Cold Packs/Hot Packs

From Chempedia

Heat therapy has become a standard treatment for ailing muscles among athletes, the disabled and elderly people. Heat packs provide relief by dilating the blood vessels of nearby muscles and allowing the soft tissue to stretch, and cold packs reduce the swelling and inflammation of injured body parts.[1]Chemical advances have led to the development of very convenient types of heat packs and cold packs, which can now be found in most emergency first aid kits.

Advantages/Disadvantages

Chemical heat packs have become more common than traditional hot water bags or electrical pads. This is because of the convenience, immediacy, and portability of chemical heat packs, which do not require electrical outlets or external heat sources. Chemical heat packs are also much safer, as they will not burn or electrocute the user. These packs are also reusable, a key benefit for health care providers. Chemical cold packs individually create instant coldness making them ideal in emergencies. Both chemical cold packs and chemical heat packs contains elements that allow them to sit in a first aid kit for months without losing potency.[2]

While heat packs are made using food grade salts and are very safe, cold packs are made using ammonium nitrate used in cold packs. Ammonium nitrate is very toxic and explosive because of its high electronegativity, and the O2 produced when it decomposes.[3]O2 is extremely volatile and is used as fuel in explosive reactions such as those used to launch the space shuttle. If an ammonium nitrate cold pack is heated, the pack will explode, as shown below in equation 

   2NH4NO3(s) + heat → 4H2O(g) + N2(g) +  O2(g) (1)

The possible dangers of an ammonium nitrate cold pack make it a less desirable alternative to the traditional ice packs that are stored in a freezer, if a freezer is available.

Heat Packs

There are a number of types of chemical heat packs used. Some packs consist of two plastic bags, the inner bag contains water, and the area between the inner bag and the outer bag is filled with a dry salt. When the inside bag is broken, the solid and the water react in an exothermic reaction, releasing heat. However, the most frequently used heat packs today involve one sealed plastic pouch containing a metal disk and a salt solution; commonly calcium chloride, magnesium sulfate, or sodium acetate.[2]

Metal Disk[7]
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Metal Disk[7]
The salt solution, typically sodium acetate, is supersaturated, containing more than the equilibrium amount of dissolved solute at a specific temperature. The sodium acetate is also a supercooled solution, therefore is cooled below its freezing point without transforming into a solid.[4] Sodium acetate has a freezing point of 54°C, or 129°F, but can easily exist as a supercooled liquid at room temperature.[3] The supersaturated, supercooled solution of sodium acetate is unstable and will crystallize if a seed crystal is present. A seed crystal is usually a very small solid portion of the material that will crystallize.[5] The sodium acetate comes out of a solution and solidifies as an orthorhombic shaped crystal.[3] This crystallization is exothermic, meaning it releases heat, and the heat of solution is negative (∆H<0).
Triggering Mechanism[7]
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Triggering Mechanism[7]
This reaction occurs even though the entropy of the system is decreasing because the entropy of the surroundings increases by an even greater amount, as expected according to the Second Law of Thermodynamics. The return of sodium acetate to solid form rapidly warms them to approximately 54°C, or 129.2°F. (The packs are insulated in order to prevent any burning.) It will stay at this temperature until all of the sodium acetate has crystallized and the stored chemical energy has been completely converted to thermal energy.

The crystallization process mentioned above is initiated using a small concave metal disk inside the plastic pouch. This disk is usually made of iron, and has minuscule slits along its surface that contain seed crystals of sodium acetate. These seed crystals act as nucleation sites for the precipitation of the aqueous sodium acetate in the pouch. Nucleation is defined as the condensation of a vapor or solid due to the presence of a small particle or molecule.[5]

The process of nucleation is illustrated the graph below. It is easy to see that once the seed crystal is introduced, such as with the snapping of the concave disk inside a heat pack, the crystallization process proceeds very quickly and the solution moves towards saturation and releasing heat (Points 4-7 on the graph).[5] Note that the graph displays the relationship between concentration and temperature of the system, not the surroundings. Thus, as the solution crystallizes, the system loses heat and the surroundings gain heat; resulting in the warm feeling of a hot pack.

