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Burning paper in ethanol

by Moogie
Tags: burning, ethanol, paper
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alxm
#19
Sep4-10, 11:10 AM
Sci Advisor
P: 1,866
Quote Quote by lightarrow View Post
Did you read what you have written?
<<it will vaporize and burn before ... vaporizes and burns>>
Obviously I meant to write "it will vaporize and burn before the liquid itself burns".

A liquid can certainly react with other chemicals even at room temperature, but a liquid cannot stay in the liquid state, above its boiling point
Yes it can. A liquid won't stay a liquid above its boiling point if it's in thermodynamic equilibrium. A fire/flame is not a system in thermodynamic equilibrium. It's pretty damn far from that.
lightarrow
#20
Sep5-10, 06:32 AM
P: 1,521
Quote Quote by alxm View Post
Yes it can. A liquid won't stay a liquid above its boiling point if it's in thermodynamic equilibrium. A fire/flame is not a system in thermodynamic equilibrium. It's pretty damn far from that.
Do you mean that you have there *liquid* ethanol at temperatures > 363°C?
Moogie
#21
Sep5-10, 06:42 AM
P: 168
I know you two are debating a different point about whether liquids can burn but could someone address my point as it is my thread :)

My point was about the autoignition temperature of ethanol and paper. Specifically my source says the ignition point of paper is below that of ethanol. If this is correct then the paper would burn before the ethanol but in the picture I have the paper is not burning and the ethanol is burning on the paper.
lightarrow
#22
Sep5-10, 07:57 AM
P: 1,521
Quote Quote by Moogie View Post
I know you two are debating a different point about whether liquids can burn but could someone address my point as it is my thread :)

My point was about the autoignition temperature of ethanol and paper. Specifically my source says the ignition point of paper is below that of ethanol. If this is correct then the paper would burn before the ethanol but in the picture I have the paper is not burning and the ethanol is burning on the paper.
The paper cannot burn until the ethanol has completely gone away and leaved the "naked" paper which it was covering. Paradoxically as it seems, the liquid here protect the paper from burning.
alxm
#23
Sep5-10, 08:23 AM
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Quote Quote by lightarrow View Post
Do you mean that you have there *liquid* ethanol at temperatures > 363°C?
I'm saying that 'temperature' does not have a well-defined meaning for a system not in equilibrium. Heat transfer occurs (mostly) via intermolecular collisions, which is the same process chemical reactions occur by. There's no distinguishing the two. You can't say "first it gets hot, then it evaporates, then it reacts". Vaporization is itself a reaction. These reactions will occur at the same time if it's hot enough, which is what I've been saying from the start.

Quote Quote by lightarrow View Post
The paper cannot burn until the ethanol has completely gone away and leaved the "naked" paper which it was covering. Paradoxically as it seems, the liquid here protect the paper from burning.
You're making stuff up. Seriously, have you studied chemical thermodynamics and kinetics at all? Beyond the introductory, equilibrium-thermodynamics level?

Again, explain at the molecular level what about the ethanol is stopping energetic oxygen molecules from reacting with the paper? You can't. And the reason you can't is because you're trying to apply simplified macroscopic, equilibrium thermodynamics to explain a non-equilibrium situation.
alxm
#24
Sep5-10, 08:27 AM
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Quote Quote by Moogie View Post
My point was about the autoignition temperature of ethanol and paper. Specifically my source says the ignition point of paper is below that of ethanol. If this is correct then the paper would burn before the ethanol but in the picture I have the paper is not burning and the ethanol is burning on the paper.
Again, if the temperature is high enough, the paper drenched in ethanol will burn before the ethanol evaporates. Stick it in an acetylene flame and see what happens.

