Talk:Gas constant

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Most recent update was mine, just created an account. R in cal/mol/K or kcal/mol/K is used in a lot of work with RNA and DNA systems.

-Zifnab

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[edit] Too many units

Do we really need that huge table of the gas constant in different units? For example, who uses slugs? Why are there so many versions that seem to be in terms of mass? I think we need to trim the list a bit. Mathfan (talk) 17:56, 2 May 2008 (UTC)

Mathfan: don't be an elitist. For reasons you may not understand, Engineers use several dozen sets of units. Why the fuck did you get rid of that table? That table has helped me for the past few months and now I'm having trouble finding R at the esoteric units that I need.

Seriously? I didn't realize those units were actually helping anyone. What's your job? Also, what elitism? Mathfan (talk) 11:35, 15 May 2008 (UTC)

[edit] Move entry to Gas constant

This entry should be moved to Gas constant or even Universal gas constant and it is the Molar gas constant that should be REDIR-ed. Whatever you do with the units etc. it does not change the sense - is there any other (universal) gas constant? Its sense lies in its molar nature and you do not need to do is again (buttery butter). If it is not molar, then it has its own name and it is Boltzmann constant, k (same but per molecule), and R = NAk. If you need sth more like specific heat (J /g K)it stops being constant ...

I know that some sources use this molar adjective - even Encycl. Britannica - but if you check sth more serious like P.W. Atkins "Physical Chemistry" (4th ed., Oxford 2000), IUPAC's Goldbook http://www.iupac.org/goldbook/G02579.pdf , Gas constant is all you get and it is all you need.

AWM~ads 21:47, 11 Mar 2005 (UTC)

An IUPAC convention is most certainly a convincing argument. However, another serious source CODATA does use the molar adjective. See also "CODATA recommended values of the fundamental physical constants, 1998", Rev. Mod. Phys., vol. 72, No. 2, 2000. So, the issue seems debatable and at least not very urgent.
That being said, your arguments are sound and IUPAC deals more with naming conventions than CODATA. Therefore, I support the move.
Jan van Male 09:36, 12 Mar 2005 (UTC)
I like your attitude. I suggest similar move for Avogadro's number which is not a number but an Avogadro's constant. AWM~ads 22:50, 25 Mar 2005 (UTC)
I don't think a similar name for Avagadro's number would be fitting, as in my experience someone involved in the field is more likely to call it Avagadro's Number than Avagadro's constant. On the other hand, the gas constant does simply get called the gas constant, with some adjective infront of gas.EagleFalconn 19:11, 22 Apr 2005 (UTC)

[edit] Specific gas constant

This page (gas constant) should not talk about the universal gas constant exclusively. Dividing the universal gas constant R by the molar mass of a gas results in a new gas constant Rs (specific to the gas). There was already some confusion on speed of sound where the text linked to gas constant explicitely claiming that Rs was in fact the universal gas constant which it clearly isn't.

gas constant should either be a disambiguation page for universal gas constant and specific gas constant or discuss both concepts on this page. Algae 20:41, 1 January 2006 (UTC)

I added a page on the specific gas constant. Feel free to review it, and make it more useful. KMossey 05:44, 8 June 2006 (UTC)

[edit] Answering Seth Ilys

The article said: The Boltzmann constant is conversion factor between gas units. It tells how many joules per kelvin make a molecule. Seth Ilys deletes it saying: I have no idea what that sentence actually means, let alone what it's supposed to mean. Well, if N is the number of molecules then Nk=PV/T is the amount of gas measured in joule per kelvin, so k is the amount of gas of a single molecule. Bo Jacoby 05:12, 31 January 2006 (UTC)

[edit] Answering 203.220.11.220

You write: Because the gas laws are so inexact for real gases, the gas constant is not known as accurately as most other fundamental constants of physics. This is not true. 6 significant digits is not bad, and in the low pressure limit the ideal gas law is precise. Bo Jacoby 09:07, 2 February 2006 (UTC)

Agreed. 6 significant digits seems pretty good compared to other constants. The gravitational constant has fewer. I wouldn't mind having list of physical constants, though, Category:Physical constants is not a great substitute. Algae 09:42, 2 February 2006 (UTC)
See Physical constant. Bo Jacoby 10:16, 2 February 2006 (UTC)
Thank you. Algae 11:35, 2 February 2006 (UTC)

[edit] R *is* the Boltzmann Constant

for the same reason. It's just usually expressed in liter-atmospheres (a unit of energy) per degree Kelvin (a unit of temperature) mole (unitless), rather than in Joules per Kelvin. Check it yourself -- when you convert the units, 0.08206 liter atmospheres per kelvin mole is 1.38e-23 J/K.

The reason for the similarity is that the pressure of an ideal monatomic gas is the same as its internal energy density, according to the kinetic theory of gas.

