# exothermic vs endothermic and why



## Geo (May 5, 2014)

An exothermic reaction is a reaction that builds heat.The greater the reaction and the more vigorous the reaction is,the hotter the solution will become.Exothermic reactions occur mainly when you are lowering the PH of a solution.Normally,when you dissolve a metal into a solution,it will build heat but this is not a rule and there are metals that dissolve with an endothermic reaction but I am getting ahead of my self.Any time you neutralize an acid,it will be an exothermic reaction taking place.The oxidation of iron (rust) is an exothermic reaction. http://en.wikipedia.org/wiki/Exothermic_reaction

An endothermic reaction is a reaction that absorbs heat and cools the solution.There are certain reactions that produce the effect that we take for granted every day.The melting of ice is an endothermic reaction.Some common chemicals we use causes an endothermic reaction and the effect is so small that we may even miss it.If any one of you has ever used baking soda for heartburn may notice the glass fogging up even though the water was from the tap.Sodium bicarbonate mixed with water gives an endothermic reaction.Other chemicals will absorb an amazing amount of energy such as dissolving sodium thiosulfate into water. http://en.wikipedia.org/wiki/Endothermic

This is a simple,or not so simple,storing and releasing of energy.When the endothermic reaction occurs,energy is being absorbed and stored for later use.The stored energy can be released by adding another chemical or element to the mix causing the exothermic reaction.This is known as "The Equilibrium Constant". http://chemwiki.ucdavis.edu/Physical_Chemistry/Chemical_Equilibrium/Le_Chatelier's_Principle/Effect_Of_Temperature_On_Equilibrium_Composition/Exothermic_Versus_Endothermic_And_K


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## Charles Connor (May 7, 2014)

When a reaction hits its equilibrium state, if there is any change made like concentration or temperature, the system will do the opposite to get to the equilibrium again, so if the reaction of gold and AR is exothermic, according to that equilibrium fact , is there no need of heating the solution and what should be done is remove or absorve that heat, with the beaker being surrounded by water and ice? that made me think about that, but since the reaction is not in the equilibrium, this does not apply... that is what i heard in my chemistry class last week...


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## Geo (May 7, 2014)

From my understanding, you have a reaction with additions of each component of an AR dissolution. When mixing the hcl and the nitric acid with water(optional as both reagents contain free H2O), heat is produced.By adding this to the metals to be dissolved, the acids are being neutralized in the dissolution thus building more heat.Heat is not required for an AR dissolution as the reaction itself creates heat. As the volume of one reagent (acid or metal) is depleted, the reaction slows and begins losing heat faster than the heat can build.By adding heat,it drives the remaining free nitrate to react.If not heated,the reaction would slow to a crawl.

As an aside, again from what I understand, when diluting a reagent that ends with ite (sodium metabisulfite) with water, an endothermic reaction occurs. When you add the hydrated SMB to AuCl2, an exothermic reaction occurs. Harold spoke of precipitating gold using SO2 gas from a cylinder.He said the reaction was hot enough to boil the solution so he would add ice to the solution before the precipitation.


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## Charles Connor (May 7, 2014)

I think that the word neutralize would not apply because the solution is still having a high pH, and probably the heat could mostly come from the giving of protons or H+ from both HNO3 and HCl when the reaction occurs


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## bmgold2 (May 7, 2014)

Kind of off topic but I have a question maybe someone here could answer. Instant cold packs often contain ammonium nitrate or urea and a breakable packet of water. Can this reaction be repeated if the solution was evaporated? What I'm thinking of is possibly removing the water using solar power and then adding water again to get a cold reaction again. I should just try it and find out but asking here was quicker and easier. If it worked, I can imagine this reaction being used as a solar powered, chemical air conditioner. Of course, buying the quantity of ammonium nitrate required would likely bring some unwanted attention but urea is still pretty available around here.

A quick experiment before clicking submit proved that the common urea fertiliser did get cool (not cold) when water was added. Without using heat, it will be days/weeks before I can see if it will get cool again once the water is evaporated. A Google search found InstaKool™ Frequently Asked Questions where I found out there is also a proprietary cooling agent (Carbamakool) included in the urea cooling packs to establish colder and longer lasting temperatures as they relate to the initial onset and duration of cold.