An ethanol-air mixture burns at a lower temperature than liquid ethanol, and a lower temperature than paper. If the temperature is low, then the ethanol will evaporate and burn in the air before the paper and liquid ethanol does, because ethanol has a low boiling point.
lightarrow
#25
Sep5-10, 10:35 AM
P: 1,521
Quote Quote by alxm View Post
I'm saying that 'temperature' does not have a well-defined meaning for a system not in equilibrium.
This is true for every real system, but chemistry still exist.
Heat transfer occurs (mostly) via intermolecular collisions, which is the same process chemical reactions occur by. There's no distinguishing the two. You can't say "first it gets hot, then it evaporates, then it reacts". Vaporization is itself a reaction. These reactions will occur at the same time if it's hot enough, which is what I've been saying from the start.
And I believe you are wrong: first ethanol vaporizes, then paper burns. Temperature of the liquid ethanol is not well defined? Well, it's not well defined globally, but it is for small portions of the liquid, and those portions wich will get hotter than 78.4°C will boil. Yes, you can have small portions of superheated substance, but not up to 363°C.
You're making stuff up. Seriously, have you studied chemical thermodynamics and kinetics at all? Beyond the introductory, equilibrium-thermodynamics level?
Instead, I wanted to ask you if you took your master by telephone
Again, explain at the molecular level what about the ethanol is stopping energetic oxygen molecules from reacting with the paper? You can't. And the reason you can't is because you're trying to apply simplified macroscopic, equilibrium thermodynamics to explain a non-equilibrium situation.
Have a read to these, regarding liquid wax instead of ethanol and candle wick instead of paper:
http://www.helium.com/items/1165845-how-a-candle-burns
http://www.helium.com/items/1118747-...okewaxparaffin
http://www.helium.com/items/1163220-how-a-candle-burns
http://www.helium.com/items/1190340-how-a-candle-burns
http://www.helium.com/items/1120994-how-a-candle-burns
alxm
#26
Sep5-10, 12:23 PM
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P: 1,866
Quote Quote by lightarrow View Post
This is true for every real system, but chemistry still exist.
What's that supposed to mean?
And I believe you are wrong: first ethanol vaporizes, then paper burns.
Yes you keep saying that, but you haven't actually explained why they can't both happen concurrently.
Temperature of the liquid ethanol is not well defined? Well, it's not well defined globally, but it is for small portions of the liquid, and those portions wich will get hotter than 78.4°C will boil.
Only if they're at the boundary layer, where they have the ability to form gas. Otherwise they'll explode. Those are called 'microexplosions'. But it doesn't matter what the temperature is inside the paper/droplet, whatever. Combustion occurs exclusively at the boundary layer of the liquid, and is dependent on the temperature, the volatility and the gas flow.
Instead, I wanted to ask you if you took your master by telephone
Says the guy who thinks boiling points and ignition temperatures are related...
Have a read to these, regarding liquid wax instead of ethanol and candle wick instead of paper
A candle is the classical example of laminar flow combustion; the liquid does not burn, not because it can't, but because the oxygen can't reach it.
lightarrow
#27
Sep5-10, 01:21 PM
P: 1,521
Quote Quote by alxm View Post
What's that supposed to mean?
That people can make precise enough computations on those systems without the need of a non-aequilibrium treatize.
Yes you keep saying that, but you haven't actually explained why they can't both happen concurrently.
And where did I say they can't? Of course you will have some oxygen diffusing through the liquid, of course you will have some liquid ethanol reacting with oxygen and thousands of other processes, but I simply say that *in this case* that it's not the reason of the ethanol combustion, the reason is the ethanol in the gas state reacting with air oxygen. If you prevent ethanol vaporization in some way, you won't have combustion at all. In this link:
http://www.cdc.gov/niosh/ipcsneng/neng0044.html
it's written that ethanol's flash point is 13°C. It means that if you have ethanol at temperatures lower than that, you cannot ignite it *even with a flame*. Why? Please, answer this question first.
Only if they're at the boundary layer, where they have the ability to form gas. Otherwise they'll explode. Those are called 'microexplosions'. But it doesn't matter what the temperature is inside the paper/droplet, whatever. Combustion occurs exclusively at the boundary layer of the liquid, and is dependent on the temperature, the volatility and the gas flow.
And have you asked yourself why is dependent on the volatility?
danman5000
#28
Sep22-10, 02:04 PM
P: 5
I think people are making this out to be way more complicated than it is.

The ethanol burns first partially because it can vaporize and better mix with air, and partially because it coats the paper. Ethanol rapidly evaporates, and it's much easier to ignite as a gas than as a liquid as it's better mixed with oxygen in the air. It's also all about surface area - the combustion reaction only takes place at the surface of anything. Therefore, if your paper is covered in ethanol, the ethanol must first burn away before the paper can start reacting with the air and also burn.

This also has nothing to do with autoignition, I'm not sure where that's coming from. Autoignition temperature is the temp. at which the material will ignite spontaneously, without any ignition source. Since we are talking about dipping paper in ethanol and intentionally lighting it on fire (not gradually heating it to high temperature), autoignition doesn't come into play here.
lightarrow
#29
Sep23-10, 01:34 PM
P: 1,521
Quote Quote by danman5000 View Post
I think people are making this out to be way more complicated than it is.

The ethanol burns first partially because it can vaporize and better mix with air, and partially because it coats the paper. Ethanol rapidly evaporates, and it's much easier to ignite as a gas than as a liquid as it's better mixed with oxygen in the air. It's also all about surface area - the combustion reaction only takes place at the surface of anything. Therefore, if your paper is covered in ethanol, the ethanol must first burn away before the paper can start reacting with the air and also burn.

This also has nothing to do with autoignition, I'm not sure where that's coming from. Autoignition temperature is the temp. at which the material will ignite spontaneously, without any ignition source. Since we are talking about dipping paper in ethanol and intentionally lighting it on fire (not gradually heating it to high temperature), autoignition doesn't come into play here.
Which is what I tried to explain too...
John_5696
#30
Oct4-10, 11:57 PM
P: 10
I think it is simple, because ethanol evaporates, it insulates paper from oxygen, or it burns and deplete oxygen around paper. Paper burns only with oxygen react with it, remember heat is not the only reason that paper burns.


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