Why do tires inflated with Nitrogen maintain constant pressure better than if inflated with standard air?

Question

Common answers to this question are:

1. The Nitrogen difuses through the tire casing more slowly than oxygen because the N2 molecule is larger, and

2. Nitrogen pressure is more consistent than normal air pressure, because air typically contains varying amounts of moisture due to changes in the relative humidity. Water causes air to be inconsistent in its rate of expansion and contraction.

My thermal physics is a bit rusty, but I don't buy either explaination. As for reason one, N2 should be a smaller molecule than O2, 2H2O2, CO2, or other common O2 molecules found in air & therefore should difuse through the tire more rapidly.

Second, PV=NRT should apply to this problem, with N,R and V remaining virtually constant so P should vary only with T.

I'm just a tire geek who forgot most of what Leon Lederman taught him at Fermi Lab. Is there a physics geek out there who can answer this question scientifically?

Thanx

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Answer 1

You're right in one respect certainly. N2 has a triple bond so the bond length will be much shorter than O2's double bond, and therefore the molecule itself is smaller. Most references I find suggest that nitrogen does leak out too, but less than oxygen instead of more. So molecule size is no explanation.

But oxygen has one property that nitrogen definitely does not: it is VERY reactive. It's bad enough at normal pressures, but at the elevated pressures in your tires it's even worse. And since it's leaking out of your tire, it's not just doing damage to the surface, but all throughout the interior as well.

You have to ask yourself - what happens to rubber when it's oxidized? It burns, on a small scale. Bonds break down, it becomes brittle and cracked, and so on. In other words, it creates an opening for another oxygen molecule to react and weaken it further (like rust on the outside of your car). And if there's a spot where it's slightly easier to go, more is going to rush through, like poking a hole in a balloon.

So I think the reason oxygen leaks out of tires six times as fast as nitrogen is that it will end up boring small, molecule-sized tubes through your tire. You won't be able to SEE the tubes (they're molecule sized!) but the atoms can find them with no problem, and slowly make them bigger as time goes on.

That's my theory, anyway. We'd need an electron microscope and some old tires to prove it, though!

Answer 2

Well I only heard about filling tyres with nitrogen a few hours ago, but the arguments in favour of it make sense. You'd think that because there is one fewer electron in the outer shell of a nitrogen atom than oxygen, it would have a smaller atom. Not so. As you move from left to right across a row of the periodic table, the atoms get steadily smaller, because you're increasing the +ve charge on the nucleus, so the electrons are held in more tightly. When you get to the group 1 metal at the beginning of the next row, the atomic diameter jumps because you've just added an electron to a filled outer shell and it's shielded from the attraction of the nucleus. The rate at which a gas diffuses through a membrane, such as a tyre tube, is inversely proportional to the square root of its atomic weight; this is because in a mixture of two gases, on average each gas molecule has the same kinetic energy, so the lighter molecules are moving faster. and also to its atomic number, because bigger molecules find it harder to squeeze between the atoms in a membrane. Dry nitrogen or, for that matter dry air doesn't contain water vapour which could condense out if the temperature drops low enough, so it makes sense that the tyre pressure doesn't change much. Nitrogen is more inert than oxygen, so I'll buy the bit about the inside of the tube not being oxidised too. Having said all this, I wouldn't have been able to work it out myself.

Answer 3

It sounds like a wive's tale to me.

Since air is 99% oxygen and nitrogen, and the two are so similar, I can't see any measurable or meaningful difference, as you already suspect.

Perhaps oxygen is more reactive with tire rubber, but now you are talking about rubber deterioration, not tire inflation.

Like you, I was taught PV/T = PV/T, regardless of the gas.

Answer 4

N2 packs better than air because of the similarity of the molecules whereas air contain many types of molecules. Therefore pressure remains const

Answer 5

Hey, you aren't supposed to answer your own question. I wanted to give you the molecules answer since I just read it this week.

Answer 6

ah you saw it on the news to?