# Endothermic Fire **URL:** \ **Category:** Factual Questions\ **Created:** [April 29, 2009, 9:56pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779 "2009-04-29T21:56:31Z")\ **Posts on this page:** 12\ **Page:** 3
**Author:** ![NightWing521](https://avatars.discourse-cdn.com/v4/letter/n/958977/32.png) [@NightWing521](https://boards.straightdope.com/u/NightWing521)\ **Post date:** [May 2, 2009, 4:03am UTC](https://boards.straightdope.com/t/endothermic-fire/494779/41 "2009-05-02T04:03:08Z")
> [@dracoi](#): > > There are several chemical reactions. Maybe the confusing thing is that they happen in equilibria that can only be maintained while you have a solvent. > > You start with solid NaBr and KCl. > > You add the NaBr to water and you get NaBr \<=\> (Na+) + (Br-) > > When you add the KCl, you get KCl + NaBr \<=\> (Na+) + (Br-) + (K+) + (Cl-) \<=\> KBr + NaCl > > (Technically, there are H2O, OH- and H+ involved, but I’m leaving the water out of the equation. It’s worth noting that adding some chemicals to water, like sulfuric acid, produces a noticeable amount of heat due to this reaction step, so it is a real reaction in its own right). > > When you remove the water, the ions in the middle of the equilibrium can no longer form spontaneously and the reactions have to settle on one end or the other. Because the equilibrium favors KBr + NaCl, that’s what you have the most of. The reaction is favored this way because of electronegativity differences; higher differences mean a stronger bond. > > So, the overall reaction is NaBr + KCl =\> KBr + NaCl, but there’s no point at which it happens quite so neatly. The trick is to NOT leave out the water and its ionic components. Ionic solution may be described most accurately by a **chemical** reaction like that below: H[sub]2[/sub]O + NaCl \<–\> H[sup]+[/sup]+OH[sup]-[/sup] + Na[sup]+[/sup]+Cl[sup]-[/sup] \<–\> H[sup]+[/sup]+Cl[sup]-[/sup] + Na[sup]+[/sup]+OH[sup]-[/sup] \<–\> HCl + NaOH All these reactions are going on simultaneously. None of them are separable until one of the original reactants is removed: i.e., by boiling off the water. ---
**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\ **Post date:** [May 2, 2009, 4:23am UTC](https://boards.straightdope.com/t/endothermic-fire/494779/42 "2009-05-02T04:23:10Z")
> [@Cardinal](#): > > Why is this different from saying that eventually with fire you pull enough energy from the fuel so that the process will not continue? It isn’t, but that’s not what I commented on. You made the claim that evaporation was self sustaining. I pointed out that it is not. > [@Cardinal](#): > > Evaporation is endothermic, but the entropy created makes the overall reaction self sustaining. There is no such thing as a self sustaining enothermic process. The energy to keep the process going must come from someplace else. > [@acetylene](#): > > Is dissociation a physical or a chemical process? Cecil, Exapno Mapcase, anyone? It makes no difference. I agree with **chaoticbear** that it is a bit of both. Either way, it obeys the same thermodynamic equations. > [@dracoi](#): > > One reason that volatiles tend to not burn people right away is that there’s a cooling effect on the surface due to evaporation. In an episode of Mythbusters, they show that you can use burning gas to cool beer cans buried in the sand. I need to see a cite for this. The energy for evaporation comes from the fire, not from anything in the beer. So why would it make the beer colder? I’m also reasonably sure that Mythbusters busted it. [See here](http://mythbustersresults.com/episode29) > [@acetylene](#): > > I see what you’re saying but the lattice energy of NaCl (787 kJ/mol) is greater than that of NaBr (747 kJ/mol) which suggests (to me anyway) that NaCl is more likely to form than NaBr. So all of the Na+ will “hook up” with all of the Cl-, then the K+ and Br- that were left out in the cold will have no choice but to get together when the water evaporates. To anthropomorphize this further, if a Na+ and a Br- did happen to hook up, any nearby Cl- is likely to kick out the Br- and take its place. Lattice energy is useless here since you are talking about an aqueous solution not gaseous ions. Instead, you need to look at solubility constants. Of course the solubility constants will be for pure water, but since you have a mixture of salts these things will be meaningless. I think this is a very complicated problem and am not certain that enough information is present to determine what precipitates out first. Most likely, **Chronos** is right that you get a mixture of each, since as one ion precipitates out, the others become more concentrated as the solution evaporates. There is likely to be a slight preference for certain ions to be paired. I could guess that potassium and bromine would like each other best due to similarity in the size of the vacant and occupied orbitals. Keep in mind that a salt crystal isn’t really a bunch of independent NaCl molecules, each Na+ is bonded to six Cl- and each Cl- is bonded to six Na+. If you percipitate two salts together, you will have a dissordered mess. I’m not certain what all the effects will be, but a lot will hve to do with the rate the solution is evaporated. > [@dracoi](#): > > Because the equilibrium favors KBr + NaCl, that’s what you have the most of. The reaction is favored this way because of electronegativity differences; higher differences mean a stronger bond. I’m not sure that electronegativity figures in here. The biggest electronegativity difference is between K and Cl, so KCl would have the strongest bond, but electronegativity has to do with atoms not ions. Chloride doesn’t have any electronegativity whatsoever since it is already negatively charged. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [May 2, 2009, 7:48pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779/43 "2009-05-02T19:48:41Z")
