H2 diffusion in water

[Anders K]

I have a question that I struggle to wrap my head around.
An H2 molecule solved in water. The water tank is tall and made of a material with a low resistance to diffusion.
The Pressure at the top of the tank is lower than ambient pressure.

Question?
Will the H2 rise to the surface (low pressure)or diffuse through the closest wall.

Thanks in advance

BR
Anders

 

[EdStainless]

You will need to find the equilibrium between that dissolved, gas in the headspace, and the amount that will diffuse.
Pressure has little effect on diffusion, it is driven by concentration gradients.

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[3DDave]

Since the container has low resistance to H2 diffusion, all the H2 will eventually leave the container and not return. The equilibrium condition has no H2 in the water.

 

[TiCl4]

Answer: Both will occur in proportions that are relative to the relative diffusion coefficients and relevant geometries.

 

[3DDave]

It doesn't say H2 is continuously generated at a certain rate. It says it starts with some.

 

[EdStainless]

3DD, there will still be some equilibrium amount of hydrogen in the container, at infinite time it will just be very, very low.
How much hydrogen are you starting with? The solubility limit in pure water at room temp is about 2mg/kg of water.


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P.E. Metallurgy, consulting work welcomed

 

[Anders K]

Thanks, everyone for the answers and reasoning. I think I´m starting to understand.
Just to be sure: the H2 molecule will not "float" to the surface if it´s solved in the water?
BR anders

 

[3DDave]

It won't really float unless there are bubbles formed. Think of this - Sodium and Chlorine are dense atoms compared to H20, but one doesn't see thick layers of sodium or chlorine at the bottom of the ocean which should happen were simple buoyancy to apply. Instead they get whacked about by collision and the eventual result is that the concentration averages out. This process should happen more rapidly with lower density/smaller weight/mass atoms/molecules, but I do not expect there to be a mass migration to the surface. Macroscopically things happen differently - get a few hundred million atoms together and make a bubble and you get some movement, but it is much much slower than molecular velocities; just much less random in direction.

 

[Anders K]

Thanks again everyone!

 

[EdStainless]

Hydrogen will diffuse to the surface until the fugacity in the headspace matches that in the water. This will happen even if the H2 is below saturation of the water as long as the headspace starts H2 free.
And since H2 in the atmosphere is low (reported as 0.0000038 % hydrogen, or 500ppbv) there will be diffusion through the walls until it is in balance with this value.

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P.E. Metallurgy, consulting work welcomed