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Does the pyrolysis of $\ce{CaCl2 · 2 H2O}$ give $\ce{CaCl2 + 2 H2O}$ or $\ce{Ca(OH)2 + 2 HCl}?$

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  • $\begingroup$ What temperatures are we talking about? $\endgroup$
    – andselisk
    Commented Dec 11, 2019 at 8:09
  • $\begingroup$ Ordinary heating $\endgroup$ Commented Dec 11, 2019 at 8:22
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    $\begingroup$ I did synthesis of uranium tungstates for quite some time and was imprinted to me that "ordinary heating" is supposed to be 900–1100 °C. An organic chemist would probably understand "ordinary heating" as some elevated temperature around the boiling point of water. So, again, what temperatures are we talking about? $\endgroup$
    – andselisk
    Commented Dec 11, 2019 at 8:26
  • $\begingroup$ About the boiling point of H2O $\endgroup$ Commented Dec 11, 2019 at 8:28
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    $\begingroup$ @andselisk Actually, I think I would consider ordinary heating reflux for whatever the solvent is. Which can go down to room temperature for ether or DCM ;) $\endgroup$
    – Jan
    Commented Dec 11, 2019 at 8:58

1 Answer 1

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All calcium chloride hydrates give away water of crystallization upon heating without forming a hydroxide, and the anhydrous $\ce{CaCl2}$ melts without decomposition [1, p. 162]:

anhydrous salts melts at 772 °C, while the mono-, di-, tetra- and hexahydrates decompose at 260 °C, 175 °C, 45.5 °C and 30 °C, respectively; the anhydrous salt vaporizes at 1935 °C.

$$\ce{CaCl2 · x H2O ->[\pu{260 °C}] CaCl2 + x H2O}$$

Calcium chloride dihydrate would remain intact until its melting/decomposition point at 175 °C. When heated quickly in the absence of desiccant, the powdered salt is dissolved in its own water of crystallization forming glass-alike solid upon cooling.

References

  1. Patnaik, P. Handbook of Inorganic Chemicals; McGraw-Hill handbooks; McGraw-Hill: New York, 2003. ISBN 978-0-07-049439-8.
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