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post #1 of 9 (permalink) Old 09-12-2008, 11:44 PM Thread Starter
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Attn Chemistry smart people.

I have a question i cant figure out, i know its a chemical reaction of some sort but i need a bit more detailed explanation. I recently Mixed some Expert Gardener Brand all Purpose Water Soluble Plant food , with tank water for a little experiment in using it to grow Aquatic Plants(no worries no fish or snails in the tank) and i got a chemical reaction the confuses me, the temperature in the cup i was mixing it up in dropped to under 40 degrees. Any one explain why? The tank has been Dosed with K2So4, CSM+B and H2KPO4 as well. Im no Chemistry Wiz so can some explain the 30+ Degree Drop in the water temperature in the cup?

The expert Brand Contains 24% total Nitrogen broke down as follows:
3.1% ammoniacal Nitrogen
1.2% Nitrate Nitrogen
16.1% Urea Nitrogen
.09% Other water Soluble nitrogen
2.7% Water Insoluble Nitrogen

Also contains:

Available Phosphate (P2O5) 8.0%
Soluble Potash (K2O) 16.0%
Total Copper(copper sulfate) .07%
Total Iron (Chelated Iron) .15%
Molybdenum .0005%
Total Zinc .06%

Before any one says it No live stock at all of any kind will be going in this tank again, i am Strictly using it as a Small Grow out tank With 4x2-liters Bottles DIY Yeast CO2, for Plants only.

So can some explain Simply and in detail what caused the Temperature Drop? Im not a Chemistry Guy so please use layman's terms if possible.
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post #2 of 9 (permalink) Old 09-13-2008, 12:08 AM
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An endothermic reaction caused the temperature to drop. I'm not sure what compounds ion there reacted to cause that specifically, though.
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post #3 of 9 (permalink) Old 09-13-2008, 12:11 AM
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Or, it was just the change in enthalpy when the urea (1st guess) or K2O dissolved (2d guess).

This is common to chromatographers -
Acetonitrile + water = cold, Methanol + water = warm - no chemical reaction involved.


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post #4 of 9 (permalink) Old 09-13-2008, 12:12 AM
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last year we did an experiment with potassium (maybe potassium permanganate) and something else, and it caused an endothermic reaction. ill see if i can figure out what else...

AP chem rocks

EDIT:

Free floating Cl ions + free floating K ions combine to form KCl which reacts with H2O, absorbing heat in the process. i believe this is what we did.

its a double replacement reaction,

KCL + H2O > KOH + HCl

shown as ions...
(H+)2(O(2-)) + (K+)(Cl-) > (K+)(OH-) + (H+)(Cl-)

but i think it would take alot of both reactants to produce a significant amount of temperature drop. i mean more than what you are dosing.

what size tank is this in?

2nd EDIT:

by no means am i a chemist, im a Junior in high school taking chemistry. im just throwing my ideas out there.
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post #5 of 9 (permalink) Old 09-13-2008, 01:15 AM Thread Starter
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Quote:
Originally Posted by chris127 View Post
last year we did an experiment with potassium (maybe potassium permanganate) and something else, and it caused an endothermic reaction. ill see if i can figure out what else...

AP chem rocks

EDIT:

Free floating Cl ions + free floating K ions combine to form KCl which reacts with H2O, absorbing heat in the process. i believe this is what we did.

its a double replacement reaction,

KCL + H2O > KOH + HCl

shown as ions...
(H+)2(O(2-)) + (K+)(Cl-) > (K+)(OH-) + (H+)(Cl-)

but i think it would take alot of both reactants to produce a significant amount of temperature drop. i mean more than what you are dosing.

what size tank is this in?

2nd EDIT:

by no means am i a chemist, im a Junior in high school taking chemistry. im just throwing my ideas out there.
Well im 33 and you seem to know more about Chemistry than i do! This is a 10g Plant Only tank. Basically to get them growing well before moving them to another tank, and as kinda a Plant QT tank to kill off excess Snails or what not that show up on the plants.
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post #6 of 9 (permalink) Old 09-13-2008, 02:06 AM
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Someone guessed urea.

Correct. Dissolution of urea produces an endothermic reaction, removing heat from the surroundings, leading to a drop in temperature.
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post #7 of 9 (permalink) Old 09-16-2008, 05:25 PM
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Quote:
Originally Posted by chris127 View Post
...its a double replacement reaction,

KCL + H2O > KOH + HCl

...
^ actually happens in the reverse, exothermically (violent, boom, boiling stuff splattering people type of exotherm if concentrations are sufficient). KCl is actually dissociated completely, K+ and Cl- ions in solution.


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post #8 of 9 (permalink) Old 09-16-2008, 05:58 PM
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Quote:
Originally Posted by Squawkbert View Post
^ actually happens in the reverse, exothermically (violent, boom, boiling stuff splattering people type of exotherm if concentrations are sufficient). KCl is actually dissociated completely, K+ and Cl- ions in solution.
Breaking all the lattice bonds in KCl requires more energy than is given off by hydrating the ions - so the net energy change results in a decrease in the temperature.

Re: the original question. The compound ammonium nitrate also has a very endothermic heat of solution (it gets cold as it dissolves). If that is one of the major components of your fertilizer (given 3% ammonia nitrogen), then that plus the urea probably accounts for much of the temp change.

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72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias

2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
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post #9 of 9 (permalink) Old 09-16-2008, 06:23 PM
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well its similar if you put mix dry table salt with water. the temperature goes down.
its has to do with the ionic bonds that are holding the atoms together breaking by taking in heat energy from the water.
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