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When magnesium chloride hydrate is heated, does $\ce{Mg(OH)2}$ form or what? I'm curious, because Wikipedia says that it decomposes to release $\ce{HCl}$ gas.

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2 Answers 2

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Wikipedia pretty much has it all already. There are numerous magnesium chloride hydrates $\ce{MgCl2 · x H2O}$ $(x = 1, 2, 4, 6, 8, 12),$ however in the temperature range from 0 °C to 115 °C the most stable one is hexahydrate $\ce{MgCl2 · 6 H2O}.$

Whether $\ce{MgCl2 · 6 H2O}$ decomposes to hydroxo-salt or only dehydrates depends on the conditions. When heating takes place in atmosphere of $\ce{HCl},$ anhydrous $\ce{MgCl2}$ is formed:

$$\ce{MgCl2 · 6 H2O (l) ->[>\pu{115 °C}][HCl(g)] MgCl2(s) + H2O(g)}$$

Otherwise, magnesium hydroxochloride is formed [1, p. 522] and $\ce{HCl}$ gas is released:

The product obtained is always the hexahydrate, $\ce{MgCl2 · 6 H2O}.$ It is dehydrated to anhydrous magnesium chloride by spray drying and heating with dry hydrogen chloride gas. In the absence of $\ce{HCl},$ heating hexahydrate yields the basic salt, $\ce{Mg(OH)Cl}:$

$$\ce{MgCl2 · 6 H2O -> Mg(OH)Cl + HCl + 5 H2O}$$

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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Andeselisk has pointed out that hydrared magnesium chloride decomposes to the basic salt $\ce{Mg(OH)Cl}$ unless the atmosphere is supplemented with hydrogen chloride gas. This basic salt decomposes upon further heating to give the oxide, a reaction that parallels the decomposition of magnesium hydroxide upon heating. From Wikipedia:

The anhydrous forms decompose when heated above 450-500 °C by decomposition of the hydroxide and chloride anions, releasing water and hydrogen chloride and leaving a magnesium oxide residue, by the reactions:[1]

$\ce{2HO^- -> O^{2−} + H2O}$ $\ce{H2O + 2Cl− -> O^{2−} + 2HCl}$

The driving force for the hydrolysis is the stronger bonding of alkaline earth cations with oxide or hydroxide ions than with chloride ions, plus the favorable entropy of evolving hydrogen chloride gas at high temperature. The effect is most notable with magnesium, whose small ion size compared with heavier alkaline earth cations magnifies the difference in electrostatic attraction to the various counterions. However, at higher temperatures calcium chloride may be reacted with water vapor or oxygen to give calcium oxide, too[2].

In the case of calcium chloride, Reference [2] reports that the thermohydrolysis to form the oxide is seen above the melting point, so any basic salt that might form would probably be part of the melt solution rather than a separate solid. Calcium oxide, the observed product which melts at a much higher temperature, is what precipitates out.

Cited References

  1. W. F. Cole and T. Demediuk (1955): "X-Ray, thermal, and Dehydration studies on Magnesium oxychlorides". Australian Journal of Chemistry, 8, 2, pages 234-251. doi:10.1071/CH9550234

  2. G. Fraissler, M. Jöller, T. Brunner, I. Obernberger (2009). "Influence of dry and humid gaseous atmosphere on the thermal decomposition of calcium chloride and its impact on the remove of heavy metals by chlorination", Chemical Engineering and Processing: Process Intensification, 48, 1, Pages 380-388. doi:10.1016/j.cep.2008.05.003

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