Metallus
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Spark and Flame temperature
This is one of those doubts that always puzzled me and that no one ever managed to exhaustively explain to me.
I studied the definitions of flash point, fire point, auto-ignition point and understood them. I also understood that they all involve a
trigger/starter like a spark or flame.
But what is the temperature and energy of a spark and a flame?
I've been taught that a spark is like a little energy container at a very high temperature. A spark itself doesn't contain enough energy to even heat
up 1ml of water to 80°C (And that is also why sparks coming from sharpening metals or from lighter's piezoelectric material do not cause burns: high
temp but not enough energy to cause wide burns) but the "localized" temperature is enough to start a "localized" reaction (which requires that high
temperature) and that can spread afterward and sustain itself.
I think I've got that. But what about a flame? What exactly is a flame? How should I consider it? I've been told that the "temperature" of a lighter's
flame using propane/buthane is around 300-400°C, which is also the "temperature" of the flame used at home to heat up your pot of water or cook your
steak. But are we talking about "temperature" or about the energy the flame can give out to heat things up? Because if we are talking about
temperature, then I'm lost and I can't understand how the hell a flame burning at "only" 300°C can start a reaction that requires at least 500-600°C
IMMEDIATELY.
Or perhaps, a flame actually burns at high temperatures like a spark (maybe not 2k but around 1k °C) but the energy it gives out is enough to only
heat things up to a maximum "sustainable" temperature of 300-400°C (because of heat dissipation with the environment/air), BUT STILL start a reaction
that requires 600°C (which will sustain itself afterwards, so no need for additional heat).
Also, I've been taught that a reaction that involves an activaction energy only occurs when you give an amount of energy superior to Ea. But if I need
300Kcal to start the reaction, what's the difference between giving 300Kcal with a spark for 1s and giving 300Kcal with another heating source at a
lower temperature during a certain period of time?
Theoretically, the only difference would be that with the spark, the reaction is immediate while with the other heating source, the reaction takes
place after, say, 5 mins. Then, always theoretically, I could ignite my thermite mix with a burst of energy from a blowtorch within 3s, or heating it
up with a lighter for 10h (random numbers). Would it actually take place or even heating it up for so long wouldn't afford any result? From what I
studied, only the kinetics would change and the reaction would happen anyways, even if not violently as I'd wish it to happen. Iron oxide would turn
to iron and aluminium would be oxidised to its respective oxide. Does this happen? I know it should but I want to confirm. These things are often
given for granted but sometimes leave you wondering.
I am really embarrassed at this point to even ask this question. This is so basic yet I fail to fully comprehend flames and sparks.
Thanks for your attention, hoping you can solve this doubt of mine and confirm/destroy what I think to know.
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blogfast25
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You're not taking into account certain things.
Lets take the example of lighting a gas flame with a spark. The spark very locally heats the air/gas mixture to the point where the combustion
reaction can take place, locally. But the combustion reaction is highly exothermic and the heat generated by the localised reaction is enough to heat
the adjacent gas/air mixture to ignition. In essence this process continues until a stable flame is formed.
Re. the relation between temperature and 'energy' of a flame, two things need to be considered. Firstly, the hotter the flame, the more heat will tend
to flow from the hot flame to the cooler pan (or other object). Secondly, the size of the flame dictates how much power (as in Watts) can flow from
the flame to the pan, hence large flames for large pans and smaller ones for smaller ones, bearing in mind of course that a considerable amount of
heat will be lost to the environment.
Incidentally, the temperature of my NG cooker flames are about 700 C.
In your thermite example it all depends on how hot the mixture needs to be to auto-ignite: if that temperature exceeds the temperature of the lighter
flame, auto-ignition temperature could never be achieved because the lighter flame can, at the very best, only heat something to its own temperature,
never above that. Using a spark could work but only if sufficiently large to locally heat the mixture to auto-ignition.
A 'spark' is a very short-lived, usually very hot, transient phenomenon. A 'flame' tends to imply a steady state, stable combustion reaction with more
or less constant temperature and stable geometry.
[Edited on 12-3-2014 by blogfast25]
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Metacelsus
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Activation energy is measured in energy per mole. This is inherently a microscale property. It means that a collision between two molecules
must have more energy than a certain amount for the molecules to react. You can't directly supply activation energy; you can only heat the reaction
mixture so that more collisions have higher energy.
With the thermite case, you have to take into account the fact that heat is being removed from your system as well by conduction and radiation. Thus,
the blowtorch will provide a higher maximum temperature than the lighter.
