Hydrogen and helium as sources of lift

December 1, 2007

in Geek stuff, Science

Here is a random counter-intuitive fact about chemistry: while the atomic weight of hydrogen is 1.00794 grams per mole and that of helium is 4.002602 grams per mole, the helium nonetheless has 92.64% of the buoyancy of the hydrogen. This is because air weighs about 1.3 grams per litre, while hydrogen and helium gasses weigh 0.08988 and 0.1786 respectively. It is the difference between the density of air and the lift gas that is important and, in absolute terms, hydrogen and helium are not that different.

Ultimately, both hydrogen and helium are capable of providing about 1kg worth of lift per cubic metre of gas at room temperature and pressure. The major reason for which helium is popular as a lifting agent for balloons and zeppelins is because it is not flammable (it is actually a remarkable unreactive element). Unfortunately, helium is a lot more costly, has other uses (such as cooling superconductors), and is in the midst of significant shortage.

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{ 9 comments… read them below or add one }

Tristan December 1, 2007 at 5:21 pm

I think this is a bit silly. What determines the mass of something? It’s density, obviously. The notion of atomic weight is only useful, and sensible, to chemists. To say what’s interesting is the weight of the individual atom is like saying, “I don’t care how densely packed this cargo ship is full of tricycles, what I want to know is the average weight of each tricycle. That’s how I’m going to determine how much fuel I’m going to need to take it across the ocean”.

Milan December 1, 2007 at 5:34 pm

Tristan,

For any particular temperature and pressure, there is the same number of atoms of gas per unit volume. That is true whether the gas is hydrogen or uranium hexaflouride.

As such, the tricycle analogy is incorrect. It is more as though the ship is full of big spheres of a set size but differing weights. If you knew one ship has spheres four times as heavy, you would intuitively expect it to be a lot less buoyant.

R.K. December 2, 2007 at 3:20 pm

From this, you can extrapolate that a vacuum balloon – as first described by Franceso de Lana in 1670 – wouldn’t be that much better than a helium balloon.

So much for some of the loftier elements of The Diamond Age.

Milan September 11, 2008 at 8:32 pm
Milan July 14, 2009 at 11:31 am

Very dangerous chemicals: Chalcogen Polyazides and Azidotetrazolate Salts

Matt July 14, 2009 at 1:50 pm

For any particular temperature and pressure, there is the same number of atoms of gas per unit volume. That is true whether the gas is hydrogen or uranium hexaflouride.

This is true for ideal gases. Most gases behave ideally at normal pressures, but at higher pressures molecular interaction can cause non-ideal behaviour. The ideal gas law is governed by PV=nRT, where P is pressure, V is volume, n is number of moles of gas you’re dealing with, R is the gas constant (there are different ones for different units) and T is temperature.

Using this equation, at temperature 15°C (~288K), 1 atmosphere of pressure, and the corresponding R value of
0.08206 L·atm·K^-1·mol^-1, you’ll find that 1 mole of any ideal gas will occupy 23.6 litres of space. Finding out what those 23.6 litres weigh is a matter of multiplying by the molecular weight. For hydrogen gas, H2, this would mean those 23.6 litres weigh 2 grams. For air, those 23.6 litres would weigh about ~29 grams. Therefore, 23.6 liters (1 mol) of hydrogen gas would lift about 27 grams.

Matt July 14, 2009 at 1:55 pm

Whoops, I wanted to add that from this, you can see a weightless 23.6 litre sphere containing a perfect vacuum would indeed only lift 29 grams, about a 7% difference from the 27 grams of hydrogen. Not completely insignificant, I guess. Not enormous, either.

. July 14, 2009 at 2:08 pm

Compressibility factor
From Wikipedia, the free encyclopedia

The compressibility factor (Z) is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behaviour. In general, deviations from ideal behavior become more significant the closer a gas is to a phase change, the lower the temperature or the larger the pressure. Compressibility factor values are usually obtained by calculation from equations of state (EOS), such as the virial equation which take compound specific empirical constants as input. Alternatively, the compressibility factor for specific gases can be read from generalized compressibility charts that plot Z as a function of pressure at constant temperature.

Milan July 14, 2009 at 10:29 pm

Fluorine is pretty awful, too.

I didn’t have one, so I’ve decided to make this my ‘dangerous chemicals’ thread, at least for the time being.

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