So, er, I'm sticking Boltzmann constant back in (it was recently diked out) zowie 15:18, 31 March 2006 (UTC)

[edit] 2/3 cut - ref please

Removed the following:

If we define the temperature of a gas as the average kinetic energy of its molecules then the constant simplifies further when the units are standardized. Instead of measuring energy sometimes in Kelvin and sometimes in Joules, the same unit could be used for both measurements. Instead of the gas constant being in Joules per Kelvin, the constant would then be dimensionless and reflect simply the physics of the gas law and not the conversion between different units. The true value of the gas constant can then be seen to be simply 2/3.
PV represents a double counting of the proportion of kinetic energy in a gas in one direction, eg up-down, left-right, or backwards-forwards. The double counting comes from calculating m*v*v whereas kinetic energy is 1/2*m*v*v. If molecules hit something and stop, one would be measuring 1/2*m*v*v. If they hit something and bounce back, then we are measuring m*v*v.
nT represents the total kinetic energy in a gas in all three dimensions and not just perpendicular to a plane. 1/3 of the energy is in each direction, up-down, left-right, or backwards-forwards.
PV/2 = nT/3
The Universal gas constant therefore has a value of 2/3.
Physicists usually don't explain that as they don't really understand it themselves.
Ivan Urwin

Provide a ref. Ivan or it stays out :-) Vsmith 18:03, 6 May 2006 (UTC)


What do you have a problem with? RHS: nT/3 - one third total kinetic energy - that only a third of the kinetic energy would be perpendicular to a given plane? LHS: PV/2 - Or that the m*v*v in pressure calculations double counts kinetic energy of 1/2*m*v*v perpendicular to a plane? Moles are dimensionless, just like a dozen is 12. You just need a calulator and to convert between Kelvin and Joules to verify it. You could call the gas constant 8 inchs per foot if you want! Of course if physicists want to put it in ergs per dozen electron volts or BTUs per mega barrel of oil equivalent or Joules per mole Kelvin instead of getting their calculators out: fine! That won't stop the fundamental physics of the gas equation saying its 2/3.

=

Here, not rigorous physics, but enough that people can see for themselves that the gas constant is 2/3 when coherent units for energy are used.

Take a particle of mass m going at velocity v, and to simplfy calculations imagine it in a sphere of radius v. Time taken from centre to perimeter is then 1 second, and time back to the centre is another second. Exchange of momentum is then 2mv in 2 seconds, and rate of change of momentum is what some guy called Newton called force. So the force is mv, and the pressure on the sphere is mv/A where A is the area. Volume in three dimensions is 1/3*base*height, so the volume of the sphere is (1/3)*v*A.

PV = (mv/A) * (1/3)*v*A = (1/3)*m*v*v = (2/3)*(1/2)*m*v*v = (2/3)*T

Sum over n particles ... The gas law is PV = n*(2/3)*T Call the constant R. R=2/3. Change the units of measurement 2/3 is 4.01E26/kmol (in the same way that 2/3 is 67 per cent) 2/3 is then 8310 J/Kelvin/kmol (in the same way that 2/3 is 8 inches per foot) R=8310 J/Kelvin/kmol

OK, we have a particle with velocity v bouncing around in a sphere of radius v to simplify calculations...and this is published in ??? Vsmith 00:19, 7 May 2006 (UTC)

The constant only has a different value if you measure the heat applied in Joules or BTUs and the temperature rise in Fahrenheit or Kelvin, etc. The mental problem people have is in switching between units: You can measure a tank's fuel efficiency in gallons per mile and the area of a field in acres. Somebody that understands what they are doing can cope with measuing areas in gallons per mile, or measuing a tank's fuel efficiency in acres or square metres. For some people tax is 20 cents in the dollar. Other people manage to cancel the two different units of currency and just specify the tax rate as a dimensionless 1/5.

Take a monatomic gas and heat it. All (ie. 100%, 1) of the heat causes a temperature change, rather than causing the molecules to spin or vibrate.

3/2R = 1.

Therefore R=2/3.

Do I have a reference for 3/2R when heating monatomic gases? No!

[edit] R=2 (2/1), R=1 (2/2) or R=2/3?

Anybody who has seen Newton's cradle - the swinging balls on strings - will know that if a particle hit another and transferred all its kinetic energy, then it comes to a stop. This is obvious anyway, without any kinetic energy, something cannot be moving. When we measure PV, (pressure times volume,) we are looking at a state where molecules collide with the surface they exert a pressure on, and bounce back. We are therefore measuring something based on twice (2!) the kinetic energy perpendicular to the surface. Assuming the kinetic energy is equally partitioned in three dimensions, that is equivalent to saying that PV measures the translational kinetic energy in two dimensions, since twice the energy in one ddimension equals the energy in two dimensions.

