Mg(OH)2 + CaCl2 = ???
May 20, 2014 6:12 PM   Subscribe

I mixed milk of magnesia (magnesium hydroxide) and pickling salt (calcium chloride) together and was very surprised when the glass suddenly heated up. I haven't taken a chem class in 12+ yrs and I can't figure out what the exothermic reaction was that I inadvertently made. Any chemists out there to shed some light?
posted by thelaze to Science & Nature (7 answers total) 5 users marked this as a favorite
calcium is a more reactive metal than magnesium. the reactivity of a metal is related to how easily it will shed the electrons in its outer ring and get naked, so to speak. the calcium got naked with the hydroxyls, leaving the magnesium and the chlorides watching on the sidelines.
posted by bruce at 6:33 PM on May 20, 2014 [1 favorite]

It's been a long time, but I thought that the dissolution of CaCl2 in water is exothermic all by itself.
posted by NoDef at 6:33 PM on May 20, 2014 [2 favorites]

also, i would expect calcium chloride to be slightly acidic, as it is the salt of a strong acid and a column two alkali earth metal, which produces a hydroxide that is slightly less basic than that of a column one alkali metal. magnesium hydroxide is, of course, basic. when you mix an acid with a base, exothermia is a thing.
posted by bruce at 6:41 PM on May 20, 2014

Hmm. Well, this would be the reverse reaction of the more common:

MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2

Now some enthalpies:

MgCl2: -641.3
Ca(OH)2: -985.2
Mg(OH)2: -924.5
CaCl2: -795.4

(-641.3 + -985.2) - (-924.5+-795.4) = +93.6 kJ/mol for the reverse of the reaction above. So, endothermic. (Wolfram Alpha says +93.4 kJ/mol, so I was pretty close!)

So, if it did get hotter, that means we are missing something. As NoDef suggests, there is the enthalpy of hydration for CaCl2 which is ~ -120 kJ/mol...and now we are negative.

Of course, we really need the entropy here as well to get the Gibbs free energy change, and I want to say the entropy change of calcium ions dissolving is positive, not negative.

I need my CRC I think... Man, this brings me back to college.
posted by Fortran at 6:41 PM on May 20, 2014 [6 favorites]

Simple test - dissolve about the same amount of calcium chloride as you used above in about the same amount of water, but WITHOUT the milk of magnesia. Does it get as warm? There's your answer, i.e. it's the dissolution of CaCl2 that's exothermic (as NoDef and Fortran have said above).
posted by hangashore at 8:05 PM on May 20, 2014 [3 favorites]

Agreeing that it's essentially just the dissolution of CaCl2 in water.

Mg(OH)2 is only very slightly soluble in water: Ksp = 5.61 x 10-12. So a saturated solution of Mg(OH)2 is about 1.1 x 10-4M aqueous Mg(OH)2, with most of it being solid Mg(OH)2 in suspension. The concentration of aqueous hydroxide ion initially is 2.2 x 10-4 M.

The potential precipitation product, Ca(OH)2 has a somewhat larger Ksp of 5.02 x 10-6. With [OH-] limited by the solubility of Mg(OH)2, no reasonable amount of Ca2+ in solution will precipitate any Ca(OH)2. (You'd need to get [Ca2+] up to around 100M before that happened, and while CaCl2 is highly soluble in water, it's not enough to get it that high.) And MgCl2 is also highly soluble in water, so that's not going to precipitate given the low concentration of Mg2+.

Bottom line: the calcium chloride dissolves, generating heat. Nothing else happens. Mostly solid Mg(OH)2 remains mostly solid.
posted by DevilsAdvocate at 8:36 PM on May 20, 2014 [3 favorites]

I did try a solution of CaCl2 and water - sure enough it's exothermic. Thanks, MeFi chemists!
posted by thelaze at 4:04 AM on May 21, 2014 [1 favorite]

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