[ptryon]

Hi

I am looking at a thermodynamics data table and I can see a value quoted for the standard entropy of steam S^Θ = 188 J/mol/K

How is it possible to quote a standard entropy value when standard entropies are measured under standard conditions (273K or 298K depending on the data). At these temperatures water is a liquid and not a gas?

I notice the same problem with a range of other thermodynamic data. The change is quoted for standard conditions, but the change simple doesn't happen at standard conditions? Do they take measurements under the real conditions and then some correct the values to work out what they would be under standard conditions?

  I dont like thermodynamics!!!!

Any help greatly appreciated
Pete

http://en.wikipedia.org/wiki/Water_(data_page)#Thermodynamic_properties

[Enthalpy]

[...] 273K or 298K depending on the data. At these temperatures water is a liquid and not a gas? [...]

This can be a good place to start debugging.

Gas and vapour exist at any temperature. It's only that, if cold (here <100°C), little pressure (here <1atm) suffices to let them condense to the liquid - or to the solid if <0°C for water.

Below the 1atm boiling temperature, the vapour pressure can make all the pressure if the gas has less than 1atm. Then the liquid can even boil, for instance atop a mountain. And conversely, the liquid reaches >100°C under pressure, in a pressure cooker or in an engine's boiler. Even at 1atm, some vapour pressure exists below 100°C - this vapour makes only a contribution to the gas pressure which is called partial pressure. It's how the Ocean puts vapour in our atmosphere; it doesn't boil but is quite efficient. Snow as well can sublime without boiling nor even melting first.

So steam at 298K can be measured, just under lower pressure.

In other cases, table values are extrapolated to produce values easier to use, but here it isn't necessary.

[ptryon]

Hi
Thank you so much for your reply. This is good! I have a follow up question.
If the values are measured at a lower pressure (i.e. less than 1 atm), wouldn't this mean the conditions aren't really standard conditions? I am teaching IB/A-level chemistry so I generally drill students with standard conditions of 101kPa and 298K.
Pete

[Enthalpy]

I don't know: that's wording detail.

[Vidya]

Hi
Thank you so much for your reply. This is good! I have a follow up question.
If the values are measured at a lower pressure (i.e. less than 1 atm), wouldn't this mean the conditions aren't really standard conditions? I am teaching IB/A-level chemistry so I generally drill students with standard conditions of 101kPa and 298K.
Pete

First thing is in standard condition ...by IUPAC definition temperature is not fixed ...second all values mentioned at standard condition are calculated one ...like if steam does not exist at 25 deg C ...but all thermodynamic values are calculated using other data under standard condition..
Pl read this page carefully ..I am also an IB chemistry teacher ...
http://en.wikipedia.org/wiki/Standard_state