Bolzmann's constant (k) in the gas law does two things. It partly converts units, between joules and Kelvin say (for example) and it partly fills the role of the gas constant, which has a value telling us that PV is measuring the kinetic energy of the gas in two of the three dimensions of space, or equivalently, twice the portion of kinetic energy in one dimension, namely twice that perpendicular to the surface the pressure acts on.

This means that we can convert between temperature energy and other energy, kelvin and joules (for example), but only once we decide on a definition for temperature. Different definitions of temperature will result in different values of the gas constant and different values of the conversion constant. If I use strange units to measure circles, then you can calculate and convert satifactorily once you measure a circle for yourself and compare with my figures. However the 'universal' constant you use includes no knowledge of whether I am measuring a radius or a diameter. Only when you understand what my measurement really means can you determine a proper constant value for converting units, and assign another constant to account for measuring different features of the circle, eg radius, diameter or even circumference.

If I measure diameter and you measure radius, then a factor of two in your universal constant will result from the physics of what is going on, and the rest will be units conversion. The conversion factor might be so many weirdo units per dozen inches. Somebody else might think 'dozen inches' is a bit strange and come up with a different constant in weirdo units per inch. This is what has happened with the gas constant in joules per mol kelvin, or joules per kelvin.

(A) If we define temperature as the energy per degree of freedom, (the kinetic energy in one dimension) then the gas constant is R=2 and the conversion between joules and kelvin is with a constant k/2.

(B) If we define temperature as twice the energy per degree of freedom, then the gas constant is 1 and the conversion between joules and kelvin is with a constant k. This I believe is an unnatural choice. It is however what happens when people take the Bolzmann constant as being simply a conversion factor and forget about the gas law.

(C) If we define temperature as three times the one dimensional kinetic energy, (ie the kinetic energy in three dimensions - the total translational kinetic energy), then the gas constant is 2/3 and conversion between joules and kelvin is with a constant 3k/2.

The first choice (A) is not a bad choice to make. For example we read for diatomic gases around room temerature that Cv~=(5/2)*R. With R=2 this becomes Cv=5. This is obvious; the total energy required to heat something with energy equally split between 5 degrees of freedom is simply five times the energy required per degree of freedom.

Poor students are then being confused by people teaching this stuff that do not understand it themselves. To elimintate the conversion constants it becomes necessary to take a ratio of two values that both include the conversion constant, eg a Cp and a Cv, and call this gamma; and then, to manipulate this to come up with stuff like (gamma-1)/gamma to get to something which should be fundamentally simple to start with.

Take a diatomic gas around room temperture and use definition A for temperature. Cv ~= 5, Cp~=7, since (PV stuff again) it takes the energy equivalent of a couple of degrees of freedom to push back the environment.

Gamma = Cp/Cv = 7/5

Gamma-1 = 2/5

(Gamma-1) / gamma = (2/5)/(7/5) = 2/7.

We are back to a simple ratio of counting a few degrees of freedom, and the clarity of this is completely remove from the students mind by doing calculations in joules per mole kelvin with figures only suitable for calculators, etc.

[edit] Merge specific gas constant

It will be useful to merge the article on specific gas constant here. I guess many readers will be interested in knowing about both the terms and it would be better to have them on one page. -Myth (Talk) 17:54, 27 February 2007 (UTC)

  • I second that. A merge would useful. Verkhovensky 05:20, 28 February 2007 (UTC)
Merger completed. -- Myth (Talk) 12:34, 19 March 2007 (UTC)

[edit] 8.314472 or 8.314482?

Slight inconsistancy in the statement of the value of the Universal Gas Constant. In the table it says 8.314482. I assume this is just a misspelling but I'm not confident enough to correct it myself so thought I'd make sure.

130.88.186.123 16:28, 17 April 2007 (UTC)

[edit] Reversion of edit from 128.252.173.197

I went and reversed a change made by 128.252.173.197. It had been changed from the proper value in terms of (L*atm)/(K*mol) (0.0820574587) to 82.0574587, which is the same value, but in terms of mL. Liter is the standard measure for gases, so that (0.082...) is the proper figure to keep.

[edit] R = 8.314472(15) J · K-1 · mol-1

In the formular, J is not denoted. —The preceding unsigned comment was added by Natasha2006 (talk • contribs) 14:48, 7 May 2007 (UTC).

[edit] Added a note on three of the constants in the table

Three of the values for the universal gas constant contained in the table are for air only. Since these three are mass-based vs. molar based constants, they can only describe a substance with a certain molar weight, which is in this case air. I added a note mentioning this, as there did not seem to be anything already in the article noting this fact. —Preceding unsigned comment added by 98.25.180.184 (talk) 03:35, 7 April 2008 (UTC)