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blogfast25
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CC:
Well said. Activation energy is one of the most misunderstood concepts of chemistry, in particular by students, lay men and the uninitiated.
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Metallus
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Ok with everything you have said, but then should I conclude that the "flames" we daily use actually burn at temperatures of around 1000°C but, due
to heat exchange with the environment, the maximum temperature achievable by a certain object heated via these flames won't be of 1000°C or 900°C
but as low as 300-400°C, am I right?
Also, about activation energy, I don't think I have it clear enough: let's assume a reaction which requires 1000cal to be activated and that doesn't
produce enough heat (afterwards) to sustain itself (no combustion). In order for 10 molecules to react with another 10 molecules, I should give my
system 1000cal: is it the same whether I give these 1000cal as 10cal every sec, for 100s or 1000cal all at once (in 1s)? Beside the rate of the
reaction, nothing should change, right? In the first case the reaction will take more time, just that. For example, in the first case 1 molecule will
react with another, step by step (1 molecule will react with another one every 10s, GRADUALLY) while in the second case all 10 molecules react at the
same time. Is this right? Because if it isn't like this, then I don't understand when my professors say that hydrogen and oxygen spontaneously react
with each other BUT, at room temperature, even though the reaction is thermodinamically favoured, the reaction will not occur, or better, it will
occur but at such little rate and in such little scale, that it's like it wasn't taking place. Still, those few molecules reacting with each other,
where did they take the energy? Was it from the environment? If it's like this, then can I look at it like reactions with an high Ea (let's say
1000cal) actually occur but go to completition only after they have accumulated the required energy from the environment after a long period (let's
say, 1cal every day for 1000days, and with a conversion of 1 molecule every sec) ? Can I see it like this or is it wrong?
I would like to know which one of my statement is wrong, which is right, which is the temp of the common flames and why said flame often acts as a
spark (as it had very HIGH temp, around 1k °C), igniting most things, when people say that the temperature is only 300-400°C (and therefore not
enough to start common reactions).
Thanks again for your attention
[Edited on 13-3-2014 by Metallus]
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Metacelsus
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| Quote: | | Can I see it like this or is it wrong? |
It is (mostly) wrong. Remember that activation energy is supplied by the random motion of the molecules due to temperature, and is not supplied
directly.
On activation energy and reaction rates:
https://en.wikipedia.org/wiki/Collision_theory
For flame temperature:
https://en.wikipedia.org/wiki/Adiabatic_flame_temperature
This is
| Quote: | | theoretical, not actual, flame temperatures produced by a flame that loses no heat. |
If you need help with the math, I can explain in more detail.
[Edited on 13-3-2014 by Cheddite Cheese]
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Metallus
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I already studied collision theory but perhaps I didn't understand it the "right way".
1) So it's just the probability that I increase by rising the temperature; it's not systematic that increasing temperature eventually and SURELY leads
to the reaction. If it's like this, then can I conclude that even if I heat up a reaction that would need 2000°C, it might happen that no reaction
occurs at all because I was very very unlucky (10^6 times unlucky  ), just like
it could happen that a reaction occurs even if I don't give it any energy because I was very very lucky at that particular moment (talking about
microscale here, not large scale reactions)? Just statistics, right? Answering yes to this question will solve a good chunk of my doubts.
Also, about flame temperatures, in that page I read that propane/butane flame temp is between 1970 and 2392°C, which would theoretically be enough to
start a thermite reaction. Yet, using a lighter to light a thermite mix will lead to nothing.
2) You have quoted the fact that those are indeed "theoretical temperatures of a flame that looses no heat" and we already know that most heat is
dissipated with the environment. STILL, you should be able to take advantage of that millisecond that your flame is at nearly 2k °C
(maybe it won't be 1970°C, but I guess the hottest part of the flame could touch 1.8k °C or 1.9k °C... or am I guessing wrong and it doesn't go
beyond, like, 800°C?) to start such reaction, just like a spark can start many reactions (temporary and localized high temperature). Then why do you
need the flame of a blowtorch? Do you mean an oxy-acethylene blowtorch whose theoretical flame temperature is 3480°C, or do you mean a propane
blowtorch whose theoretical flame temperature would "just" be 130°C above a lighter's one?
3) What I also fail to understand is why, for example, 1 ml of ethanol put in a container with a temperature of 360°C, in the absence of a
flame/spark will not ignite while a flame burning presumably below 350°C will set it on fire at room temperature. Ethanol autoignition point is
363°C and things would make sense only if the flame actually burned at a temperature superior to 363°C, enough to start the reaction (AT LEAST
LOCALLY).
