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Have an experiment I want to try out utilizing thermoelectric power and I want to generate the heat via a simple chemical reaction. Ideally I'd like to mix 2 liquids together (one being water or alcohol would be great) that can get to around boiling water temperature for a few minutes or longer. Byproducts need to be only other liquids that can be drained easily or gasses that aren't dangerous to humans. No solids wastes and non corrosive / carcinogen / caustic.

So far I've found one that's close to my needs but still produces a solid waste product and isn't particularly safe, which is Calcium Oxide + Water -> Calcium Hydroxide + Heat. This might work if there's another liquid I could add after the main reaction to turn the Calcium Hydroxide into a liquid for disposal.

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    $\begingroup$ I think the most helpful thing I can say here is that this seems unsafe. $\endgroup$
    – Zhe
    Dec 22, 2016 at 19:32
  • $\begingroup$ It would only be unsafe if it didn't fit my requirements. The reaction would take place in a beaker surrounded in thermoelectric mesh and once the reaction is done it would be drained to another tank for safe disposal. $\endgroup$
    – ABlankenship
    Dec 22, 2016 at 19:46
  • $\begingroup$ Concentrated sulfuric acid plus water would do a great job. It would readily kill your or gouge out your eyes, too. $\endgroup$
    – Ivan Neretin
    Dec 22, 2016 at 19:57
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    $\begingroup$ I've worked in lab long enough to know if you are generating that big of an exotherm that you hope to make sustained, usable electric power from it, a beaker and some mesh is not even close to counting as containment. $\endgroup$
    – Zhe
    Dec 22, 2016 at 20:07
  • $\begingroup$ Good point! Look at them chemists, what do they use when they have access to all those fancy reagents and want to generate some intense, but controllable heat? Electric heater, that's what. $\endgroup$
    – Ivan Neretin
    Dec 22, 2016 at 20:15

2 Answers 2

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The first thing that comes to mind, if you have access to a stock-room, is mixing two solutions: (1) NaOH; (2) HCl. This can release a lot of heat if your solutions are concentrated enough, and it forms salt-water if your NaOH and HCl are of the same molarity.

Edit: I'm on my lunch break, so I did some of the math...

The chemical reaction of interest in this case is:

$$\ce{OH- + H+ -> H2O}$$

The standard enthalpies of formation ($\Delta H_\mathrm f^\circ$) of these species are:

$$\begin{array}{lr} \hline \text{Species} & ΔH_\mathrm f^\circ/\pu{kJ mol-1} \\ \hline \ce{OH-} & -229.99 \\ \ce{H+} & 0.00 \\ \ce{H2O} & -288.83 \\ \hline \end{array}$$

Thus, the change in enthalpy for the reaction is:

$$\Delta H^\circ = -288.83\ \mathrm{kJ/mol} - (-229.99\ \mathrm{kJ/mol}+0\ \mathrm{kJ/mol}) = -58.84\ \mathrm{kJ/mol}$$

Therefore, for $1\ \mathrm{mol}$ of $\ce{NaOH}$ + $1\ \mathrm{mol}$ of $\ce{HCl}$, you get $58.84\ \mathrm{kJ}$ of heat. Say you want to release enough heat to get the net solution up to $100\ \mathrm{^\circ C}$.

Water has a heat capacity of $4.18\ \mathrm{J/(g\ ^\circ C)}$. Say you have $1\ \mathrm L$ of $\ce{NaOH}$ + $1\ \mathrm L$ of $\ce{HCl}$, you'll need enough heat to raise the temperature of $2\ \mathrm L$ of water to $100\ \mathrm{^\circ C}$. I'll assume the water starts off at $25\ \mathrm{^\circ C}$, so you have $2\,000\ \mathrm g$ and $75\ \mathrm{^\circ C}$ to go.

$$4.18\ \mathrm{J/(g\ ^\circ C)} \cdot 2\,000\ \mathrm g \cdot 75\ \mathrm{^\circ C} = 62\,700\ \mathrm J = 62.7\ \mathrm{kJ}$$

How many moles of $\ce{NaOH}$ + $\ce{HCl}$ do you need for that much heat?

$$\frac{q}{\Delta H^\circ} = \frac{62.7\ \mathrm{kJ}}{58.84\ \mathrm{kJ/mol}} = 1.065\ \mathrm{mol}$$

That would mean that you can mix $1\ \mathrm L$ of $1.065\ \mathrm M$ $\ce{NaOH}$ + $1\ \mathrm L$ of $1.065\ \mathrm M$ of $\ce{HCl}$, and would theoretically expect to get a temperature close to $100\ \mathrm{^\circ C}$.

