Why do cornflour explosions occur?

[Moogie]

Hi

What exactly is an explosion? How is it different from something burning?

When you get an explosion with cornflour as described here why is the lid blown off? Is it due to rapid production of carbon dioxide? I presume cornflour is just starch so the reaction is essentially combustion of glucose.

The cornflour ‘bomb’
Cornflour is sprayed into the flame of a candle burning inside a large tin can with the lid on. The resulting small explosion caused by rapid combustion of the cornflour blows the lid off the tin. The reaction dramatically illustrates the conversion of the chemical energy stored in foodsuffs into heat and other forms of energy. It can also be used to show the effect of surface area on the rate of chemical reaction.

thanks

 

[mgb_phys]

In the wonderful world of blowing things up burning, deflagration and explosion (ie detonation) are different.

Burning is subsonic - the heat from one burning bit of material travels to the next one which then starts to burn.
Explosions are when the energy is carried by a shock wave and is usually supersonic (exact defintions vary depending on the industry).

The burning corn flour is a rapid burning (as is gunpowder!) - although the damage it does to a building is still pretty impressive.

 

[Borek]

The burning corn flour is a rapid burning (as is gunpowder!)

Yes and no - gunpowder oxidizes itself, flour needs air oxygen to burn. So while both cases are related, they are also very distinct.

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methods

 

[Moogie]

What do you mean gunpowder oxidises itself?

 

[Moogie]

Also, does the lid come off because of the CO2?

 

[Borek]

What do you mean gunpowder oxidises itself?

What are gunpowder ingredients?

 

[Borek]

Also, does the lid come off because of the CO2?

Unlikely - when starch burns number of moles of CO₂ produced can't be larger than number of moles of O₂ consumed.

What do you know about gases behavior?

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[Moogie]

Gunpowder = sulfur, charcoal, and potassium nitrate.

I don't know why that means it oxidises itself.

I'm not sure what I know about gas behaviour. I'm returning to science after a long absence. I know general gas laws and kinetic theory of gases. Nothing more than that.
I presume you are getting at pv=nRT . I don't know the number of moles of gas involved but like you said it won't be an increase so the increased pressure cos of increased temperature blows the lid off?

 

[alxm]

Gunpowder contains its own oxidant. (as a separate component, whereas high explosives such as TNT are their own oxidants) How else would it burn in a closed tube such as a gun?

In any case, the basic difference between oxidation, combustion and detonation is the rate at which it occurs.

 

[Moogie]

I didn't know a gun was a closed tube. I've never really given guns much thought :)

Sorry but i still don't understand what this means:
Gunpowder contains its own oxidant. (as a separate component, whereas high explosives such as TNT are their own oxidants)

What is the oxidant in gunpowder? What do you mean TNT is its own oxidants? Perhaps i need to take a step back and ask what precisely is an oxidant in this context. There are lots of definitions of oxidants and reductants but i always think of an oxidant as something that removes electrons from another substance. I presume oxidant means source of oxygen in this context?

 

[Moogie]

according to this:
http://cnx.org/content/m33088/latest/#id24247221

TNT has a negative oxygen balance as it doesn't contain enough O2 for complete formation of h20 and co2. However that's probably being pedantic and I only care about the underlying concepts.

I hope what you have been saying is that some compounds contain enough oxygen (or other oxidant) for their own combustion. Never heard of this before but its not exactly left-field.

It would seem saltpeter is the oxidant in gunpowder. Why makes this a good oxidant?

 

[cesiumfrog]

For what it's worth, "flour suspended in air" also contains its own oxidant.

 

[alxm]

TNT has a negative oxygen balance as it doesn't contain enough O2 for complete formation of h20 and co2. However that's probably being pedantic and I only care about the underlying concepts.

(H-twenty? hope that's a typo!

Well it's right about the balance, but the linked text is dead wrong in saying:

"Thus, despite its reputation as an explosive, TNT is only efficient in the presence of an external oxidant, which may be air or another compound that provides a positive oxygen balance. "

TNT works just fine without an external oxidizer. It just doesn't reach complete combustion, forming some free carbon and carbon monoxide. "Efficiency" is not your main concern with an explosive. It means more energy liberated in total, but what you're interested in with an explosive is the power, i.e. energy per unit of time, which means a high reaction rate. (Corollary: Gas-guzzling 'muscle-cars' versus low-horsepower vehicles with good gas mileage)

I hope what you have been saying is that some compounds contain enough oxygen (or other oxidant) for their own combustion. Never heard of this before but its not exactly left-field.

