It’s in the bag – Energetics and Consumer Products

Does your elbow hurt from too much tennis?  Are your hands or feet cold after a day of skiing?  Pull out a cold pack or a hot pack.  How do they work?

Cold pack generally contains the salts of ammonium nitrate and water in two separate packages.  When the inner bag containing water is broken, ammonium nitrate, which is readily soluble in water, dissolves.  The dissolution process, however, is strongly endothermic, and heat is transferred from the surroundings – your elbow – to the system – the dissolving salt.
NH4NO3(s)  +  aq.    NH4+(aq)  +  NO3-(aq)
											H = +25.69 kJ mol-1

Hot packs come in several varieties, one of which uses sodium acetate, CH3CO2Na.  When the salt crystallizes from water, heat is evolved and is transferred to the surroundings.
	Na+(aq)  +  CH3CO2-(aq)  +  3H2O(l)    CH3CO2Na . 3H2O(s)
												H = -37.86 kJ mol-1

Anther variety of hot pack uses an oxidation-reduction reaction.
	4Fe(s)  +  3O2(g)    2Fe2O3(s)
												H = -1648.4 kJ

This very exothermic reaction allows the hand to maintain a temperature of 57 oC to 69 oC for several hours if oxygen flow is somewhat restricted, as by keeping the warmer in a glove or pocket.

It’s in the bag – Energetics and Consumer Products

Does your elbow hurt from too much tennis?  Are your hands or feet cold after a day of skiing?  Pull out a cold pack or a hot pack.  How do they work?

Cold pack generally contains the salts of ammonium nitrate and water in two separate packages.  When the inner bag containing water is broken, ammonium nitrate, which is readily soluble in water, dissolves.  The dissolution process, however, is strongly endothermic, and heat is transferred from the surroundings – your elbow – to the system – the dissolving salt.
NH4NO3(s)  +  aq.    NH4+(aq)  +  NO3-(aq)
											H = +25.69 kJ mol-1

Hot packs come in several varieties, one of which uses sodium acetate, CH3CO2Na.  When the salt crystallizes from water, heat is evolved and is transferred to the surroundings.
	Na+(aq)  +  CH3CO2-(aq)  +  3H2O(l)    CH3CO2Na . 3H2O(s)
												H = -37.86 kJ mol-1

Anther variety of hot pack uses an oxidation-reduction reaction.
	4Fe(s)  +  3O2(g)    2Fe2O3(s)
												H = -1648.4 kJ

This very exothermic reaction allows the hand to maintain a temperature of 57 oC to 69 oC for several hours if oxygen flow is somewhat restricted, as by keeping the warmer in a glove or pocket.