Weird Science

Endothermic Process

Polish ver­sion is here

Some­thing to Cool You Down

An endo­ther­mic reac­tion is a che­mi­cal reac­tion that results in a net absorp­tion of heat from its sur­ro­un­dings. This means that as the reac­tion pro­gres­ses, it draws in ther­mal energy. It is the oppo­site of an exo­ther­mic reac­tion, an exam­ple of which I discus­sed here. Con­trary to com­mon belief, endo­ther­mic reac­tions do not always requ­ire exter­nal hea­ting to pro­ceed. In many cases, addi­tio­nal pro­ces­se­s—such as entropy chan­ges or exo­ther­mic dis­so­lu­tio­n—oc­cur along­side the reac­tion, mea­ning that the ove­rall heat exchange with the envi­ron­ment may still be posi­tive.

There are many exam­ples of endo­ther­mic reac­tions. In this expe­ri­ment, we will focus on the dis­so­lu­tion of ammo­nium nitrate.

What Do We Need?

We will need the fol­lo­wing mate­rials:

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Tap water will do just fine. But where can we get this exo­tic-soun­ding che­mi­cal com­po­und? It turns out that ammo­nium nitrate is the main ingre­dient in a com­mon nitro­gen fer­ti­li­zer. It is rea­dily ava­i­la­ble at well-stoc­ked gar­den sup­ply sto­res.

Ilustracja

Source: http://www.anex-wie­li­chowo.pl/oferta-Nawozy,1,nazwa-Sale­tra-amo­nowa-ZAK,66.html, acces­sed: 10/12/2011

While this is not pure ammo­nium nitrate, as it con­ta­ins small amo­unts of anti-caking agents, it is still per­fec­tly sui­ta­ble for our expe­ri­ment. Ammo­nium nitrate is usu­ally found in pow­de­red or gra­nu­la­ted form, ran­ging in color from white to off-white. It is highly hygro­sco­pic, mea­ning it absorbs moi­sture from the air, so it must be sto­red in a tigh­tly sea­led con­ta­i­ner. To mea­sure tem­pe­ra­ture chan­ges, we will also need a ther­mo­me­ter. A medi­cal ther­mo­me­ter won’t work, as its range is too nar­row. Once we have eve­ry­thing, we can pro­ceed with the expe­ri­ment.

War­ning: While ammo­nium nitrate used in this expe­ri­ment is not highly toxic, gene­ral safety pre­cau­tions sho­uld always be fol­lo­wed when wor­king with che­mi­cals. The author assu­mes no respon­si­bi­lity for any poten­tial damage or inju­ries. You pro­ceed at your own risk!

Pre­pa­ra­tion

Pour a small amo­unt of room-tem­pe­ra­ture water into a con­ta­i­ner and mea­sure its tem­pe­ra­ture with a ther­mo­me­ter:

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As we can see, the water tem­pe­ra­ture is about 23°C (73°F). Now, add a few tea­spo­ons of ammo­nium nitrate and stir. You will feel a distinct drop in the solu­tion’s tem­pe­ra­ture when tou­ching the con­ta­i­ner. Let’s take ano­ther tem­pe­ra­ture rea­ding:

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The tem­pe­ra­ture has drop­ped to just 7°C (45°F), a decre­ase of 16°C (29°F)! By mixing ammo­nium nitrate with ice, we can achieve tem­pe­ra­tu­res as low as -15°C (5°F) or even lower. This makes it an excel­lent coo­ling mixture.

Why Does This Hap­pen?

The pro­cess occur­ring here is the dis­so­lu­tion of ammo­nium nitrate in water. This com­po­und is highly solu­ble, with 214 g (7.55 oz) of NH4NO3 dis­so­lving in 100 g (3.53 oz) of H2O at room tem­pe­ra­ture (about 25°C or 77°F). The energy requ­i­red for this dis­so­lu­tion is absor­bed from the sur­ro­un­ding envi­ron­ment in the form of heat. As a result, the solu­tion beco­mes noti­ce­a­bly col­der.

Fur­ther rea­dings

Marek Ples

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