Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: that mad man on 21/02/2008 16:47:27
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There are plenty of chemical reactions that can produce a quantity of heat but..
Are there any that can produce a cold reaction (lowering of temperature) under normal conditions?
If not why not?
Er,,,,,forget the above and look below at paul.fr's post. [:I]
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Not a chemical reaction, necessarily, but a much simpler process is possible.
NOTE: This experiment is only possible with enormous strength.
Supplies: a seringe with a cap for the end.
Put the cap on the end of the seringe. Pull out the injection shaft, and make sure you get a good amount of air in there.
While holding the cap on really tight, shove the shaft in as hard as you can until you can't any more. Immediately pull the cap off the end and feel the air that comes out. It should be cold.
WHY THIS IS: Density and temperature have a direct relationship. If a gas is compressed into a liquid (example: liquid nitrogen) the temperature will decrease greatly.
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http://chemistry.about.com/cs/howtos/ht/endothermic.htm
Most endothermic reactions contain toxic chemicals, but this reaction is safe and easy. Use it as a demonstration or vary the amounts of citric acid and sodium bicarbonate to make an experiment.
Difficulty: Average
Time Required: Minutes
Here's How:
1. Pour the citric acid solution in a styrofoam coffee cup. Use a thermometer or other temperature probe to record the initial temperature.
2. Stir in the baking soda (sodium bicarbonate). Track the change in temperature as a function of time.
3. The reaction is: H3C6H5O7(aq) + 3 NaHCO3(s) --> 3 CO2(g) + 3 H2O(l) + NaC6H5O7(aq)
4. When you have completed your demonstration or experiment, simply wash the cup out in a sink. No toxic chemicals to mess with!
Tips:
1. Feel free to vary the concentration of the citric acid solution or the quantity of sodium bicarbonate.
2. An endothermic is a reaction that requires energy to proceed. The intake of energy may be observed as a decrease in temperature as the reaction proceeds. Once the reaction is complete, the temperature of the mixture will return to room temperature.
What You Need:
- 25 ml citric acid soln
- 15 g baking soda
- styrofoam cup
- thermometer
- stirring rod
Endothermic chemical reactions are a rarity, but they do exist.
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Chemical reactions producing cool temperatures are rare because cold is not a thing-it is a lack of a thing, and that thing is thermal existance.
For example, zero kelvin is complete thermal absence. You cannot go below zero in kelvin because zero is complete thermal absence. Thus, it is very difficult to produce cold. The only way is to somehow dispel the heat, in which case most metals are ideal.
Refrigerators use the pressure system above, not a chemical reaction. However, anothersomeone has proven that a chemical reaction is possible.
Good luck with whatever you plan to use this knowledge for.
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also, it is nigh impossible without some kind of acid.
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Thanks for the info folks. [;)]
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http://chemistry.about.com/cs/howtos/ht/endothermic.htm
Most endothermic reactions contain toxic chemicals, but this reaction is safe and easy. Use it as a demonstration or vary the amounts of citric acid and sodium bicarbonate to make an experiment.
Difficulty: Average
Time Required: Minutes
Here's How:
1. Pour the citric acid solution in a styrofoam coffee cup. Use a thermometer or other temperature probe to record the initial temperature.
2. Stir in the baking soda (sodium bicarbonate). Track the change in temperature as a function of time.
3. The reaction is: H3C6H5O7(aq) + 3 NaHCO3(s) --> 3 CO2(g) + 3 H2O(l) + NaC6H5O7(aq)
4. When you have completed your demonstration or experiment, simply wash the cup out in a sink. No toxic chemicals to mess with!
Tips:
1. Feel free to vary the concentration of the citric acid solution or the quantity of sodium bicarbonate.
2. An endothermic is a reaction that requires energy to proceed. The intake of energy may be observed as a decrease in temperature as the reaction proceeds. Once the reaction is complete, the temperature of the mixture will return to room temperature.
What You Need:
- 25 ml citric acid soln
- 15 g baking soda
- styrofoam cup
- thermometer
- stirring rod
It doesn't say how to prepare the citric acid solution. Maybe it's a saturated solution?
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Another interesting endothermic reaction is:
2NH4NO3 + Ba(OH)2*8H2O --> Ba(NO3)2 + 2NH4OH
It's interesting because it involves solid compounds. As soon as you thourogly mix them, you have the cold.
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"Not a chemical reaction, necessarily, but a much simpler process is possible."
Nonsense. All reactions are exothermic in one direction and endothermic in the other so there are plenty of endothermic reactions which get colder as ther reaction goes. Reacting bicarbonate of soda with vinegar is one of the easiest to try out The one cited above (citric and bicarbonate) is another)
I'd also like to know what this "also, it is nigh impossible without some kind of acid." is suposed to mean.
One of the most widely used reactions in the past was the production of hydrogen by the reaction of hot coal with steam. That reaction is endothermic and doesn't involve anything conventionally though of as an acid.
BTW Lightarrow, NH4OH barely exists- what you have is NH3 and H2O
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BTW Lightarrow, NH4OH barely exists- what you have is NH3 and H2O
Certainly. Sometimes, even in chem laboratorys, a water solution of NH3 was indicated as ammonium hydroxide (maybe because it's faster to write than "water solution of NH3"). [:)]
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NH3 Aq is not much more difficult than NH4OH.
The real reason for labeling solutions as NH4OH is that you can pretend there's more of it on a % basis.