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Thermodynamics/Which chemical(s) will produce a certain exothermic reaction the best?

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QUESTION: Dear Kevin,

Suppose I'm constructing a disposable flask which holds a liquid (let's assume its water). Which chemicals, held between the walls of the flask, could produce an exothermic reaction to heat up the liquid to about 70-80'C (hot enough for teas and coffees), within a few minutes?

It would be ideal if such chemicals:

1. Aren't toxic nor react violently (just efficiently releasing energy as heat energy, not kinetic or sound energy).

2. Are easy for a layman to obtain and use, and use with a product (so hopefully none of the chemicals nor their chemical products are corrosive or emit toxic fumes, or are otherwise hazardous).

3. Are very cheap to obtain.

Thank you for your time,

Li

ANSWER: Hi Li,

Commercial self-heating cans use one of three exothermic reacions.

1. CaO + H2O -> Ca(OH)2

2. CuSO4 + Zn -> ZnSO4 + Cu

3  CaCl2  + H2O ->  CaCl2 (solution)


I hope this helps.
Best wishes
Kevin

---------- FOLLOW-UP ----------

QUESTION: Hi Kevin, thanks for your help. Am I correct in thinking that the first reaction/equation is the most suitable?

The first one has cheap and easily obtainable reactants (Calcium Oxide and water), and relatively little will have to be used (only 56g/mol of Calcium Oxide needs to react with 18ml/mol of H2O, compared with 111g/mol of Calcium Chloride). Also, the Zinc and Copper (II) sulphate (aq) may be relatively more expensive than the others (I don't know for sure though).

However, the third equation has a greater change of enthalpy of formation (which I assume means that I will get more heat out of it when the reaction takes place, compared to the first equation. I'm also assuming that the product is Calcium Chloride Monohydrate, as I can get even greater amounts of heat energy from the more hydrated versions of Calcium Chloride).

I don't know which equation produces the most efficient reaction. My knowledge in chemistry is not good. What are your thoughts Kevin? I hope to increase the water temperature in my container from 20'C/room temperature to 70'C-80'C within 4-5 minutes.

Thank you :)

Li

Answer
Hi Li,

The most suitable combination for the home experimenter is anhydrous CaCl2(s) + H2O(l) It is least toxic, cheap and least corrosive.

Exothermic chemical reactions can get VERY hot, very quickly so wear suitable gloves and eye protection. Think of any heat effects on working surfaces. Also it is best to add the solid to water rather than the other way round to avoid steam generation.

I hope this helps
Best wishes.
Kevin

Thermodynamics

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Kevin

Expertise

Thermodynamics, chemistry, chemical reactions, kinetics, chemical reaction safety, dust explosion technology, static electricity. General science.

Experience

Over 40 years experience as a practicing thermochemist in industry. Head of the fire and explosion laboratory of a major European chemical company (Ciba-Geigy). Now retired.

Organizations
Institute of Chemical Engineers. Royal Society of Chemistry.

Education/Credentials
Chartered Chemist, Member of the Royal Society of Chemistry (C.Chem MRSC). Msc Sheffield.

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