(Image by theveryquietroom )
How many of you have applied a cold pack on your forehead before when you have a severe headache? Or perhaps you are a sportsman/woman that has used cold packs for your joints and muscles before. It certainly helps to relieve the pain isn’t it? But have you ever wondered how it works?? It is in fact Chemistry in actions!
Some cold packs contain chemicals which takes in heat when they react. This is why cold packs have a cooling effect on their surroundings, i.e. cooling effect on our forehead or our body muscles.
This phenomenon is due to the reactions that takes in and give out heat, and we call it formally as Energy Changes in Basic Chemistry studies.
Ammonium salts such as ammonium nitrate (V) dissolve in water with the absorption of heat. This example of a spontaneous endothermic reaction is the basis of those commercial product you get at pharmacy, an Ice Pack that doesn’t have to be kept in the freezer. Cool huh!!!??
These ice packs contain a small quantity of ammonium nitrate (NH4NO3) or ammonium chloride (NH4Cl), which is separated from a sample of water by a thin membrane.. When the pack is struck with the palm of our hand, the membrane is broken, and the salt dissolves in the water.
NH4NO3(s) + H2O(l) –> NH4+(aq) + NO3-(aq)
Enthalpy Change = 25kj/mol
or
NH4Cl(s) + H2O(l) –> NH4+(aq) + Cl-(aq)
Enthalpy Change = 20kj/mol
Because the reaction is Endothermic (+ve Enthalpy change), it absorbs heat from its surroundings, and the ice pack (which?is the plastic membrane)?can get cold enough to treat minor athletic injuries and headache.
More cool stuff about chemistry-related phenomenons will come soon. Stay Tuned!
I am designing acold pack and i would like to know the environmental perspectives of ammonum chloride.
Thanks a lot u are truly a genius. You make it look so easy
Great content, really good stuff.
Regards, Harrison Fwaaa
Wouldn’t it be an exothermic reaction since it’s losing heat?
An endothermic reaction would be for a hot pack because it absorbs heat. As for the enthalpy change, it would need a negative sign in front of it and would appear on the right hand side of the equation displaying that it is exothermic.