solkoe
February 20, 2008, 8:48pm
1
These are the bags that are use to treat injuries. You squeeze the bag to break the inner bag and the chemicals mix.
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]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.
http://dopamine.chem.umn.edu/chempedia/index.php/Cold_Packs/Hot_Packs