Why hand warmers get hot
Oxidation of iron · Redox · Exothermic
A hand warmer is a packet of rust-in-a-hurry: the same reaction that eats bridges, sped up to keep your fingers warm.
Reactants approach
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ΔH = -1,648 kJ · exothermic, releases heat
ΔH per 4 mol Fe.
Meet the molecules
- Reactantsolid
Fe
iron
- Reactantgas
O2
oxygen
- Productsolid
Fe2O3
iron(III) oxide
What you notice
Tear open a disposable hand warmer and within a few minutes it reaches about 50–60 °C, staying warm for several hours. The pouch holds iron powder, salt, water soaked into vermiculite, and activated carbon, behind a cover that lets air in slowly. The heat comes from rusting, the same reaction that eats away car bodies and bridges, made fast by using a powder with a huge surface area and salt water to carry ions. Seal a warm pack in an airtight bag and it cools down, because it runs out of oxygen.
Try it
Open a hand warmer, let it heat up, then seal it in a zip bag with the air squeezed out; feel it cool over the next hour, then open the bag and feel it warm again.
Safety: Hand warmers can cause low-temperature burns if held against bare skin for a long time, so keep them in a glove or pocket.
Step by step
- 1
Air gets in
Opening the outer wrapper lets oxygen diffuse through the porous inner pouch to millions of tiny iron grains.
- 2
Iron loses electrons
At the surface of each grain, iron atoms give up electrons and become Fe2+ ions. Salt water acts as an electrolyte, letting ions move between nearby sites.
- 3
Oxygen gains them
O2 molecules, together with water, take up those electrons to form hydroxide ions, which combine with the iron ions.
- 4
Rust and heat
Further oxidation turns Fe2+ into Fe3+, ending as hydrated iron(III) oxide, written here simply as Fe2O3. Every 4 mol of iron (223 g) releases about 1650 kJ.