Question

How do hot packs work?

Answer 1

Typical air-activated hot packs release heat through the reaction between iron powder and oxygen.

Explanation:

Typical air-activated hot packs contain the following substances [1]:

• Fine iron powder
• Activated carbon (like graphite)
• Salt (e.g., table salt `"NaCl"`)
• Water along with absorbent

The reaction mostly responsible for releasing heat in a hot pack would be the exothermic oxidation of iron:
`4"Fe"(s)+3"O"_2(g)->2"Fe"_2"O"_3(s)` `Delta"H"=-1648 color(white)(l)"kJ"*"mol"^(-1)` [2]

Each mole of the reaction involves six moles of electrons transfer and releases `1,648 color(white)(l)"kJ"` of energy. The reaction is so exothermic that it is also applied to make thermites.

Activated carbon, salt, and water catalyze the reaction by creating down-scaled galvanic cells within the hot pack. Salts would dissolve in water and act as electrolytes to promote the conductivity of the solution. The drenched mixture serves as a binding site for atmospheric oxygen come into contact with iron, which is powdered to maximize the contact area with oxygen. The configuration help ensure a reasonable rate of heat release. Additionally, activated carbon serves as a conductor of heat maintaining a relatively uniform temperature throughout the packet.

References
[1] What’s inside disposable hand warmers? Chemical & Engineering News, https://cen.acs.org/articles/88/i4/Hand-Warmers.html
[2] The most exothermic reaction,
http://people.bu.edu/straub/courses/demomaster/thermite.html