This might be off a bit because I've made some assumptions:

  1. $\Delta H^\circ$ is constant with respect to temperature from $25\ \mathrm{^\circ C}$ to $100\ \mathrm{^\circ C}$. This may not be true.
  2. the dissolved salts in water dont significantly affect its heat capacity
  3. You have $2\,000\ \mathrm g$ of water in $1.065\ \mathrm M$ $\ce{NaOH}$ + $1.065\ \mathrm M$ $\ce{HCl}$

I think this could get you close though? I'm a bit surprised that the molarities aren't higher... It's a starting point at least.

Disclaimer of course: be careful with the $\ce{NaOH}$ + $\ce{HCl}$ solutions, they can be dangerous. Use proper chemistry hygeine protocols. The mixture should be benign, but you should confirm this with pH paper.

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  • $\begingroup$ This is interesting, salt water would be a fine waste product though my only worry is it being damaging or cause salt build up within the tubing for disposal. $\endgroup$
    – ABlankenship
    Dec 22, 2016 at 19:51
  • $\begingroup$ Depending on what institution you're at, some places allow you to dispose of neutralized acid/base solutions down the sink, as long as the acids and bases are benign and the pH is ~7. (Source: This is how it worked at 2 of my 3 institutions.) Your stockroom manager or assistant should be able to tell you. $\endgroup$
    – NMJD
    Dec 22, 2016 at 20:01
  • $\begingroup$ Just saw your previous comment about what you meant by tubing: NaCl is incredibly water-soluble. If you're very worried, you can flush the system 1-2x with DI water between experiments, that should remove any salt left behind. $\endgroup$
    – NMJD
    Dec 22, 2016 at 20:05
  • $\begingroup$ This is great, thanks so much for the expansion of the answer. $\endgroup$
    – ABlankenship
    Dec 22, 2016 at 20:28
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    $\begingroup$ My institute even allowed pouring more concentrated acids down the drain since we had a neutraliser in the cellar. $\endgroup$
    – Jan
    Dec 23, 2016 at 15:44
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As Zhe says, this isn't safe. Your question mentions "draining" chemicals as if it's OK to dump chemicals down the drain or out on the ground. It's not. What possible connection is there between an exothermic chemical reaction and an experiment in thermoelectrics, I can't imagine. Use a hot plate. Here's a hint: heating water to its boiling point requires a lot of energy (per gram). Another name for a chemical reaction that produces a lot of energy is "an explosion". I can't think of any which someone who thinks "if its liquid, I can just dump it down the drain" would be safe to implement.

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  • $\begingroup$ The chemicals don't fit my requirements if it can't go down a drain. Though by drain I simply meant its own drip system to clear the system to be reloaded. It could drip into a bucket for safe disposal later. $\endgroup$
    – ABlankenship
    Dec 22, 2016 at 19:50
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    $\begingroup$ My assumption was @ABlankenship is doing an electricity-generation experiment, where a heat source is used to generate electricity. I think there's a very obvious connection because an exothermic chemical reaction and such an experiment. Also, I have spent several years managing and assisting in stockrooms. Many liquids can go down the drain, but you have to be informed and careful. $\endgroup$
    – NMJD
    Dec 22, 2016 at 20:04
  • $\begingroup$ Most chemicals can't be dumped down the drain. Technically, you shouldn't be able to dump large amounts of salt down the drain. $\endgroup$
    – Zhe
    Dec 22, 2016 at 20:08
  • $\begingroup$ Most chemicals, but this is not a universal fact, especially for waste like aqueous sodium chloride. The rules depend on the specificities of your institutions sewer handling. State and federal regulations for most institutions I worked at allow unrestricted disposal of diluted NaCl solutions formed from the neutralization of acid solutions. Many universities only allow the stockroom staff or EHS to do this, to ensure it is done correctly. I reiterate that you should check with the relevant authorities at your institutions before determining your waste management plan. $\endgroup$
    – NMJD
    Dec 22, 2016 at 20:17

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