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## Geo (May 7, 2014)

The only reagent that is not consumed in the reaction would be the catalyst, whatever that would be.The acids are consumed,neutralized,converted to, whatever. Even though the PH remains low,acid is consumed in the dissolution.


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## Charles Connor (May 7, 2014)

Well if we are talking theorically, yes, both acids are cosumed and no deNOxing will be needed and the solution would not the that acidic... so this could be an exothermic reaction that in some cases needs heat to get the theorical equilibrium?
I have not tryed to dilute urea so i do not know if it gets cold, when i add it to my gold solution it gets a little bit hot, but when i let the solution to cool down and add the urea i does not generates heat.... also if you have ammonia, HCl and KNO3 you can make ur own NH4NO3, but i suggest to grab some books and learn more about the amazing world of chemistry when you understand what is going on it is even better


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## solar_plasma (May 8, 2014)

bmgold2 said:


> Kind of off topic but I have a question maybe someone here could answer. Instant cold packs often contain ammonium nitrate or urea and a breakable packet of water. Can this reaction be repeated if the solution was evaporated? What I'm thinking of is possibly removing the water using solar power and then adding water again to get a cold reaction again. I should just try it and find out but asking here was quicker and easier. If it worked, I can imagine this reaction being used as a solar powered, chemical air conditioner. Of course, buying the quantity of ammonium nitrate required would likely bring some unwanted attention but urea is still pretty available around here.
> 
> A quick experiment before clicking submit proved that the common urea fertiliser did get cool (not cold) when water was added. Without using heat, it will be days/weeks before I can see if it will get cool again once the water is evaporated. A Google search found InstaKool™ Frequently Asked Questions where I found out there is also a proprietary cooling agent (Carbamakool) included in the urea cooling packs to establish colder and longer lasting temperatures as they relate to the initial onset and duration of cold.



Interesting question. Also NH4Cl takes energy when it gets dissolved, and too, when the water gets evaporated again. Where does the energy go... Too early morning to think clearly, but I will try to find out later.


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## macfixer01 (May 8, 2014)

bmgold2 said:


> Kind of off topic but I have a question maybe someone here could answer. Instant cold packs often contain ammonium nitrate or urea and a breakable packet of water. Can this reaction be repeated if the solution was evaporated? What I'm thinking of is possibly removing the water using solar power and then adding water again to get a cold reaction again. I should just try it and find out but asking here was quicker and easier. If it worked, I can imagine this reaction being used as a solar powered, chemical air conditioner. Of course, buying the quantity of ammonium nitrate required would likely bring some unwanted attention but urea is still pretty available around here.
> 
> A quick experiment before clicking submit proved that the common urea fertiliser did get cool (not cold) when water was added. Without using heat, it will be days/weeks before I can see if it will get cool again once the water is evaporated. A Google search found InstaKool™ Frequently Asked Questions where I found out there is also a proprietary cooling agent (Carbamakool) included in the urea cooling packs to establish colder and longer lasting temperatures as they relate to the initial onset and duration of cold.





Yes the process would be reversible. You could dry out the Ammonium Nitrate then use it again to produce cold. However it is also such a hygroscopic chemical that drying it out could be a problem unless you use heat or you're located in a very dry environment. Also I consider it to be deliquescent (absorbs enough water from the air to dissolve in), although some attribute that to impurities and claim that if pure it isn't deliquescent. When I was a kid I had various chemistry sets and used to buy 1 ounce bottles of Perfect brand chemicals at the hobby shop to experiment with. More than once an unopened bottle of Ammonium Nitrate prills was found a couple months later in a drawer, still sealed but it had been reduced to a bottle of solution.

Another interesting and reversible reaction is used in the Heat Pack bags which contain a super-saturated solution of Sodium Acetate. When triggered by a shock wave from the little metal clicker inside the bag, the Sodium Acetate starts to solidify and heats up. You can reverse the process and reuse the pack by microwaving the bag to re-dissolve the Sodium Acetate.


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