> [@WarmNPrickly](#): > > There is no such thing as a self sustaining enothermic process. The energy to keep the process going must come from someplace else. Ok, it’s spontaneous. I don’t mind splitting hairs, it’s what I do for a living, but if we’re going to split that one, and I guess we should, that’s what I meant. ---
**Author:** ![Public\_Animal\_No.9](https://avatars.discourse-cdn.com/v4/letter/p/b9e5f3/32.png) [@Public\_Animal\_No.9](https://boards.straightdope.com/u/Public_Animal_No.9)\ **Post date:** [May 2, 2009, 11:03pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779/44 "2009-05-02T23:03:01Z")
> [@Brendone](#): > > This may fit the bill. I work for a company that formulates and manufactures fire retardant coatings. When exposed to fire they char and create a protective barrier that prevents the fire from spreading. In some of the testing for it, the coating is put on what is essentially a hot plate heating it hundreds of degrees and then it is exposed to flame. What would normally not burn will now burn, only as long as the hot plate is active (that is part of the test, will it continue to burn without the heat source?) and without the extra heat from the plate, the fire extinguishes… Technically could be endothermic as it requires heat for the fire to occur. It’s still not endothermic. It’s just that the amount of heat generated by the combustion reaction is not enough to maintain a stable flame. There are a lot of instances where a flame goes out because there is not adequate heat generated, but the combustion process itself is still endothermic. What happens when a solid burns is that the heat of combustion causes the solid material to thermally decompose. The decomposition process requires heat, and the resulting products become gas-phase. The gas-phase decomposition products then become the fuel for the flame, and are further decomposed at high temperature, react with oxygen, and form CO2, water, and what are usually called combustion by-products (soot, various hydrocarbons, etc.). If the flame doesn’t generate enough heat to drive the first step, additional heat - in this case the hot plate - can provide the necessary heat to decompose the surface material, generate the gas-phase fuel, and sustain the flame. The flame itself is still exothermic, but it’s not generating enough heat to be stable on its own. It’s similar to the case where a gas grill runs out of fuel - the flame is still exothermic, but there isn’t enough fuel going into the flame to maintain temperatures high enough for the flame to be stable, and it goes out. There is too much air, not enough fuel, and the heat generated isn’t adequate to keep the reactions going. I can’t answer **Sunspace** ’s question - I don’t know. I suspect what is being asked is whether there are reactions that generate flames, but not at temperatures that are hot enough to burn flesh or other materials we’d like to have not burn. There may be, but I don’t know what the reactants would be. The argument against something like that happening is that the energy level has to be high enough for the reactants to release light energy, and that level may be too high for a cold flame. ---
**Author:** ![Testy](https://avatars.discourse-cdn.com/v4/letter/t/c37758/32.png) [@Testy](https://boards.straightdope.com/u/Testy)\ **Post date:** [May 3, 2009, 8:52am UTC](https://boards.straightdope.com/t/endothermic-fire/494779/45 "2009-05-03T08:52:09Z")
Definitely not endothermic but still impressive and fun. A friend and I used to make hydrogen balloons using a sodium hydroxide and water solution and adding aluminum foil. Mix this in a narrow neck bottle and fit a balloon over the neck. When it fills with hydrogen tie off the neck of the balloon and you have a nice toy. Some balloons are just not filled enough and have a more-or-less neutral buoyancy. We would touch these with a kitchen match and the resulting fireball was impressive. If we kept doing it, the fire would eventually crinkle the hair on the backs of our hands. Safety glasses are a REAL good idea when doing this. Regards Testy ---
