# Is diborane lighter or heavier than air?

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1. Jun 23, 2017

### xpell

Hi,

I have been reading about diborane (B2H6, a gas at standard conditions) and while the sources provide different density values (!), as in 1.2475 g/L at standard conditions, or 1.18 g/L at 15ºC and 1.216 g/L at 25ºC (sounds crazy?), all of them are lower than dry or moist air at the same temperatures if my info is correct. Actually, almost every source says that diborane's vapor density ranges from 0.96 to 1, air being 1 (BTW, any source for good data about diborane? Every source seems to say a different thing! But in any case, all of them seem to agree that its density is lower or equal to air density.)

In despite of this, I have found not a few serious sources (NOAA, NIH, etc) stating that diborane vapors are heavier than air and precautions must be taken to prevent its accumulation in low-lying areas. How is this possible? Actually, if you search in Google for "diborane" "lighter than air" and "diborane" "heavier than air", you will find plenty of sites saying both things! Can anyone please clarify, if you're so kind...? And if you have solid data about diborane's density (and other properties) without those wild variations, I would appreciate it a lot! :)

Last edited: Jun 23, 2017
2. Jun 23, 2017

The density of diborane is nearly the same as nitrogen $N_2$ and not much lighter than $O_2$. Thereby even though it may be a trifle lighter than air, it will not rise into the upper atmosphere like helium would. The Wikipedia mentions diborane as being able to ignite and explode spontaneously in moist air, so that if you have a source that is producing it, it would be necessary to take means to prevent it from accumulating. Perhaps @Borek could comment further on this.

3. Jun 23, 2017

### xpell

Thank you very much, Charles, I'd just need to learn if it would "hug the ground" or even tend to accumulate in low-lying places as some sources say; or if it would rise even if slightly (or tend to stay above ground if already there) as it would be logical given that it seems to have a slightly lower density than air...

...and get some solid info about diborane's density at different temperatures, since the sources I've been able to find vary wildly! :)

4. Jun 23, 2017

To compute the density, the equation $PV=nRT$ works quite well. The density in grams $\delta_m=(M.W.)( \frac{n}{V})=(M.W.) (\frac{P}{RT})$ where $P$ is the partial pressure. If you think of it as being at 1 atmosphere in a container that had no weight to it, you could determine its buoyancy compared to air. $N_2$ has M.W.=28, so that it has density $\delta_m=28 (\frac{P}{RT})$, (use $P=1$ atm), and $O_2$ has M.W.=32, so it has density $\delta_m= 32( \frac{P}{RT})$, so that air (78% $N_2$ and 21% $O_2$) will have density somewhere in between these two numbers. Diborane has M.W.=27.6 if I computed it correctly, thereby there will be little or no buoyant effect. You can see by the same formula why helium (A.W.=4) will rise upwards and will not remain near the surface. $\\$ Note: The above densities are in grams per liter using $R=.08206$ ( liter atm)/(mole degK). $T$ is the temperature in degrees Kelvin $=T_{centigrade}+273$.

Last edited: Jun 23, 2017
5. Jun 23, 2017

### xpell

Thank you again and I appreciate it a lot, Charles, but... I didn't understand this, so sorry! I'm just your average guy with a huge interest in science but I wasn't fortunate enough to get a good education. So to start with, I don't even get why helium will rise (or even what A.W. means) but another less-lighter than air gas won't. Please consider me like kinda of a middle-school pupil, even if I'm 47!

6. Jun 23, 2017

### Staff: Mentor

When you have questions like this, pubchem is the gold standard for information. It always tells you where it got the values.
Do not foget to factor in temperature and elevation
The density of air at 15 deg C:
So diborane is lighter than air slightly --

It is also toxic at levels of 2.5ppm in air. ...per Pubchem

7. Jun 23, 2017

That's ok. :) It's using something called the universal gas law equation $PV=nRT$, and M.W. is a molecular weight. A.W. is the atomic weight of helium. Nitrogen and oxygen which make up air are each molecules consisting of two atoms. That's why the use the symbol $N_2$ for nitrogen and $O_2$ for oxygen. You might also be familiar with the water molecule, which is two hydrogen atoms and one oxygen atom so they call it $H_2 O$.

