burning paper in ethanol

lightarrow

Did you read what you have written?
<<it will vaporize and burn before ... vaporizes and 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 (I'm assuming atmospheric pressure), so liquid ethanol cannot exist above 78.4°C. Do you really think ethanol could burn at such temperature? Ethanol autoignition temperature is 363°C:
http://www.cdc.gov/niosh/ipcsneng/neng0044.html

alxm

Obviously I meant to write "it will vaporize and burn before the liquid itself burns".

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

Do you mean that you have there *liquid* ethanol at temperatures > 363°C?

Moogie

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

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

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.

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

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

This is true for every real system, but chemistry still exist.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.Instead, I wanted to ask you if you took your master by telephone 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

What's that supposed to mean?
Yes you keep saying that, but you haven't actually explained why they can't both happen concurrently.
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.
Says the guy who thinks boiling points and ignition temperatures are related...
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

That people can make precise enough computations on those systems without the need of a non-aequilibrium treatize.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.And have you asked yourself why is dependent on the volatility?

danman5000

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

Which is what I tried to explain too...

John_5696

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.