I need to confirm what I "think" I have understood. I'm an university student and for long I've carried with me these doubts. There are many things
that I think to have understood the right way and everytime my professors confirmed I had understood it right. But when getting back home and
analysing what I had said, I realized that some things weren't always clear and that perhaps it was just a coincidence.
I like chemistry even outside the university and while 99% of my colleagues don't give a flying shit about fully understanding it, I would like to
know the most I can when it comes to my favorite field, which also seems to be the one I haven't fully and clearly comprehended. These are severe
holes in my knowledge for me and I NEED to correct them.
Thanks again for your help and, if you can, I would be pleased if you could also tell me what is "right". So far I've only understood what is "wrong".
Thanks again for your availability, it's much appreciated.
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blogfast25
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Quote: Originally posted by Metallus  | [
1) So it's just the probability that I increase by rising the temperature; it's not systematic that increasing temperature eventually and SURELY leads
to the reaction. If it's like this, then can I conclude that even if I heat up a reaction that would need 2000°C, it might happen that no reaction
occurs at all because I was very very unlucky (10^6 times unlucky ), just like
it could happen that a reaction occurs even if I don't give it any energy because I was very very lucky at that particular moment (talking about
microscale here, not large scale reactions)? Just statistics, right? Answering yes to this question will solve a good chunk of my doubts.
Also, about flame temperatures, in that page I read that propane/butane flame temp is between 1970 and 2392°C, which would theoretically be enough to
start a thermite reaction. Yet, using a lighter to light a thermite mix will lead to nothing.
|
I'm still not sure you understand the statistical part of it.
Molecules (in a gas, for instance) at a given temperature have a statistical distribution of kinetic energies, something like a bell ('Gauss') curve,
with many having kinetic energies around the mean.
Raise the bulk temperature of the gas and the distribution essentially moves to a higher mean.
For reactive collisions (as opposed to more or less elastic) to occur, the molecules need a minimum amount of kinetic energy.
It follows that at higher temperature a higher proportion of molecules have kinetic energy exceeding that threshold, thus more collisions will be
reactive (rather than elastic) and the reaction proceeds faster.
If almost no molecules have energy that surpasses the threshold then no appreciable reaction takes place.
As regards these propane/butane flame temperatures, they are a NONSENSE, sheer and utter. If that were true one could melt alumina with those flames.
In reality they are barely hot enough to bend glass!
They are so-called 'adiabatic' temperatures and have little bearing on an actual, real burner temperature.
[Edited on 13-3-2014 by blogfast25]
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blogfast25
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| Quote: Originally posted by Metallus |
3) What I also fail to understand is why, for example, 1 ml of ethanol put in a container with a temperature of 360°C, in the absence of a
flame/spark will not ignite while a flame burning presumably below 350°C will set it on fire at room temperature. Ethanol autoignition point is
363°C and things would make sense only if the flame actually burned at a temperature superior to 363°C, enough to start the reaction (AT LEAST
LOCALLY).
|
I don't know where you get that value from. But assuming, for argument's sake, that it is correct, then most flames will ignite ethanol in the
presence of oxygen because most flames exceed 363 C.
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Metallus
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by Metallus | [
1) So it's just the probability that I increase by rising the temperature; it's not systematic that increasing temperature eventually and SURELY leads
to the reaction. If it's like this, then can I conclude that even if I heat up a reaction that would need 2000°C, it might happen that no reaction
occurs at all because I was very very unlucky (10^6 times unlucky ), just like
it could happen that a reaction occurs even if I don't give it any energy because I was very very lucky at that particular moment (talking about
microscale here, not large scale reactions)? Just statistics, right? Answering yes to this question will solve a good chunk of my doubts.
Also, about flame temperatures, in that page I read that propane/butane flame temp is between 1970 and 2392°C, which would theoretically be enough to
start a thermite reaction. Yet, using a lighter to light a thermite mix will lead to nothing.
|
I'm still not sure you understand the statistical part of it.
Molecules (in a gas, for instance) at a given temperature have a statistical distribution of kinetic energies, something like a bell ('Gauss') curve,
with many having kinetic energies around the mean.
Raise the bulk temperature of the gas and the distribution essentially moves to a higher mean.
For reactive collisions (as opposed to more or less elastic) to occur, the molecules need a minimum amount of kinetic energy.
