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Old Aug 21st 2017, 10:13 AM   #1
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experimental thermodynamics properties

Hello everybody,
I am using an equation of state to calculate the thermodynamical properties of a material. I have a problem which is the ideal part of the equation. Actually, I cannot be sure that the ideal contribution in my equation is the really ideal contribution, so I decided to calculate non-ideal thermodynamic properties by subtracting the nominal ideal part my equation. I want to validate the thermodynamic properties of my equation with experimental data. are the experimental data non-ideal or total(ideal+ non-ideal)????
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Old Aug 21st 2017, 10:42 AM   #2
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Go on. Are you talking about a gas?
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Old Aug 21st 2017, 10:58 AM   #3
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Ideal often implies that this is what you will get if your experiment is very strictly controlled to avoid various issues that would make it non-ideal.

Non-ideal equations will therefore often include additional terms to allow for the less rigidly defined experimental conditions.

You still can't necessarily assume that the non-ideal equations will perfectly describe your experiment;
There will still be assumptions built into the equations regarding their range of applicability.
The less the experiment matches the assumptions of the equations, the worse the match between calculation and experiment.

Lastly there will always be noise,
this can often be recognised, because it should be random,
but, ideally, the noise should be quite a bit less than the primary effect you are looking for.
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Old Aug 21st 2017, 11:27 AM   #4
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Originally Posted by Woody View Post
Ideal often implies that this is what you will get if your experiment is very strictly controlled to avoid various issues that would make it non-ideal.

Non-ideal equations will therefore often include additional terms to allow for the less rigidly defined experimental conditions.

You still can't necessarily assume that the non-ideal equations will perfectly describe your experiment;
There will still be assumptions built into the equations regarding their range of applicability.
The less the experiment matches the assumptions of the equations, the worse the match between calculation and experiment.

Lastly there will always be noise,
this can often be recognised, because it should be random,
but, ideally, the noise should be quite a bit less than the primary effect you are looking for.
In gases at least, non ideal behaviour is not to do with experimental technique.

It is to do with gases not observing the very basic assumptions that lead to 'ideal behaviour'.
In particular ideal behaviour means that the internal energy depends only upon temperature.

The main reasons for non ideal behaviour are

1) The volume of the molecules is ignored. This is OK at modest pressures and volumes, but becomes significant when they are squeezed together.

2) The assumption that all collisions are perfectly elastic and there are no cohesive forces acting is not obeyed by the molecules.

The result of these are to introduce some form of dependence on other thermodynamic state variables such as volume. For example Van Der Waals or the Virial equations.
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Old Aug 21st 2017, 12:01 PM   #5
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Hi Studiot,
Yes I agree, but I was trying to indicate that:

An experimental set-up will reasonably follow the "Ideal" gas laws
at modest pressures for "simple" gasses where the intermolecular interactions are a reasonable match to the ideal gas approximations.

Move to very high pressures, or to molecules with complex intermolecular interactions (or any of the other "real" situations that are beyond the ideal gas assumptions) then the Ideal Gas laws might no longer sensibly match the observed behaviour.

Under those circumstances a more complete, more complex, model would be required,
one that moved away from the idealised simplifications implicit in the assumptions made in the "Ideal" law.
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