Author Topic: standard conditions on thermodynamic data  (Read 1320 times)

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stratosphere

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standard conditions on thermodynamic data
« on: January 14, 2004, 01:39:00 AM »
im not sure if this is the appropriate forum for this question, but its seems a bit to acedemic for any of the others.

on thermodynamic data charts (e.g. H,G,S) they usually say the data for gases is at 1atm and 25C, but for some substances its vapor phase cannot occur at 1atm and 25C, because it would condense (e.g. water vapor).
in these cases should one presume the data is taken at the minumum temperature at which a vapor pressure of 1atm could exist?
for example 100C for water

Rhodium

  • Guest
physical chemistry sux
« Reply #1 on: January 14, 2004, 02:54:00 AM »
I've never heard that a substance in the gas phase would be a requisite, only that the heat of formation measurement is taken at 25°C/1 atm. If the substance is a liquid under such conditions, so be it.

If you for example perform 1/2 O2(g) + H2(g) -> H2O(g) with 25°C/1 atm as starting conditions in a closed container, you have to take into account both the change in pressure as well as the heat of vaporisation for water to be able to figure out the resulting temperature change to be able to figure out the change in temperature, using the values for the compounds' heat of formation.


stratosphere

  • Guest
but on thermo tables it gives the data for...
« Reply #2 on: January 16, 2004, 08:12:00 PM »
but on thermo tables it gives the data for example for both H20(l) and H20(g).
for H20(l) the 1atm, 25C are both the starting and ending conditions (for example imagine the bomb has a movable piston), but for H20(g) 1atm and 25C could not possibly be the ending conditions.