Siddy
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Max H2(g) psi from 2 known methods
After details on the maximum PSI of Hydrogen that can be formed from common methods.
1. Electrolysis of water, tube over cathode to trap Hydrogen and pipe to small storage container (5L volume).
2. Mg(s) + H2SO4 > H2(g) piped to a similar storage container.
Which method produced a higher pressure of hydrogen?
What is the max pressure created?
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microcosmicus
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Hmm . . . nice little exercise. I'll start with no.2 and
then sketch what to do with no.1
The trick here is no note that at maximum pressure,
the dissolving and redox reactions will be in
equilibrium with each other.
The equilibria involved here are as follows:
H2(g) <-> H2(aq)
H2SO4 <-> 2H+ + SO4--
2H+ + Mg <-> H2 + Mg++
The redox reaction involves the transfer of two electrons
and has potential 2.37 --- remember that the potential
for reducing hydrogen is defined as the zero of the
redox potential scale.
Therefore, by Nernst's equation, we have
[H2(aq)] [Mg++] / [H+]^2 = 10^80.3
Since that number on the left is astronomically huge ---
something on the order of magnitude of the number of
hydrogen atoms in the known universe (!) --- this basically
means that, in equilibrium, all your acid will be used up.
Therefore, we can pretty much assume that all the H+
becomes H2, so the question becomes how much H+
can we have in the system. Since the H+ comes from
the acid, we want as much acid as possible.
That would happen if we used really concentrated acid.
In such a case, we would pretty quickly get to a situation
where the liquid was saturated with MgSO4, the salt
precipitating out as fast as the metal got eaten and the
hydrogen being liberated as fast as the acid dissociates
At the end of the day, we would be left with hydrogen gas,
salt, and a bit of water.
To get maximum pressure, we wouldn't want extraneous air
in our vessel since that is unnecessarry volume which would
only lower the pressure. Therefore, the maximum possible
pressure would, to a reasonable approximation be the pressure
of a volume of H2 equal in to the volume of the same number of
moles of H2SO4. I'll let you look up the density of H2SO4,
compute molecular weights, etc, to come up with a number.
Since this is going to be some huge amount, the assumptions that
went into making it are open to question, in particular using
concentratons in the Nernst equations. We really should use
activities in sulfuric acid at high pressure. However, since the
equilibrium constant is so ridiculously huge, I sincerely doubt this
is worth fussing over --- the conclusion that virtually all the
acid gets spent should not change.
As for the first method you propose, by the same sort of reasoning
with Nernst't equation, we can conclude that all the water will get
electrolyzed for any reasonable potential. Thus, the maximum pressure
would similarly be given by the pressure of gas occupying the space
which would be occupied by an equivalent number of moles of water.
However, I seriously doubt you can get near this pressure. As you
increase the pressure, more gas dissolves in the water so you have
to worry about the hydrogen dissolving in the water and evaporating
into the oxygen and vice versa. Hydrogen mixed with oxygen is going
to be unstable against explosion, especially at high pressure. I leave it
to you to look up solubility as a function of pressure, diffusion constants,
etc,. and figure out what pressures could realistically be attained before
this would become a problem.
Therefore, I guess that procedure 2 would be able to give the higher
pressure because it could go to completion without such a complication
of mixing gases.
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chemrox
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I can't find a vector drawing program to work in these pages. Consider a cylinder with a sealed piston that has the H2 on one side and a hydraulic or
pneumatic pressure on the other. Don't know what the apparatus is called but needed for high pressure hydrogenation. How do they pressure Parr
machines above the gas source pressure?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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