User talk:WikiPidi
Heat capacity: Constant volume or pressure
[edit]Why is it that hardly anybody cares to specify if a listed value for the heat capacity of a gas is measured under constant volume or constant pressure? The numbers are significantly different, and lead to very wrong results if one condition is implicitly assumed while the figure in fact was measured under the other. Since He is very close to an ideal gas it is fairly easy to confirm that the listed number in the article is for constant pressure, I will change the article to specify that. In many other articles the given numbers are useless! WikiPidi (talk) 14:32, 8 November 2010 (UTC)
- Hmm, seems it's not that easy, as nested templates are used. I've brought the issue up on the Template:Infobox_element talk page WikiPidi (talk) 15:51, 8 November 2010 (UTC)
- This particular heat capacity of 20.786 J/mole/K is exactly 2.5000 R (to 5 sig digits!), so obviously it's constant pressure heat capacity for a monatomic gas. Furthermore, exactly the same figure is given for the the other 4 noble gases Ne,Ar,Kr,Xe. This is fishy, as real substances rarely show the same heat capacity to 5 sig digits, even when it is the correct one. Even fishier still, radon is also listed as having the same heat capacity of 20.786 J/mole/K, which means this is certainly a calculated not a measured value, since nobody has collected enough radon to measure its heat capacity to that value (radon itself produces so much heat from its own decay that this would be a difficult and perilous measurement to even get an estimate for). SO at this point I have to say is TILT. Calculated values from theory should be marked so. SBHarris 17:29, 8 November 2010 (UTC)
- These values are not necessarily wrong. Kinetic gas theories show that either constant pressure or constant volume heat capacity are simple functions of physical constants (as you found, it's R*5/2, or R*3/2, respectively), i.e. independent of gas dependent parameters. So this boils down to the question of to which degree the noble gases behave like an ideal gas. AFAIK at normal conditions the deviations are really quite tiny, so I tend to believe these numbers.WikiPidi (talk) 14:03, 31 January 2011 (UTC)
- This particular heat capacity of 20.786 J/mole/K is exactly 2.5000 R (to 5 sig digits!), so obviously it's constant pressure heat capacity for a monatomic gas. Furthermore, exactly the same figure is given for the the other 4 noble gases Ne,Ar,Kr,Xe. This is fishy, as real substances rarely show the same heat capacity to 5 sig digits, even when it is the correct one. Even fishier still, radon is also listed as having the same heat capacity of 20.786 J/mole/K, which means this is certainly a calculated not a measured value, since nobody has collected enough radon to measure its heat capacity to that value (radon itself produces so much heat from its own decay that this would be a difficult and perilous measurement to even get an estimate for). SO at this point I have to say is TILT. Calculated values from theory should be marked so. SBHarris 17:29, 8 November 2010 (UTC)
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UTRS appeal #19718 was submitted on Nov 08, 2017 13:28:41. This review is now closed.