Words: 1065
* . Introduction Endothermic reactions are accompanied by the absorption of heat. The dissolving of ammonium nitrate in water is an example of an endothermic reaction. The solution resulting from this mixture is colder than either the ammonium nitrate or the water. This is the simple explanation of what happens in an instant ice pack. The more detailed information will be discussed in the following paragraphs. Cold Packs Most cold packs come with a fabric cover made to absorb condensation and to protect the skin from contacting the surface of the cold pack.
Cold packs are used for injury or muscle relief. The cold pack contains two bags one containing water and the other containing ammonium chloride. Once the ammonium chloride is broken it reacts with water causing a endothermic reaction. Cold packs remain cold for different lengths of time through the reaction extending up to fifth teen minutes precession, giving users relief from inflammation and pain at the site of injury. Many cold packs are reusable, by refreezing the e pack. Once the pack is refrozen it contains stability in the reaction.
It can then be reapplied to the injury causing an endothermic reaction giving comfort to the surrounding area. Cold Pack Reactions Cold packs have a endothermic reaction that usually contains water and a packet of ammonium chloride. The Cold pack is activated by breaking the barrier separating the water and ammonium allowing the chemicals to mix. Once the cold pack is placed to the skin, energy is released keeping the cold pack reacting for a period of time. Endothermic reaction absorbs heat causing the cold pack to keep reacting until the cool of the pack is warm up to the surrounding area.
Endothermic reaction is the opposite of exothermic reaction absorbing the surrounding heat. When the ammonium chloride reacts with water it has a chemical reaction that will give off a cold effect, when soaking up heat from the body. How Cold Packs Work The cold packs have two bags, a strong outer plastic bag with a thin plastic bag inside. When the inner bag is squeezed the bag ruptures, and the water it contains mixes with the ammonium nitrate and the endothermic reaction occurs. This cooling reaction occurs instantaneously but lasts for about an hour.
How does this reaction sustain itself for twenty minutes? This question will be answered later in this explanation. The type of cold pack presented here contains ammonium nitrate, which is a white crystalline substance. When these crystals react with water they split into positive ammonium ions and negative nitrate ions. Due to the energy expended by the water in order to dissolve these crystals, the water becomes colder. Chemical Reaction Ammonium nitrate (NH4NO3), is classified as a salt. The salt family of chemicals contains ions, which are particles with electrical charges.
Due to the fact that opposite polarity ions attract each other, they form a solid crystal that is called salt. This when seen by the naked eye seem like a simple reaction. Bu, in reality this occurs in two different steps and each step a change of energy takes place. The first step is the separation of the solid crystals back into separate ions, a positive ammonium ion and a negative nitrate ion. The break these ionic bonds requires a lot of energy which means heat must be taken from the surrounding water. The second step the water molecules, which are H2O, are attracted to the ions and attach themselves to the ions.
The second step actually causes heat to be produced to the surrounding liquid mixture. .Even though several water molecules may attach themselves to each ion as indicated in the second equation below the overall chemical reaction causes the surrounding liquid to become colder. Step 1: NH4NO3 + Heat ? NH4 + + NO3 – Step 2: NH4 + + NO3 – + H2O ? NH4 + (H2O)x + NO3 – (H2O)x + Heat The first step requires six kilocalories per mole of ammonium nitrate to occur and since this absorbs the heat from the water the surrounding liquid is significantly colder.
Entropy Heat is usually produced in a stored bond relation so the reaction that occurs in cold packs does not seem to be possible. Since this is an endothermic reaction the chemical bonds that form actually have more stored energy than what they had before the reaction. This does not seem to follow the natural order of things. When water is dissolving the ammonium nitrate it spontaneously gets cold. Why does this occur? According to chemists this occurs due to a process called entropy. The measurement of disorder in a system is entropy.
In chemical reactions the molecules and ions change rapidly from orderly arrangements to disorderly ones. The disorder for some reason is escalated by nature itself. When this occurs in the majority of reactions to increase entropy, heat must be released or an exothermic reaction occurs. But when this disorder occurs at great levels, thus increasing entropy greatly, the heat flow is reversed, an endothermic reaction occurs. This only occurs when the reaction allow the entropy to increase greatly. Let us examine the cold packs again, the materials are orderly arranged.
The pure water in the inner bag and the ammonium nitrate arranged in solid crystals in the outer bag, each substance was stored in its own container and orderly arranged. Then the inner bag is suddenly ruptured and the water and ammonium nitrate ions change suddenly into a disorderly reaction. This orderly arrangement of ions in the inner and outer bag now contaminate each other, the once pure water now mixing with both separated ions. This large disorder resulted in a large change in entropy, which caused an endothermic reaction to occur.
This large change of entropy instantly cools the water to about 36 degrees and maintains this reaction for about 20 minutes. Conclusion This paper has explained the various chemical reactions and makeup of an instant ice pack. First it covered the how the product was made. Then it was discussed at length the type of reaction occurring, on both the visible and molecular level. After that the chemical bond changes were discussed. Finally, the entropy discussion allowed the reader to understand the chemical and physical reactions of a cold pack as it pertains to nature.