Sodium acetate and water (heat)?

[Guest link]

Ok im new to all this and im not to sure im putting this in the correct froum but here goes!

 

I was wondering if anyone could describe to me how I would go about making one of the hand warmers I have seen in camping shops I no that Sodium acetate and water are activated by bending a metal disk inside a pouch which then creates heat but after that im at a loss, if anyone can help me out I would be very greatfull?

[wolfson]

Heat packs contain sodium acetate and water. Sodium acetate is good at supercooling. It "freezes" at 130 degrees F or 54 degrees C, but it is happy to exist as a liquid at a much lower temperature and is extremely stable. Clicking the disk has the ability to force a few molecules to the solid state, and the rest of the liquid then rushes to solidify as well. The temperature of the solidifying liquid jumps up to 130 degrees F or 54 degrees C in the process.

When you boil the solid you melt it back to the liquid state. You have to completely melt every crystal or the liquid will quickly re-solidify. You can repeat this cycle thousands of times, just as you can freeze and melt water as many times as you like. So snapping the disk causes the crystallization process. The crystallization of a supersaturated sodium acetate solution is an exothermic process (gives out heat).

Hope that helped.

[YT2095]

at the opposite scale Ammonium Nitrate`s equaly fascinating, the addition of water to the AN crystals will make it freeze

it`s the same stuff used in the Cold Packs for emergency sports injuries and such, It`s also been exploited in Canned Ice-Cream ideas

[Guest link]

That is quite intersting, at the moment the only was to re use these bags after they have ran out is to boil them or to re heat them in a microwave is there a chemical allternitave to the problem?

[YT2095]

no, sorry

you only get out what you put in sadly (as far as the sodium acetate bags go anyway).

there are other alternative methods of generating heat chemicaly that would be perfectly usable and safe as a hand warmer. they are only good for one time use though.

 

Calcium Oxide and water is very good at making heat and quite safe

[Guest link]

Ok thanks for your help its much appreciated. Do no of anywhere online I could purchase some Sodium acetate im new to this so I dont no where to begin really?

[YT2095]

well, online I don`t really know, as I buy all my supplies from a Lab equipment warehouse just up the road from me.

you COULD however make your own Sodium Acetate

 

simply you would need a few things available at most hardware stores, namely: Sodium Hydroxide (red Devil drain cleaner if you`re in the USA) or plain old "Caustic Soda" from B&Q if you`re in the UK

then you`ll need a gallon of white spirit vinigar (used for pickling onions and stuff).

then you`ll need some litmus paper or indicator and a rather large plastic or glass jar.

into which you add the vinigar (keep at least one cupfull back as a reserve).

then make a strong solution of the Sodium Hydroxide, remembering to add the crystals to the water, not the water to the crystals!

while stirring your vinigar, add some of the hydroxide and test with the litmus paper, if it`s still red, add a bit more hydroxide, and keep doing that untill there is little color change.

IF the paper goes blue, add the cup of vinigar you kept in reserve

do it slowly and keep testing, you want little color change and preff in RED side if you can

then you have to warm this mix and let as much water eveporate as possible, from your gallon of spirit vinigar and hydroxide, you`ll get enought crystals for 2 small heat packs

when you have a bout a pint (half a liter) left, filter it through a coffee filter paper to keep out impurities (they mess up the crystalisation proccess).

then evapoarate some more untill there are crystals start to form

from here you`re well on your way

when you get to that stage let us know, and we`ll take it from there

 

all the best

[Cap'n Refsmmat]

I thought it was the sodium in it, because sodium burns on contact with water. But I must be mistaken. Pity that happens a lot.

[YT2095]

Sodium is indeed very reactive in water and generates alot of heat, often enough to ignite the Hydrogen gas that`s liberated from the Water molecules. but in this instance it`s using a different mechanism, the heat given off upon rapid crystalisation.

The Sodium used here is the Hydroxide (the stuff the water and sodium becomes after they react).

[Leroy]

i'm busy with my own project making my own heat pack and cold pack but since i study chemistry i'd also like to know why sodium acetate releases heat and ammonium nitrate absorbs it.