8. Jun 23, 2017

### xpell

Yes, yes, I know, this, but once we start with the equations...

9. Jun 23, 2017

Basically the equations tell you why a helium balloon will float upwards while a balloon filled with ordinary air does not rise. A balloon filled with diborane would behave very similar to a balloon filled with air. It would be very hard to tell the two apart.

10. Jun 23, 2017

### xpell

I (think) I understand that one, the density of air is equal to the density of air so it doesn't have any reason to rise, but... why a slightly lighter gas would not...? "Common sense" (yeah, yeah, I know...) would suggest the opposite!

11. Jun 23, 2017

### xpell

Yeah, I got interested in diborane because I was reading about pyrophoric materials and I found it to be the only one which is lighter than air... and I basically wondered if it would rise in flames! (Instead of staying close to the ground and killing someone or something...)

12. Jun 23, 2017

The difference isn't enough to make it rise. Nitrogen and oxygen are close enough to the same density when at the same pressure that they mix quite readily. The air pressure is slightly lower at altitudes, but I think the oxygen content is still present there even though oxygen is slightly heavier than nitrogen and might show a slight tendency to stay closer to the ground.

13. Jun 23, 2017

### xpell

OK, I get that. I still don't understand why, but I get it! :)

So, if it was (let's say) "propelled into the air", it would stay up there and would not rise or fall, wouldn't it...? (Until it starts burning, at least!)

14. Jun 23, 2017

It would tend to mix freely with air. In general it would propagate everywhere so that if it was in an enclosed room you would find it in equal amounts near the floor vs. up near the ceiling. If you propelled it upwards into higher altitudes, it wouldn't fall back down=it would tend to spread out simply because that's what gases will do.

15. Jun 23, 2017

### xpell

Thank you very much for your (big) patience, Charles, now I'm getting the idea. So I could consider it like sort of a "neutral buoyancy" gas, couldn't I...?

[But I still don't understand why a gas which is lighter than air (even if slightly) will not rise (even if slowly!)]

16. Jun 23, 2017

If it were to rise up, (there is a slight tendency to do this), there is a stronger tendency in the atoms or molecules of the gas to fill the void that was left behind. These gases have a tendency to mix because the individual atoms and molecules are moving very quickly all over the place. Unless the buoyancy is tremendous, like it is for helium, the mixing tendency will be the dominant factor.

17. Jun 23, 2017

### Staff: Mentor

You can understand density using the concept of floating.

Density is the weight (mass is the correct term) of a fixed volume of something. A child's toy block made of wood, it floats on water. A block the same size made of iron sinks. If you could magically make a block of water it would neither sink nor float - it would be neutrally buoyant.
Wood is less dense than water. Iron is denser than water. Obviously water is the same density as water.

You express density as mass/volume. Let's use grams/milliliters for our unit, weight==grams, milliliter == volume, 1000th of a liter ( a cm3 ) is the same volume as a milliliter - you could use either one.
Density of iron=7.87g/ml
Density of water=1.00g/ml
Density of white pine (kind of wood) = 0.40 (average because wood varies). Other species like oak are more dense, balsa wood is one of the least dense woods.
All species of wood with densities less than one float. And yes there are some few wood species that are denser than water and do not float, examples:
http://blog.mischel.com/2012/10/21/woods-that-sink/

18. Jun 24, 2017

### Staff: Mentor

As Jim wrote - wood that is less dense than water goes up in water, iron that is more dense goes down. That's quite easy to follow as it is in an agreement with our experience. Gases are not different - the one that is less dense than the air will go up, the one that is more dense will go down. Same principle, we just rarely deal with gases, so we don't "feel" them as we do with water, planks and nails.

However, there is an additional problem with gases. Note, that a piece of wood or a piece of iron are well separated from water, they don't mix. Gases mix easily. Comparing their densities can be easily used to predict how would the balloon filled with gas behave - but not necessarily "just the gas". Sure, you can have an invisible pocket of gas raising or going down, but you won't be able to easily spot it, plus it will rather soon mix with the air and disappear. Puddles of heavy gases can exist as they are in contact with the air on one side only and there is no convection that would mix them with the air.

Once the gases are mixed lighter molecules don't go up (or rather: even if they go, air is mixed fast enough by the wind and thermal motion of the molecules so that the separation is negligible).