Life is not meant to be easy, my child; but take courage: it can be delightful. George Bernard Shaw
Idea Transcript
HOT PADS AND COLD PADS Hot pads and cold pads ................................................................................................................................. 1 Hot pads ........................................................................................................................................................ 1 Reversible types ........................................................................................................................................ 2 Solidification of hydrates ...................................................................................................................... 3 Irreversible types ....................................................................................................................................... 4 Mixture enthalpy ................................................................................................................................... 4 Note 1. Why acid and water mixtures are exothermic .......................................................................... 5 Note 2. Why is acid always added to water, and not the reverse .......................................................... 5 Note 3. Why alkali and water mixtures are exothermic ........................................................................ 5 Oxidation enthalpy ................................................................................................................................ 5 Cold pads....................................................................................................................................................... 6 Comparison of hot / cold processes .............................................................................................................. 8
HOT PADS AND COLD PADS Hot/cold packs are handy items designed to provide local heat/cold. Main application is therapeutic (to alleviate pain), with desired temperatures of just above body-temperature (say 38 ºC) for heat-therapy, and just above freezing (say 1 ºC) for cold-therapy. Hot pads must be exothermic, dH0. But for the evolution of any unpowered system, its Gibbs function, dG=dH-TdS