It follows that at higher temperature a higher proportion of molecules have kinetic energy exceeding that threshold, thus more collisions will be
reactive (rather than elastic) and the reaction proceeds faster.
If almost no molecules have energy that surpasses the threshold then no appreciable reaction takes place.
As regards these propane/butane flame temperatures, they are a NONSENSE, sheer and utter. If that were true one could melt alumina with those flames.
In reality they are barely hot enough to bend glass!
They are so-called 'adiabatic' temperatures and have little bearing on an actual, real burner temperature.
[Edited on 13-3-2014 by blogfast25] |
Yes, Maxwell-Boltzmann distribution, I studied it and now that I think about it, it does make sense. I just didn't do the mental work of linking the
two things together effectively. What I wanted to say is that there are particles which are "lucky" and will find themselves at an high temperature
and unlucky ones who will be at low temp. This would simply translate in the extremes of M-B curve (low population of molecules having high/low temp).
Anyways, I took the autoignitin point of ethanol from here http://en.wikipedia.org/wiki/Autoignition_temperature
Then, last question (to which I hope I'll get answered "yes"): does the temperature of the spark/flame need to be above the autoignition point of said
substance in order to start the reaction (let's assume it's a combustion that, once started, will sustain itself). Talking about liquids/gases mainly.
If I have an oil with an autoignition point of 600, will my 580°C hot flame be able to put it on fire or not (from what I understood so far, it
should NOT). In this case, if the temperature of a common flame isn't as low as 300-400°C (because otherwise methane wouldn't ignite when put into
contact with a lighter's flame) and it isn't as high as those tabulated obscene results that go beyond 2k °C, which is the actual real temperature
of a propane/butane flame?
[Edited on 14-3-2014 by Metallus]
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blogfast25
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Individual molecules don't have temperature, only kinetic energy (translational, rotational and vibrational). Collectively a large number of molecules
contain enthalpy (the sum total of these kinetic energies) and temperature measurement represents kind of an average of the individual molecules'
energies.
One has to be careful with definitions and 'auto-ignition temperature' is no different. In the absence of air or oxygen, no matter how combustible the
gas may be, there can be no auto-ignition. Similarly, a small amount of combustible in a sea of air will not ignite either.
But roughly in that area where combustion is possible, sources that are hotter than said 'auto-ignition temperature' should be able to ignite the
mixture.
[Edited on 14-3-2014 by blogfast25]
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Zyklon-A
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Does the auto-ignition temperature change depending on the concentration of oxygen in the atmosphere?
My old science book says that for every increase of 1% oxygen in the air, there is a ~75% higher chance of spontaneous ignition. (Spontaneous ignition
of what? This is a very vague statistic.) I'm not sure if this is because the auto-ignition temperature is lowered, or if other variables are at play.
(Or if this statistic is correct at all.)
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blogfast25
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Zb:
I would expect intuitively that, within the flammability limits, higher concentration of oxygen in the mix would lower ignition temperature a bit but
I wouldn't want to put my hand in the fire about that! 
[Edited on 14-3-2014 by blogfast25]
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Metallus
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| Quote: Originally posted by blogfast25 | Individual molecules don't have temperature, only kinetic energy (translational, rotational and vibrational). Collectively a large number of molecules
contain enthalpy (the sum total of these kinetic energies) and temperature measurement represents kind of an average of the individual molecules'
energies.
One has to be careful with definitions and 'auto-ignition temperature' is no different. In the absence of air or oxygen, no matter how combustible the
gas may be, there can be no auto-ignition. Similarly, a small amount of combustible in a sea of air will not ignite either.
But roughly in that area where combustion is possible, sources that are hotter than said 'auto-ignition temperature' should be able to ignite the
mixture.
[Edited on 14-3-2014 by blogfast25] |
Stupid me.
These were all concepts I once learnt and even understood but never had the occasion to actually apply to practice. Now, after some years passed, I
found myself dealing with the practical problem and ignored all the things I had learnt and relied on things that I was giving for granted.
Thanks for having refreshed these memories of mine. The thing is that something doesn't leave my head when I actually put it into practice. When I
started experimenting, I was giving for granted that everything that I was taught applied to real life to the letter. Mixing A + B will give C. Done
and nothing happened, or the reaction took place but I obtained things I wasn't supposed to obtain. By actually carrying out the reactions I learnt
that they don't always take place, learnt why they don't take place and I learnt how to make it happen.