 

Has it got to do with crystallisation enthalpy or something else. Could anyone explain exactly what happens or redirect me to a webpage which explains it in detail.

 

I'd also like to test if it can be done with other saturated salt solutions maybe something like NaCl or other salts that are readily available.

 

thnx

[wolfson]

Leroy, i think i answered that question, if you have anymore just ask.

[Leroy]

i don't see the post where you explain in thermodynamically so either i'm blind or maybe i didn't make it clear enough in my previous post.

[wolfson]

Sorry I didnt know you wanted in in "thermodynamics" so you should know that the chemical bonds are broken heat is released so the reaction is exothermic, and in the case of Ammonium Nitrate, endothermic, as when you dissolve the Ammonium Nitrate in water energy (heat) is requires about 26KJ/mol,

 

sodium acetate(aq) ==> sodium acetate(s);

 

So becoming:

 

a/_\ ====> B~C/_\

 

So the two weak pi-bonds are converted into two stronger sigma bonds and the large negative entropy shows the change from pi to sigma.

[VendingMenace]

Sorry I didnt know you wanted in in "thermodynamics" so you should know that the chemical bonds are broken heat is released so the reaction is exothermic,

 

I just wanted to clarify something in this statment. Energy is never realeased when a bond in broken. Bonds form becuase they are thermodynamically downhill of the seperate chemicals. Thus, energy is released when bonds form. Becuase enthalpy is a state variable, it follows that enery must be absorbed to break a bond. This means that all bond breaking events are endothermic. It just so happens that right after this endothermic event, an exothermic even occurrs (forming of a new bond). If this new bond is more stable than the old one, then the entire process will be exothermic.

 

Anyways, i just wanted to be clear that breaking of bonds always required energy and, hence, is always endothermic

 

 

(I know you know this wolfson, but i thought i should make the point extremely clear, so that one learning chemistry would not be confused )

[Leroy]

so let me see if i got this right

 

if the disolving enthalpy is + then it requires heat to dissolve and when it crystallizes the heat is released. And with some substances its the other way round and that's why ammonium nitrate needs heat to crystallize.

 

But what i want to know as well is why don't they use sodium chloride or any other more accesable salts, even though sodium acetate isn't hard to come by.

 

And one other thing, how do they get that strip of metal in the supersaturated solution when it is something the crystals can 'grow' on.

[VendingMenace]

Now as for the question;

since i study chemistry i'd also like to know why sodium acetate releases heat and ammonium nitrate absorbs it.

I think i will take a stab at it

 

 

So, as far as sodium acetate goes, we must first consider what is happening. WEll, the sodium acetate is forming a solid and is heating up the sorounding solution, but hte question is, "why?"

 

WEll, that is a supurb question and is excellent for demonstrating the power of thermodymanics (as it can easily be answered with just a basic grasp of enthalpy and entropy and gibbs free energy).

 

So we know that gibbs free energy of a reaction (delta G --> /_\G) is equal to the sum of the enthalpy of that reaction and the negative entropy of a reaction multiplied by the temperature at wich it happens, right? That is, we have;

 

/_\G = /_\H - /_\S (T)

 

ok, we also know that for a process to proceed spontaneously, it must have a negative delta G. Right?

 

Ok, so durring the formation of the solid, the sodium acatate is falling out of solution and forming an ordered lattice (crystals). IN the course of this, the soduim acetate is having it degrees of freedom (spacially) severely diminished (going from free in solution, to trapped in a latice). THis means that in the course of the reaction the system is loosing entropy (becoming more ordered).

 

Ok, so we know that the system is becoming more ordered, right? That must mean that /_\S is negative. But since we subtract out the entropy in the equaion, we see that the overal contribution of the entropy will be a positive number.

 

Well, we know that the reaction does proceed spontaneously (once it is started) becuase when you click the metal disc the pack gets hot. So, /_\ G must be negative. In order for /_\G to be negative, we see that /_\H must be nagative as well (in order to counter-act the positive contribution of the entropy). So we know that /_\H must be negative.