**Author:** ![sergeirichard](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@sergeirichard](https://boards.straightdope.com/u/sergeirichard)\ **Post date:** [May 4, 2009, 4:05am UTC](https://boards.straightdope.com/t/endothermic-fire/494779/46 "2009-05-04T04:05:45Z")
> [@AK84](#): > > Ok, is there such a thing? Back in my school chemistry days, someone claimed that he had seen a video of someone burning a flammable liquid in their hand, and being unhurt since it was an endothermic reaction. > > Juvenile legend, or fact? Well I’ve accidentally spilled flaming sambuca on my hands, during a drinking game at a party. As it was in a darkened room, my hands were covered with a dramatic - and delicious! - blue flame. The effect was impressing my friends so I let it burn for several seconds before clapping the flames out. In truth I wasn’t feeling much pain at the time… but there were no burns in the morning. If I remember correctly, the flames were warm, even hot - though perhaps it was actually the liquid that was hot - but in no way enough to be painful. So I’m sure something like this was the origin of the juvenile legend. Young chemistry students saw it happen and surmised that the lack of burns was due to an endothermic reaction. But it was a wild and wrong guess. And after all, if it were really an endothermic reaction it would have actually felt cold. ---
**Author:** ![Alex\_Dubinsky](https://avatars.discourse-cdn.com/v4/letter/a/dbc845/32.png) [@Alex\_Dubinsky](https://boards.straightdope.com/u/Alex_Dubinsky)\ **Post date:** [May 4, 2009, 5:20am UTC](https://boards.straightdope.com/t/endothermic-fire/494779/47 "2009-05-04T05:20:53Z")
Acetone burning in your hand is two reactions. One of them is endothermic. The fire is not endothermic in sum, but it’s endothermic where it counts. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 5, 2009, 1:31am UTC](https://boards.straightdope.com/t/endothermic-fire/494779/48 "2009-05-05T01:31:33Z")
> [@acetylene](#): > > Chemistry teacher here. This is one of my favorite demonstrations. it’s called the methane mamba. > > Run a tube connected to a source of natural gas (methane) into a container with soapy water. Bubbles will form. The bubbles are filled with methane instead of air, so they will rise. but they stick together so you get a rising column of very flammable, and very stinky, bubbles. WET YOUR HAND! Then break off the column at the bottom so it sticks to your hand, going upwards. Light it and listen to the oohs and aaahs. It doesn’t burn your hand because a) the heat rises and your hand is underneath it, and b) your hand is wet > > The combustion of methane is definitely exothermic, though, as anyone standing near it can tell you. > > [Here is a video.](http://www.youtube.com/watch?v=AopbZW_w_q8&feature=related) There are lots of them on youtube. Here’s [another](http://www.youtube.com/watch?v=T-mOJ9wHmNY&feature=related) one ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [May 5, 2009, 9:05pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779/49 "2009-05-05T21:05:17Z")
Eh, maybe it’s time for a warning/disclaimer, for any among us who may be sincerely foolish. Don’t assume that these stunts are always safe, every time! You have the right to be a damn fool, but we’d rather you didn’t hurt yourself doing it. In many of these experiments, showy displays of fire didn’t hurt anyone, because the flame was brief and the fuel was quickly spent. It doesn’t always happen so neatly. For example, an old friend of mine used gasoline to start a brushpile on fire. In the time it takes to say _whump_, he was engulfed in flame, and the fire was gone. He still had his hair, his eyebrows, and his clothes. His bare legs, though, had 2nd degree burns. That fart-lighting thing? Don’t do it in the nude, and do NOT do it while wearing synthetic fabrics! Just as a screen over a Bunsen burner stops the flame from going further, your cotton jeans will stop the flame from burning the hair around your anus. Synthetic fabrics, rather than protecting you, will melt to your skin unless everything goes just right. ---
**Author:** ![acetylene](https://avatars.discourse-cdn.com/v4/letter/a/f19dbf/32.png) [@acetylene](https://boards.straightdope.com/u/acetylene)\ **Post date:** [May 5, 2009, 11:45pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779/50 "2009-05-05T23:45:43Z")
Amen to that, brother! I am constantly advising students not to do **any** of the things they see on Youtube. Dry ice bomb, The Works bomb, whatever. Don’t do it. You WILL get hurt. I have to constantly evaluate my demonstrations the ensure that: A) The dangerous ones cannot be imitated because the students will not have the chemicals or apparatus and B) There is some instructional purpose. ---
**Author:** ![Alex\_Dubinsky](https://avatars.discourse-cdn.com/v4/letter/a/dbc845/32.png) [@Alex\_Dubinsky](https://boards.straightdope.com/u/Alex_Dubinsky)\ **Post date:** [May 6, 2009, 8:25pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779/51 "2009-05-06T20:25:35Z")
> [@AskNott](#): > > cotton jeans will stop the flame from burning the hair around your anus. Are you sure that’s a benefit? ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [May 6, 2009, 9:23pm UTC](https://boards.straightdope.com/t/endothermic-fire/494779/52 "2009-05-06T21:23:16Z")
This [classic thread](http://boards.straightdope.com/sdmb/showthread.php?t=147416) speaks of the logical conundrum that is endothermic fire 🙂 [Previous page](https://boards.straightdope.com/t/endothermic-fire/494779.md?page=2)