Progression of Crystallization[5]
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Progression of Crystallization[5]

One of the main advantages of chemical hot packs is their reusability. The solid pouch can be submerged in boiling water to bring the crystallized sodium acetate back to a liquid state.[2] Again, this can be seen in the diagram above; movement along the curve in the direction of increasing temperature restores the ’metastable’ or supersaturated state. When boiling the pouch it is crucial to allow enough time for the entire crystalline lattice to be broken back into solution or it will quickly resolidify.

Cold Packs

A cold pack comes in a plastic bag made of tough white plastic. This bag is filled with a smaller bag and ammonium nitrate crystals. The smaller bag contains water, and is made of a thin weak plastic, so it is easy to break. When a cold pack is used, it must be "broken" by rupturing the inner bag. Breaking the bag releases the water, which dissolves the ammonium nitrate. The water and ammonium nitrate react completely, and within fifteen to twenty the cold pack will no longer feel cold.

Cold packs make use of the heat transfer that occurs during chemical reactions, but in contrast to heat packs, utilize endothermic reactions. In the endothermic reaction between ammonium nitrate crystals and water, the heat required for the reaction to proceed from reactants to products is absorbed from the surrounding environment, resulting in a decrease in temperature of the pack noticeable to the touch. This reaction, equation (2) 

   NH4NO3(s) + H2O(l) + heat  → NH4+(aq) + NO3-(aq)     (2)
Cold Packs[8]
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Cold Packs[8]

occurs spontaneously despite the positive change in enthalpy because the change in entropy is more influential.[9] The heat lost by the surroundings of the cold pack result in a lowering of the entropy of the surroundings and even larger increase in entropy of the system. The system becomes so much more disordered with the dissolved ions than the original crystal powder that the change in enthalpy is negated. By the Second Law of Thermodynamics, the entropy of the universe is always increasing; ∆Ssys + ∆Ssurr > 0. Therefore, for a spontaneous reaction to occur given that ∆Hsurr > 0, and ∆Ssurr < 0, ∆Ssys >> 0 has to be true.[6] Chemical cold packs all make use of this incredibly convenient, useful occurrence. The spontaneity of the reaction provides an expedient means of providing medical care in emergencies.

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Footnotes

  1. ^ Spine-health.com. health.com/topics/conserv/heat_therapy_and_ice/heat_therapy_and_ice.html-http://www.spine-health.com/topics/conserv/heat_therapy_and_ice/heat_therapy_and_ice.html (accessed October, 2005).
  2. ^ Howstuffworks.com. http://www.howstuffworks.com/question290.htm (accessed September, 2005).
  3. ^ Answers.com. acetate%22 (accessed September, 2005).
  4. ^ Wikipedia. [1] (accessed November 2005).
  5. ^ GlobalSpec. [2] (accessed September, 2005).
  6. ^ Silberberg, Chemistry The Molecular Nature of Matter and Change, 3rd Ed; McGraw-Hill: New York, 2003; chapter 13 & 20.
  7. ^ Rogerson, Mansel A., and Silvana S.S. Cardoso. "Solidification in Heat Packs: III. Metalic Trigger." AIChE Journal. \[Online\] 2003, 49, 522-529.
  8. ^ Dry Pack Industries. [3] (accessed November 2005).
  9. ^ Cengel, Yunus A. Introduction to Thermodynamics and Heat Transfer; McGraw-Hill: New York, 1997; pp. 243-245.

10. Science.ca. [4] (accessed September, 2005).
11. Answers.com. chloride (accessed September 2005).
12. Wikipedia. [5] (accessed October 2005).

Researched and Written by Patricia Schlaefer, Scott Seaburg, Megan Spear, JongHak Seo, and Lindy Simonsen

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