Is there any way to make an endothermic reaction that is not combustion but looks kind of like it?
I recognize that this is intuitively right, but I suspect that you aren’t considering a closed system at constant pressure. If you have ever pulled a vacuum on benzene, you know that the benzene will quickly freeze simply by losing so much energy through evaporation. At that point, it basically stops evaporating. It does slowly evaporate, but then it isn’t a closed system either. If you run an endothermic process in a closed system at constant pressure long enough, the process will quench itself assuming the process doesn’t go to completion first. Eventually, you will pull enough heat from the system that TdeltaS is less than the enthalpy required. The process will not sustain itself.
Again, I would have to say no. There are reactions that produce light (think of glow sticks) instead of heat, but light is another form of energy so these are exothermic as well.
The thing to realize is that there really aren’t that many examples of enothermic reactions. Ammonium nitrate and water is a common example. This is used in some chemical cold packs. But this is not a chemical reaction. It is forming a solution, which is a mixture. Melting and vaporization are endothermic but they aren’t chemical reactions, either. And none of these look like fire.
I suppose you could get some dry ice, which sublimes (goes from a solid directly to a gas). Put it in water so it creates fog even faster. Drop a red or orange glow stick in the water or shine a red light on the fog.
That’s about as close as you are going to get.
I disagree. This is an example of the dissociation of an ionic compound, which involves breaking chemical bonds. It is therefore a chemical reaction.
The chemical equation for this would be written:
NH[sub]4[/sub]NO[sub]3/sub -------> NH[sub]4[/sub][sup]+/sup + NO[sub]3[/sub][sup]-/sup
Contrast this to dissolving sugar in water, which is indeed a physical reaction. Unlike ionic compounds, the sugar molecules do not dissociate in water.
Why is this different from saying that eventually with fire you pull enough energy from the fuel so that the process will not continue?
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.
Hmmm… Interesting point. I guess it depends on who you ask. According to wikipedia,
Bonds are indeed broken but are new bonds formed? Are you saying that the solvation sphere formed between the NH[sub]4[/sub]+ ion and the water molecules is a new compound? If that is true then this is indeed a chemical change. Dictionary.comagrees with you.
But are they chemically bonded? Is a new compound being formed? I say no.
We are talking about ion-dipole attractions, which are a type of intermolecular force. As I was taught, these are not true chemical bonds. They are momentary and much weaker than covalent or ionic bonds.
This guyagrees:
Now I’m waiting for a PhD to come along and smack me down, for this and for using dissolution, solvation, and dissociation interchangeably.
Consider: I have a beaker of water, to which I add some pure sodium chloride. Then, I add some pure potassium bromide. Then I stir everything up, and then evaporate away the water. At the end of this process, I’ll have a mix of sodium chloride, potassium chloride, sodium bromide, and potassium bromide. My precipitate includes two compounds (sodium bromide and potassium chloride) which I didn’t have to start with, so obviously a chemical reaction occurred at some point in the process. When?
This sounds like something I’d be asked in a chemistry class, but if you forced me to answer, I’d say that the first chemical reaction took place when the salts dissolved and dissociated into ions, the second when the mixture becomes saturated and ions start to crystallize out of solution. But keep in mind I’m a fairly B average kind of student.
[quote=“Kobal2, post:14, topic:494779”]
How about liquid nitrogen in your mouth or a bed of hot coals under your feet ? That works too 
<snip> “The point is that it can’t hurt the sandwichee…” <snip>
LOL!! 
Beer snorted wildly out of my nose upon reading this!
MMD! (Made my Day) 
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.
I think.
This is a very interesting hijack we have going on here. I would like to know the definitive answer to this.
Is dissociation a physical or a chemical process? Cecil, Exapno Mapcase, anyone?
I really didn’t want to click on this link (or to read it when I did), but it was so worth it. I loved the end!
On second thought, Chronos, I see your point. If you add NaBr and KCl to water, then by my reasoning you will get NaCl and KBr when the water evaporates. The reactants are different than the products. Sounds like a chemical reaction to me.
I wish an inorganic chemist would come along and settle this.
It is very easy to see what an endothermic fire looks like. Just play the video of a normal fire backwards.
When I worked in printing we would routinely clean our hands of ink with ink-thinner.
One of my cow-orkers was severely (possibly even terminally) blonde. One time she had just finished cleaning her hands with a very wet thinner rag and then tried to light a cigarette. There was an audible ‘poof’ and her hands were fully engulfed in raging flames. She stood there staring wide-eyed at her hands and screaming a ragged high note that would scare off a grizzly bear.
I walked over to her calmly, grabbed her wrists and clapped her hands together. The fire went out. I felt no heat as I did this. The hair on the back of her hands wasn’t even singed.
I’m sure the flames generated heat, but so little as to be harmless and practically unnoticable.
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.
You’d have felt more heat if you held your hands half an inch away from hers, rather than touching her skin directly. (Even then, it probably would not have been a lot of heat).
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.
I would argue that it’s both - the crystalline salts physically break down and disappear in the solution, and it’s also chemical because the water molecules are interacting in some way at an atomic level in the solvation process.
I know this isn’t exactly what you’re looking for, but once a high-mass star builds up an iron core, it can only fuse iron into higher elements with an endothermic reaction. It’s like a giant nuke going off and making everyone chilly, if you don’t think about it too much.
dracoi:
Are you sure they happen in equilibrium? I’m pretty sure that equilibrium only occurs when the solute is slightly soluble. Not one of the four compounds we are discussing is slightly soluble. Unless the solution is saturated, they will completely dissociate and there is no equilibrium established. The reaction, or whatever it is, goes to completion.