Instant heat pack

PAD HEAT TREATMENT INSTANT HEAT PACK KWIK HEAT BECOMES 110 DEG 4 6530013813648 PG 14.22 ... 

You are designing an instant heat pack for a food application. Your design includes a nontoxic ionic salt that dissolves exothermically in water. You need to control the amount of heat released. Initial prototypes get too warm, even with a fairly small amount of salt used. What else could you do to reduce the heat released in your product Identify any key limitations that would have to be met. 

A process that releases heat into the surroundings is called an exothermic process. Most common chemical reactions—and all combustions, such as those that power transport and heating—are exothermic (Fig. 6.8). Less familiar are chemical reactions that absorb heat from the surroundings. A process that absorbs heat is called an endothermic process (Fig. 6.9). A number of common physical processes are endothermic. For instance, vaporization is endothermic, because heat must be supplied to drive molecules of a liquid apart from one another. The dissolution of ammonium nitrate in water is endothermic in fact, this process is used in instant cold packs for sports injuries. 

Heats of solution are the basis for instant cold packs and instant hot packs used for the first-aid treatment of minor sprains and pulled muscles. These packs have two separate compartments. One contains water, and the other contains a salt NH4 NO3 for cold packs and MgSO. or CaCl2 for hot packs. Kneading the pack breaks the wall between the compartments, allowing the salt to mix with water. As the salt dissolves to form an aqueous solution, the temperature of the pack changes. Heat is absorbed or released only as the salt dissolves, however, so after all of the salt has dissolved, the pack gradually returns to room temperature. Further manipulation of the pack has no effect. 

To provide cold therapy for cuts, bruises, sprains, and lacerations, an instant ice pack for first-aid treatment uses a chemical reaction that requires heat in order to occur. An instant Ice-Pack contains two compartments — one containing liquid water, the other a solid. The pack is activated by squeezing the liquid compartment to break an inner seal that permits the mixing of the two compartments. Heat is withdrawn from the surroundings by the reacting chemicals, lowering the temperature of the ice-pack contents. 

An instant ice pack for first-aid treatment uses the endothermic nature of the dissolution of an ionic salt in water to provide cold therapy. Two typical materials that absorb heat as they dissolve in water are ammonium nitrate and ammonium chloride ... 

Assuming that we start at 298 K, the final T will be around 270 K or —3°C or 25°F. Brrr Our calculation has also involved a number of other assumptions, including that we have assumed a temperature-independent enthalpy of reaction and a temperature-independent heat capacity for the water. We have also assumed that the water does not freeze (would release some heat). Nevertheless, the calculation gives a fairly reasonable estimate of the temperature drop that provides the cooling therapy of an instant ice pack. 

An instant cold pack has a vial of ammonium chloride inside a bag that contains water. To activate the cold pack, the inner vial of ammonium chloride is broken, and the ammonium chloride dissolves in the water. Since the heat of solution for ammonium chloride is endothermic, this causes the system to get colder. 

Solid ammonium nitrate, NH4NO3, dissolves in water in a very endothermic process, absorbing heat from its surroundings. It is used in instant cold packs for early treatment of injuries, such as sprains and bruises, to minimize swelling. 

The dissolution of anhydrous calcium chloride, CaCl2, in water is quite exothermic. This dissolution process is utihzed in commercial instant hot packs for quick treatment of injuries requiring heat. 

In both cases, the salt and water are separated by a thin membrane. All you have to do is squeeze the pack to mix the components and you have instant heat or cold at your fingertips. 

Athletic trainers use instant ice packs that can be cooled quickly on demand. Squeezing the pack breaks an inner container, allowing two components to mix and react. This reaction makes the pack become cold. Describe the heat flow for this spontaneous process. 

A Figure 13.5 An instant ice pack, containing ammonium nitrate, used to treat athletic injuries. To activate the pack, the container is kneaded, breaking the seal separating solid NH4NO3 from water. The heat of solution of NH4NO3 is positive, so the temperature of the solution decreases. 

Ammonium nitrate (NH4NO3) dissolves in water with an endothermic heat of reaction of 26.4 kJ mol. How much ambient heat will be absorbed when 2.0 kg of ammonium nitrate dissolves in water (This reaction is utilized in instant ice packs for treating injuries. Solid ammonia nitrate is contained inside a thin-walled plastic bag, which is sealed inside a thicker bag that also contains some water. The inner bag can be broken by pressing the outer bag, thereby allowing the ammonium nitrate to form a solution with the water which gets quite cold.)... 

Athletes know that the apphcation of heat or cold to a strain or sprain usually relieves the pain and may lessen the severity of the injury. Instant hot and cold packs allow you to quickly and easily apply the appropriate remedy to the injury. 

Instant hot and cold packs create aqueous solutions that form exothermically or endothermically and therefore release or absorb heat. A hot pack generates heat when a salt such as calcium chloride dissolves in water that is stored in the pack. The calcium chloride dissolves exothermically. A cold pack absorbs heat when a salt such as ammonium nitrate dissolves in water. The ammonium nitrate dissolves endothermically. 

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