Is there an exothermic reaction which produces 1 to 2 minutes of heat at around 175 degrees Fahrenheit, no toxic gasses and is considered to be more or less safe?
closed as too broad by A.K., bon, Geoff Hutchison, Klaus-Dieter Warzecha, Jan Jun 10 '16 at 13:11
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Crystallisation of supersaturated sodium acetate is a good one, as seen in my answer here.
Another is the reaction of water with calcium oxide, which is commonly seen in self-heating coffee and soup cans. This reaction can get very hot, though.
Simplest thing that comes to my mind is dissolving sodium or potassium hydroxide in water, and/or neutralizing it. Both steps are exothermic, and also rather safe (given that you don't just blindly throw things together). Should become hot enough, I'd guess.
Use a mix of ammonium chloride, sodium nitrite and acetic acid. It produces water, nitrogen and heat.