Cold Packs Introduction : When many chemicals are dissolved in

water, they steal energy from their surroundings. The net effect is that the surroundings get cold. This kind of process is called "endothermic reaction". The chemical has a "positive heat enthalpy".

A number of commercial cold packs consist of ammonium chloride in one bag and water in another bag. Breaking the ammonium chloride bag will cause it to dissolve in water. Since it has a large positive heat of solution, the surroundings (a sprain for instance) will get cold.

Commercial ProductCommercial Product

Cold packs:

TheoryTheory

When the ammonium chloride is dissolved in water to from infinitely dilute aqueous solution , there is a enthalpy change of solution .

In particular , the enthalpy of solution of ammonium chloride is positive , which indicate that energy is taken from the surrounding , therefore if we put the reaction bag on our skin , we can feel cold as energy is taken from our skin into the solution.

Enthalpy change diagram

ProceduresProcedures10 grams of ammonium chloride

( white powder ) is put into a stout plastic bag ( Glad - Lock Storage Bag )

A second smaller & thinner bag which is full of water ( 50g ) is put into the Glad - Lock Storage Bag , then seal the bag properly to make sure no leakage

When cold is required , the user can simply give the bag a sharp blow to burst the inner bag of water .

Put the bag on your skin , then you can feel the bag is cold as a result of temperature drop of the solution

DiscussionDiscussionWhen the ammonium chloride dissolves in

water ,a strong endothermic reaction is taken place . The actual temperature drop depends on the quantity of ammonium chloride and water.

For example : 10g of ammonium chloride (M=53.50) in 50g of water

10g of ammonium chloride is 0.187 moleTotal heat absorbed = 0.187 x 15.2 KJ

= 2.842 KJ

On our skin

Heat = m c (T)

= 0.08 ( 4.2 ) ( T )

- 2.842 = 0.05 (4.2 ) ( T )

T = - 13.5 K

This temperature drop is cool

enough to release pain.

C=specific heat

capacity of water

Reduce inflammationReduce the risk of internal bleedingReduce painAids in slowing down cellular activity

in order to reduce bacteria production due to inflammation.

Brings on temporary

The medical use of cold The medical use of cold packs :packs :

Reusability of the cold Reusability of the cold packspacks

Actually the cold pack is not reusable as we can not easily ‘undissolve’ the ammonium chloride . However , ammonium chloride is quite cheap and the only other ingredients are water and plastic bags.

Shung Tak Catholic English CollegeShung Tak Catholic English College

May, 1999May, 1999