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Background: Well, it's summer here in Brazil, and a great way to keep cool is drinking a so called Tereré, and, of course, we do need cold water for that.

Problem: I was looking for endothermic spontaneous reactions so I can cool the water in a bottle using an inner aluminum bottle in which the endothermic reaction occurs. I already knew that Urea's dissolution is endothermic and spontaneous, but I couldn't find how much endothermic it is, i.e., how many Joules it absorbs per Mol of urea dissolved. Another problem related with urea is that it's dissolution is a slow process (I think, not sure), additionally, the urea I can find is the one use as fertilizer, and this one has lots of impurities that I'll need to take into account when calculating how much I'll need to react.

What I know is that the urea dissolves in the same amount of water in weight, that the reaction time increases as the concentration increases, but have no idea how much energy it absorbs neither the how much time it takes to react, of course it depends on the concentration, granularity, etc.

Question: Considering all of that, are there any other reagent(s) I can buy or synthesize without being a licensed chemist that can be used to cool the water? Or, do you have the information that is missing about urea's dissolution (Joules/Mol absorbed in dissolution, approximated reaction time) so I can figure out if I can use it for this purpose?

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  • $\begingroup$ Ther is no chemical reaction around! You should learn that dissolution is physics! Last not least, for centuries common salt was used as a coolant, so why would you use the much more expensive urea? Your gueseeing around fertilizer ureas purity is nonsense. All urea is produced by the same plants, impurities might be introduced on distribution and repackaging. $\endgroup$ – Georg Oct 21 '13 at 14:54
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    $\begingroup$ @Georg Please consider the tone of your comments before posting them. Comment about the posts, not the user that has posted them. Consider this a warning. $\endgroup$ – jonsca Oct 21 '13 at 23:00
  • $\begingroup$ Yeah I am aware of that @Georg, I was just trying not to restrict the answer for dissolution processes, i.e. I could accept a reaction as answer if it was feasible. I really don't know about urea, that's why I made it clear by saying "I'm not sure", what does matter about it is the solution enthalpy that I already managed to find for salts trb456 mentioned on his answer. $\endgroup$ – HericDenis Oct 22 '13 at 10:20
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    $\begingroup$ @Georg slat does not affect the cooling rate of water but the cooling rate of ice..you can't cool the water with only salt! And in fact reacting hydrated barium hydroxide with ammonium chloride is an endothermic reaction! $\endgroup$ – G M Oct 24 '13 at 8:00
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Unfortunately, many aqueous endothermic reactions involve some chemicals you may wish to avoid. Ammonium nitrate is the classic example, but in the US it is becoming more and more unavailable because of its potential use as an explosive. Another frequently mentioned possibility is reacting hydrated barium hydroxide with ammonium chloride, but $\ce{Ba(OH)2}$ is very caustic and poisonous.

So of what I've read, your best choices might be mixing water with a chloride salt like ammonium or potassium. I do not know how cold this will get your container. I suspect it will not be as good as ammonium nitrate, which would probably be ideal given the former use of this compound in medicinal cold packs.

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There are some Homemade reactor for water cooling that can do what you want.

Mohammed Bah Abba had the same problem in Niger so he invented the Abba refrigerator: a pot-in-pot refrigerator that uses the evaporation of the water from a porous material to refrigerate the things inside the pot. The image belowe is not to of the Abba refrigerator but the principle is quite similar and here is well explained. enter image description here

I think you can make a mod cuia/guampa with a pot-in-pot refrigerator embedded.

To speed up your cooling I should use a pressurized gas, this can cold the thing in two ways:

  1. the gas expansion is an endothermic process
  2. if the flow of gas go above the wet surface you can boost the evaporation process

NOTE: the Abba refrigerator works well in dry and ventilated ambient, I hope you are not in Amazonia!!

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  • $\begingroup$ This is a great idea, but seems to be very slow process so it does not fit on my needs. I need something to work in a park in some minutes, if it will take too much time I can just go back home and get ice from the refrigerator. Anyway, it would work since I live in the south of Brazil haha $\endgroup$ – HericDenis Oct 20 '13 at 21:02
  • $\begingroup$ @HericDenis I have improved the answer is it hot before you cool it? $\endgroup$ – G M Oct 21 '13 at 9:15
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Beside construction of a solar power driven DIY cooler for beverages (like here), G M's post directing to use the heat of evaporation as a vehicle to cool a liquid may be indeed advantageous. What about using molar sieves in an outer shell of a barrel -- again, at the place of consumption, no electricity is needed. Yet, CoolKeg claims to be scaleable (7 to 20 L) and both barrel and the active ingredient (zeolithe) is able to be recycled. A video describes how this may be implanted into a production chain, this site around beer provides some to read to understand the function.
Of course, to regenerate the molar sieves, alternative / more ecofriendly sources of heat may be used, too.

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I have used very large quantities of urea (fertiliser) mixed with water. No matter how warm the water when mixed thoroughly the water temp will drop to minus 4 degrees. We had to this as part of the supplementary diet to feed thousands of cattle in drought areas of Australia. It will chill water extremely effectively for low cost.

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  • $\begingroup$ I knew about urea, but I didn't know that it could drop the temperature so significantly! -4°C is way below what I expected. $\endgroup$ – HericDenis Nov 3 '16 at 22:14
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Recently has been developed an ammonium nitrate free cold pack. It consists of a mixing of Water, Urea, and Carbamakool™ this is the secret ingredient you should search for. This is a safe alternative to posioning chemical. See this site for more information.

Maybe you can try to analyze one of this pack to find the information you need.

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