 

Well, what do we call a reaction with a negative /_\H? Exothermic ANd a reaction that is exothermic releases heat.

 

So we have just shown, thermodynamically that the reaction of sodium acetate (or any other substance, for that matter) forming crystals out of solution must be exothermic. It just so happens that for sodium acetate, it is quite exothermic.

 

 

 

 

Now as far as Ammonium Nitrate (A.N.) goes, there is not quite the same elegant explination that will show why it must be endothermic. But lets look at it anyways.

 

OK, so for A.N. what do we have? WE have a solid inside of a container that when it is broken it gets dissoved? (I think that is right). So, using the same reasoning as above, we see that when we are disolving a crystal, we are increaing the entropy of the system. That is becuase the molecules are freed from a constricted latice and put into solution.

 

Now that must mean that entropy is positive (inacreasing). That means that the contribution of entropy to the gibbs free energy will be negative. Right? WEll, if entropy is negative, enthalpy could be positive and the overall gibbs free energy would still be negative.

 

So we see that the reactoin could be negative, though it is not forced to be. Thus, in the case of A.N. it just so happens that the enthalpy of disolving actually is negative.

 

Of course it turns out that all disolving of substances is endothermic, just like all crystalizing is exothermic. But I am not quite sure that you are in a place where i could expalin it (an i am not sure that i have time to write all that right now anyways, sorry)

 

 

The take home message being that, do to a decrease in entropy durring freezing (crystalization), thermodynamics shows us that crystalization is nessesarily an exothermic reaction.

 

 

 

Hope that answers your question -- ask more if you got 'em

[Leroy]

it does clear up a lot of things, thanx

 

and i have one more question since you've said that crystallization is an exothermic process, could you say that it wouldn't be possible to make a cold pack that which can be reused like the sodium acetate pack.

 

Because the ammonium nitrate pack is irreversable as far as i know

[YT2095]

it is irreversable, unless you take out the AN and dry it thourougly, then you may use it again sure

[Leroy]

but would taking another salt say NaOH (as an example) and make it reversable is would all endothermic reactions like AN reacts be irreversable.

 

Instead of heating it like your do with the sodium acetate heat packs can't you just freeze the crystallized salts to get it back the way it was.

[YT2095]

I`m not quite sure of the question dude?

 

AN crystals will go VERY cold when you add water, dry this mix up on a heater or in dry air (it doesn`t have to be HOT) and make them back into crystals, they will again go cold when you add water, you may do this 100`s of times

 

same with NaOH, add water it will get very hot!, let them dry as the same method above untill they form crystals again, add water and it will get Very Hot! again

 

these reactions ARE reversible as is the sodium acetate, BUT you must always reactivate them again, either by Drying or by heating to re-liquify.

 

freezing AN will work, but the saturation of the soln has to be critical, it`s often used in the blue freezer packs that you stick in polystyrene boxes to keep your food cold on camping trips

[nerosrevenge]

Revisiting the hand warmer idea…what is the purpose of the little metal disk? How would one simulate the reaction caused by “clicking” it (or where would on obtain a similar metal disk?).

 

Steve.

[YT2095]

it`s purpose is to create a tiny shockwave that will slam 2 molecules together making the beginings of a crystal "Seed", once that happens, more fall into line as the crystal grows.

 

you can get such discs out of Thermal sensors, the type used to break a circuit when they get to a certain temp, the disc distorts and "snaps over" breaking the circuit. those are quite usable. or a pair of coins or marbles hit together will have the same effect, sometimes just dropping one on the floor can start it or flicking it with a pencil or something

[olmpiad]

About freezing the AN. If your goal is to end up with an ice pack, why freeze it? It takes much less of a hassle to freeze water, and it's free.

[jdurg]

About freezing the AN. If your goal is to end up with an ice pack, why freeze it? It takes much less of a hassle to freeze water, and it's free.

 

 

Heh. If you have no electricity around it's going to be a bit hard to freeze the water.