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Standard state pressure
Putting together the new Template:Chembox I had to research whether standard state pressure is 100 kPa or 101.325 kPa. I found a great deal of confusion, but the later and more "in-depth" literature seems to be unambiguous that it is supposed to be 100 kPa. The 1985 JANAF Thermochemical Tables state that "in all previous tables, the standard-state pressure was 1 atmosphere (101 325 Pa). For this publication, the standard-state pressure is changed to one bar (100 000 Pa)." Yet I found a later (1994) secondary publication, the Concise Encyclopedia of Chemistry (de Guyter) still referring to the older 101 325 Pa standard in their brief entry on standard state. The latest CRC Handbook of Chemistry and Physics unequivocally states that "The standard state pressure is 100 kPa (1 bar)." Their tables are now corrected to this pressure. Therefore I'm going to amend the wording in the article to match this. Walkerma 17:33, 16 May 2005 (UTC)
- What this article should match is basically what you told us here. Not everybody looking for information about "standard state" is going to be looking for information compiled in the last decade. The fact of the change should be clear in the article itself. Gene Nygaard 03:38, 24 August 2005 (UTC)
I received a correction via email that the current IUPAC definition does not include temperature, and this appears to be the case , so I have removed that statement. Mindspillage (spill yours?) 20:13, 22 August 2005 (UTC)
Removal of stubbiness
No one's worked on this page in a year, and it seems to be fairly complete, or at least no longer a stub.--Atemperman 03:55, 25 September 2006 (UTC)
I'm not sure what the convention here on wikipedia is, and don't have time right now (in the middle of chemistry coursework) to find it, but bar is not an SI unit; neither is mol/L for concentration. Perhaps it's just high-school idiocy, but the International Baccalaureate organisation is fairly adamant that temperature is not in the definition, as [User:Mindspillage] said but crucially it is 1 atm rather than 100 kPa. I'll hold off making this last change until someone can provide a good source either way. Simplyw00x 15:54, 4 March 2007 (UTC) Consider reverting to atm. There are 101.3kPa in 1 atm, not 100kPa.
- Bar is certainly closer to being an SI unit than atm! (It is actually cgs an earlier incarnation of SI). The correct SI unit is Pascal and 1 bar = 100kPa. The temperature of interest is usually to be specified separately.
- The problem with the whole page is that it misses the whole point of standard states rather miserably. They are aribitrarily chosen NON-ZERO states. That is the opposite of a natural zero as e.g. zero Kelvin or zero Pascal, because there is an infinite number of equally suitable or unsuitable states that equally qualify. And no that is not a messy error-to-be-corrected (but IUPAC committe or otherwise), it is deliberate choice that is vital for the functioning of the system.
- The easiest way to see that is to look at the definition of activities. They are defined as ratios, e.g. between the concentration of a solution and a standard concentration. This is a trick to get rid of dimensions, because e.g. the ratio between 3 moles per liter and a(!!) standard concentration of 1 mole per liter is simply three. Period. No dimension.
- The point here is that we cannot apply this trick if we take the standard to be a natural zero, because dividing by zero is not a wise thing to do.
- (It may well be that the IUPAC in its infinite wisdom is missing this point too, because the old plimsoll was chosen for good reasons: it indicates anything-but-nought. So, declaring that noughts and plimsolls are equally acceptable is educationally far from wise).
- Moreover, the wisest choice of standard state depends strongly on the problem at hand. E.g. in very non-dilute solutions it is not very sensible to think in molarity (volumes contract, solutes become solvents at some point etc.). So taking 1 mole per kilogram or a molar ratio X of unity (the pure substance) as standard is much more workable. (To fix the standard state at 1 mol/lit makes working with phase diagrams impossible...)
- In conclusion: there is no and should be no 'the' standard state only a standard state, properly stated and defined.
- Jcwf (talk) 01:52, 8 December 2008 (UTC)
The page doesn't actually say anything different… of course you can define any standard state you like, but there are certain standard states which are much more widely used than others. You will see that the superscript is not a naught but a circle: it doesn't denote a natural zero any more than °C or °F are based on natural zeros. Indeed, the non-thermodynamic temperature scales are very good examples on conventional reference points, and much more widely known than standard states! Your definition of activities is missing a vital point: they are concentrations (or molalities, or mole fractions, or pressures if you're using fugacities) divided by the standard concentration (etc.) but multiplied by an empirical coefficient which accounts for non-ideal behaviour. Physchim62 (talk) 02:18, 8 December 2008 (UTC)
- I had left out the latter because in dilute solutions this factor is unity and no it is not the vital point of activities: the removal of dimensionality is. Without that there is no RTlnP or RTlna because the logarithm does not make much sense if P has a dimension. (Energies in J*logarithmic(pascals)? Yikes)
- Your reference to the temperature scales also rather misleading because they are related by addition and no they do not remove dimensionality.
- If the nought is not a nought why does everybody call it that way? I have never heard anybody call something 'delta G circle'. Please do not make things up. Jcwf (talk) 02:29, 8 December 2008 (UTC)
Please don't imply that I'm making things up, you'll only look silly. I've never heard anyone refer to 'delta G naught' either – that would imply a variation in the conventional terrestrial acceleration due to gravity (Δg0) – instead people talk of 'delta G standard', usually referring to one of the conventional standard conditions described briefly in the article. If you really think that the main point of activities is for dimensional homogeneity then you need to learn a bit more thermodynamics: defining activities as dimensionless merely simplifies a few equations later down the line, it is of no scientific consequence whatsoever. Physchim62 (talk) 02:45, 8 December 2008 (UTC)