# Enthalpy of formation at non standard conditions

1. Oct 24, 2014

### Julius

Hi,

I want to know the enthalpy of formation of Ga2O3 and Ga2O at low pressure (ultra high vacuum) and high temperature.

Temperature seems easy, but I'm not sure how to get the enthalpy values for low pressures.

Does anyone know if there are books with enthalpy - pressure diagrams of these compounds?

It should be also possible to calculate it from the value at atandard conditions using ∂H/∂P=(V−T∂/V∂T)dP, but one needs a PVT relation. For Ga2O maybe the van der Waals equation for non-ideal gases? And for Ga2O3?

Cheers,
Julius

2. Oct 24, 2014

### Staff: Mentor

When it comes to data and diagrams, best starting point is often a library - librarians are trained to know where to look for information (they may not know what it means, but they know where to look for it ;) ).

3. Oct 25, 2014

### DrDu

If the Ga compounds are solid at the T and p in question, you can probably neglect them, as $\int V dp$ is very small due to V being small.
If they are gaseous, they are certainly described well by the ideal gas law which works best at low p and high T. So the pressure dependence vanishes, too, as $V-T \partial V/\partial T=0$ for an ideal gas.

4. Oct 27, 2014

### Julius

Thank you for the replies! :)

Yes, at low pressures I can use the ideal gas law, but I need to start from standard atmosphere pressure, so I think i still need an other PVT relation, right?

5. Oct 27, 2014

### DrDu

No, as T is high

6. Oct 28, 2014

### DrDu

I pondered about this question a bit more and there occured something to me: For gasses, the standard state is a hypothetical state where the ideal behaviour at infinitely low pressure is extrapolated to 1 atmosphere.
So you can assume ideal behaviour for the gas by definition.
This is not very relevant in your case as gasses at standard pressure and temperature usually behave quite ideally.