High explosives are compounds which contain oxygen or oxidizing groups chemically bonded, where the chemical bonds to the oxidizing groups are broken so that oxidation can occur if they're subjected to heat or mechanical shock. This has two benefits: 1) The detonation occurs more violently and at greater speed, because the oxidizer and fuel are 'mixed' at the molecular level. 2) Greater stability, since it takes more energy to break these bonds. Which is good for safety. And also why you need an explosion in the form of a blasting cap to set them off.

It would seem saltpeter is the oxidant in gunpowder. Why makes this a good oxidant?

It's a good oxygen donor and when doing so, acts as an oxidant. It can be reduced to nitrite, nitric oxide, and nitrogen.

Cesiumfrog: Well, okay.. But you can't carry a suspension of dust around in a convenient cartridge :)

 

[Borek]

But you can't carry a suspension of dust around in a convenient cartridge :)

But you can generate it in-situ...

https://www.youtube.com/watch?v=GmRASCHJe2Q

 

[Moogie]

Being a girl I never thought I would find explosions so interesting.

What does he mean by flour suspended in air contains its own oxidant? Does he just mean the surrounding air.

Returning to my original question, despite the very interesting detour, why are dust bombs so dangerous/easy to make? It is because the high surface area of the dust enables the reaction to proceed very quickly?

 

[Borek]

What does he mean by flour suspended in air contains its own oxidant? Does he just mean the surrounding air.

Yes. Think in terms of the reaction - you need both something that burns and oxidant (be it air oxygen or something willing to give away oxygen).

Returning to my original question, despite the very interesting detour, why are dust bombs so dangerous/easy to make? It is because the high surface area of the dust enables the reaction to proceed very quickly?

That's an important part of the story. Other is - if you don't need oxidant to be part of the bomb, you can squeeze more energy into the same mass of the charge. Finally there are important differences between explosions of point charges (point meaning relatively small volume, like a cubic feet) and explosion of charges of a large volume (like 10k cubic feet). Not that I know what exactly these differences are, I just know they do exist.

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methods

 

[Moogie]

thank you - very interesting

 

[cesiumfrog]

But you can generate it in-situ... [video]

Do you know how the ignition works in that video? Wikipedia seems to suggest that the dust is superheated beforehand, and spontaneously ignites only when sufficiently diffuse, but I don't see what prevents parts of the cloud igniting prematurely?

 

[Borek]

As far as I can tell this is not dust but liquid fuel. I was just referring to the fact that you don't have to deliver the mixture to the target, you can prepare it just before the explosion. And no, no idea about technical details. Most likely by superheating they mean liquid is heated to the point where it is not boiling because it is kept in a slightly pressurized container, but its temperature can be still well below ignition point.

 

[cesiumfrog]

Sorry, I probably shouldn't have used the word "superheated" (nor "dust"), but still, the impression I get is that the fuel is indeed above that ignition point (as it spreads out, before it ignites). WP:

A thermobaric bomb's effective yield requires the most appropriate combination of a number of factors; among these are how well the fuel is dispersed, how rapidly it mixes with the surrounding atmosphere and the initiation of the igniter and its position relative to the container of fuel. In some cases separate charges are used to disperse and ignite the fuel.[citation needed] In other designs stronger cases allow the fuel to be contained long enough for the fuel to heat to well above its auto-ignition temperature, so that, even its cooling during expansion from the container, results in rapid ignition once the mixture is within conventional flammability limits.

 

[Borek]

OK, I see what you mean.

No idea - the only answer I can think off is that the explosion can be limited then by the flammability limits - more precisely by the upper explosive limit.

 

[cesiumfrog]

Ok, so for concentrations higher than the upper flammability limit, the oxygen is so diffuse that the energy liberated (if or when oxygen molecules of one layer of the mixture are initially caused to react with the fuel) dissipates so much (in the time it takes this heat to reach the oxygen molecules in the next layer of the mixture) that it is insufficient to maintain a chain reaction (and so the portion of oxygen present in the mixture basically stays unreacted). Do I understand this?

If so, than naively it seems to me that the flammability limits would move apart as temperature increases (this is in fact correct, right?). And for temperatures above the auto-ignition temperature, even if the mixture is above the upper flammability limit, I would expect all of the oxygen present to spontaneously react? Does that in fact happen? But if so, why wouldn't this deplete the mixture of the oxygen it is gradually accumulating, and spoil the suddenness of the explosion?