Reef Chemistry Question of the Day #261 DIY Heat Pack

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #261

A recent thread discussed DIY heat packs to keep a tank warm during a power failure.

Which of the following chemicals would make the worst DIY heat pack by mixing it with fresh water in a reef safe container?

A. Magnesium hydroxide
B. Magnesium chloride
C. Magnesium sulfate
D. Magnesium nitrate
E. Magnesium bromide
F. Magnesium iodide

There are actually two reason why the correct choice is unsuitable. Bonus points for knowing both reasons. The reasons have nothing to do with toxicity or damage to the container.

The chemicals are exactly as written, with no additional hydrating water molecules.

Good luck!





















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[JimWelsh]

Over four hours later and not one taker? Answer is A. Magnesium hydroxide, because: (1) It is quite sparingly soluble in water, and (2) Its enthalpy of solution is quite low.

 

[Chuk]

I'll go F Mag Iodide it has the highest enthalpy of solution I could find (I found -178 in Perrys interestingly I couldn't find Mg(OH)2 in there)

 

[Randy Holmes-Farley]

I'll go F Mag Iodide it has the highest enthalpy of solution I could find (I found -178 in Perrys interestingly I couldn't find Mg(OH)2 in there)

"would make the worst DIY heat pack"

 

[Chuk]

Whoops. Thanks for the dope slap, reading the problem the whole way through is always what gets me.

 

[Randy Holmes-Farley]

Whoops. Thanks for the dope slap, reading the problem the whole way through is always what gets me.

lol

Happy reefing.

 

[Randy Holmes-Farley]

And the answer is...

Which of the following chemicals would make the worst DIY heat pack by mixing it with fresh water in a reef safe container?

A. Magnesium hydroxide
B. Magnesium chloride
C. Magnesium sulfate
D. Magnesium nitrate
E. Magnesium bromide
F. Magnesium iodide

Except for A, all of these are quite soluble (meaning a lot dissolves in fresh water, typically hundreds of grams per liter) and each gives off a lot of heat per amount that dissolves.

It turns out that magnesium hydroxide is not soluble at all (wikipedia lists it as 0.0064 g/L at 25 °C) and it actually takes up a little heat (gets cold) when it does dissolve. As an aside, this value is why you cannot put much magnesium in limewater/kalkwasser: it just precipitates out as magnesium hydroxide.

Here's a typical set of data. The more negative the enthalpy of dissolution is, the hotter it gets when dissolving. Positive values means it actually gets colder.

https://chem.libretexts.org/LibreTe...hemistry/Chapter_9.05:_Enthalpies_of_Solution