state for ideal gas while also accounting for variation in the heat capacities C_p and C_v with temperature (which can be significant).

I am not sure where to include this in the article. So I am mentioning it here. -Myth (Talk) 08:12, 25 February 2007 (UTC)

I agree. But lets get the terminology right first. If a gas is thermally perfect but calorically imperfect is it still an ideal gas? I believe it is. Can we say a perfect gas is thermally and calorically perfect, while an ideal gas is thermally perfect and may or may not be calorically perfect? PAR 17:56, 25 February 2007 (UTC)

It is common to refer to a thermally perfect gas as ideal gas, because for many applications one is just interested in using the ideal gas equation. I just have one reference which makes the distinction in the definition. I think the context will make the definition clear in most cases.

Reference: Vincenti, Walter G.; Kruger Charles H. Jr. [1965] (2002). Introduction to Physical Gas Dynamics. Krieger Publishing Company, pp. 8. ISBN 0-88275-309-6. 

-Myth (Talk) 04:25, 26 February 2007 (UTC)

I cannot find any reference which says that the specific heats of an "ideal" or "perfect" gas are independent of temperature. All the references I have use "ideal" and "perfect" interchangeably, and only refer to a gas for which PV=NkT. I think we should overhaul the article to reflect this. PAR 06:17, 26 February 2007 (UTC)

The definitions are important when we are dealing with large temperature ranges over which the specific heat capacity can change appreciably. This is usually the case in combustion and gas dynamics. That's why many references will not be concerned in trying to distinguish between the different definitions.

btw here is another reference which clearly distinguishes between a calorically and thermally perfect gas (again it is in relation to gas dynamics)

Another reference: Anderson, John D. Jr. [1989] (2000). Hypersonic and High Temperature Gas Dynamics. AIAA, pp. 388. ISBN 1-56347-459-X.  Google search result for this reference.

I don't think it is necessary to change the article to say that ideal or perfect gas only refers to PV = NkT, because that won't be the correct definition. It would be better to just add a statement clarifying that many authors refer to a thermally perfect gas as an ideal gas or a perfect gas and that the reader should be aware/careful of this. -Myth (Talk) 07:01, 26 February 2007 (UTC)