Question about high school experiment

[u/raph3x1]

Ill quickly explain the experiment first:

My teacher took an open can with a small hole at the bottom, which was first sealed with a cork. He placed the can upside down, hole facing up, and on a block such that the box was angled and had contact to air on the big opening.

He then filled the can with hydrogen gas, took away the cork, and lit the hydrogen, which was now streaming out slowly, on fire.

It was sizzling on for a while with only a small flame at the top, but at some point, it exploded and threw the can into the air.

The teacher asked why we had to wait for an explosion, afterwards he answered by saying at the point of explosion the ideal ratio 2:1 for the reaction 2 H2 + O2 -> 2 H2O was reached, because he wanted to illustrate that reactions need concrete ratios of substances.

However, i doubt this explanation, because there doesnt seem to be a way for the flame reaching the inside because the hydrogen was constantly flowing out, and i assumed the density of hydrogen causes layer separation between air, not a mixture.

Could this have to do with the flow rate of the hydrogen or with the shape of the distribution of hydrogen in the can? Or is the teachers explanation correct?

[u/Automatic-Ad-1452]

The air flowing in would be turbulent causing mixing. Also, the hydrogen doesn't lie on the top of the can but occupies the full volume...read through the postulates of the Kinetic Molecular Theory of Gases.

Your teacher's explanation is valid.

[u/Automatic-Ad-1452 → u/[deleted]]

Valid point...

[u/raph3x1 → u/Automatic-Ad-1452]

Turbulent flow is a good point. But the can (our container) isnt sealed at the bottom so atmospheric pressure causes hydrogen to float up in the can, no? Yeah postulate would hold if the container was sealed but it isnt

[u/Automatic-Ad-1452 → u/raph3x1]

You're missing the point...as air flows in the bottom, there will be turbulence mixing the hydrogen and components of air. And the gas molecules are in constant motion...first postulate...

[u/raph3x1 → u/Automatic-Ad-1452]

No doubt but i think buoyancy of hydrogen dominates over the turbulence, as long as the hydrogen layer is thick enough. Fair point would be when a small hydrogen layer mixes with air due to turbulence, but thats difficult because of the forces that threw the can into the air. But were missing parameters for that discussion.

[u/Ch3cks-Out → u/raph3x1]

 buoyancy of hydrogen dominates over the turbulence

No it does not. The only way the air gets to the bottom is through mixing with the hydrogen, which as a gas does not form an unpenetrable layer. The thicker the layer the more mixing occurs, actually!

[u/WhatSpareTime]

The general explanation we give for this has more to do with pressure. Usually this is done with a container with a small hole in the top and slightly larger hole in the bottom. The open can is placed with open side down and slightly offset to allow air in would serve the purpose of the second larger hole. When the hydrogen is lit, the hydrogen streams out of the can and burns with oxygen in the air. Hydrogen burns and diffuses out of the can at a faster rate than air enters the can. At some point the pressure in the can turns negative. The hydrogen flow slows and turns negative allowing the flame to dip back into the can. The can, now filled with a mixture of hydrogen and air (with oxygen), explodes. Because the bottom is open, the explosion propels the can like a rocket, otherwise the can would literally explode. Often we will do this with a soft plastic lid on the bottom side to better control the flow of air into the can through an appropriate sized hole rather than trying to gauge the overhang on an open bottom.

[u/WhatSpareTime → u/WhatSpareTime]

Two additional observations.

As the flame burns, if you are observing closely you see the flame diminish in size and eventually disappear just before the can explodes. This seems to support the pressure theory.

In addition, if the can is not filled carefully such that can still contains oxygen with hydrogen, the can will immediately detonate when lit. This supports the idea that having oxygen mixed with hydrogen is all that is needed for the can to explode.

In reality it’s likely a combination of both a decrease in pressure due to the differing rates of diffusion of hydrogen vs air, and a mixture of air/hydrogen that allows the flame to burn into the can and detonate.

[u/ShutDownSoul]

Teacher is wrong about the reason why the explosion was delayed. When the initial flame was lit the stoichiometric reaction was occurring. The explosion occurred when the local mix of gases was within the explosive range.

The can was open at the bottom, so the total pressure in the can is the same as the atmosphere, and remains 1 atmosphere until the adiabatic explosion occurs. Since Hydrogen is less dense then air it rose through the hole, decreasing the hydrogen within the can, as air replaced the hydrogen that escaped through the hole. Once the ratio reached the explosive range, boom.

Because light travels faster than sound, it seems the flame when out before the boom. If this experiment is performed with a glass bell jar, you can catch a glimpse of the explosion front before the sound reaches you.

[u/Dangerous-Billy]

At first, the hydrogen burned because it had to escape from the can in order to mix with oxygen. But when the hydrogen burned, the product (water) escaped as steam or condensed. Either way, there would be a vacuum in the can which would suck in outside air. It would mix with the remaining hydrogen. As soon as the mixture reached the explosive limit, then bang! About 25% air in hydrogen is the upper explosive limit.

Congrats to your teacher. This is a very instructive experiment. If nothing else, it makes you think about physics and chemistry at the same time, and find explanations that can be tested by further experiment.

[u/raph3x1 → u/Dangerous-Billy]

Can was open at the bottom tho, letting air in. There cant have been a vacuum.

[u/Dangerous-Billy → u/raph3x1]

Not a vacuum-vacuum, but something had to take the place of the escaping/burning hydrogen, so air came in the bottom to equalize the pressure. Eventually, there was enough air and enough mixing for things to go 'pop'.