Only by actually carrying out oxidations/reductions I understood redox potential. Only by actually using the different concentrations of acid/base I
understood why some metals needed a more diluite/concentrated solution and why a lump of metal didn't react at all while few stripes with a big
surface dissolved quickly. I ended up learning all the colors of metal complexes because I actually made them and still remember them vividly.
However, many theoretical things that I've studied and never put to use are just sitting there, waiting to be used and eventually ending up getting
forgotten.
But getting back to sparks and flames: theoretically speaking, wouldn't a simple sparkle be enough to start a thermite reaction? I mean, the sparks
are highly energetic and come in big amounts... and wouldn't those same sparks be enough to start many other reactions that only occur when activated
by an hot temperature? From what said until now, it should work, yet I never see anyone mentioning it or actually using it so I guess it doesn't work.
The question would be: why?
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HgDinis25
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Not sure if it helps, but "Tabelas para a determinação de minerais" by C.Romariz (can´t finde referecnes in english) describes bunsen burner flame
as having three distinct parts:
-Internal zone, pale blue, rich in non burned gas;
-Medium zone, pale violet, combustion of gases happens here, rich in CO, recution zone;
-External zone, redish, rich in CO2, oxidation zone;
It also states that the medium zone has the highest temperature, not stating what temperature exactly.
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blogfast25
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| Quote: Originally posted by Metallus |
But getting back to sparks and flames: theoretically speaking, wouldn't a simple sparkle be enough to start a thermite reaction? I mean, the sparks
are highly energetic and come in big amounts... and wouldn't those same sparks be enough to start many other reactions that only occur when activated
by an hot temperature? From what said until now, it should work, yet I never see anyone mentioning it or actually using it so I guess it doesn't work.
The question would be: why?
|
‘In theory’ there’s nothing that would stop a spark from igniting a thermite mixture, as long as certain conditions are met.
Firstly, assuming a well defined temperature needs to be achieved for the mixture to start reacting locally then obviously the spark would
need to be at higher temperature than this ignition temperature, otherwise heat transfer would stop at below ignition temperature and no ignition
could occur.
The spark would also have to be big enough and long lasting enough to transfer enough heat to that small amount of mixture so that it reaches ignition
temperature. Small sparks would take too long and as they’re short lived would probably fail to induce the temperature required.
A large enough, hot enough spark could certainly ignite a termite mixture. But it’s not necessarily a very practical way of doing things…
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Metallus
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by Metallus |
But getting back to sparks and flames: theoretically speaking, wouldn't a simple sparkle be enough to start a thermite reaction? I mean, the sparks
are highly energetic and come in big amounts... and wouldn't those same sparks be enough to start many other reactions that only occur when activated
by an hot temperature? From what said until now, it should work, yet I never see anyone mentioning it or actually using it so I guess it doesn't work.
The question would be: why?
|
So sparklers would do the job anyways? Why no one uses them then? They are cheap and readily available...
‘In theory’ there’s nothing that would stop a spark from igniting a thermite mixture, as long as certain conditions are met.
Firstly, assuming a well defined temperature needs to be achieved for the mixture to start reacting locally then obviously the spark would
need to be at higher temperature than this ignition temperature, otherwise heat transfer would stop at below ignition temperature and no ignition
could occur.
The spark would also have to be big enough and long lasting enough to transfer enough heat to that small amount of mixture so that it reaches ignition
temperature. Small sparks would take too long and as they’re short lived would probably fail to induce the temperature required.
A large enough, hot enough spark could certainly ignite a termite mixture. But it’s not necessarily a very practical way of doing things…
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blogfast25
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Sparklers are used quite frequently for lighting thermites. They are not really used for their sparks though: rather because they burn very hot and
don't need air...
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Metallus
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| Quote: Originally posted by blogfast25 | | Sparklers are used quite frequently for lighting thermites. They are not really used for their sparks though: rather because they burn very hot and
don't need air... |
In the previous post I fucked up with the quote
Anyways, I thought it didn't work because I often see people using other things as starters for their reactions, like magnesium strips that, honestly,
would be harder to acquire than sparklers, at least here where I live.
Thanks for your answers, I hope to have solved my doubts on the matter 
PS:
I still haven't understood which is the real temperature of a propane/butane flame (not 300 but neither 1k)
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blogfast25
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People don't often use sparklers because they have some drawbacks too. But I've seen them being used for lighting themites and it works.
Re. real propane/butane flames, Bunsen propane (or blowtorch) with air on full: about 600 - 700 C. Butane slightly lower. Butane cigarette lighter
flame: much lower because air and gas aren't premixed and air supply is sub-optimal: probably 400 - 500 C.
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