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Why does $\ce{MgCl2.8H2O}$ undergoes internal hydrolysis? $$\ce{MgCl2.8H2O(s) ->[\Delta]MgO(s) + H2O + HCl}$$

while Calcium, Strontium, and barium undergoes dehydration and form anhydrous chloride. $$\ce{MCl2.xH2O ->[\Delta] MCl2 + xH2O (M = Ca/Sr/Ba)}$$

Even though unlike $\ce{BeCl2}$, all of them have same ionic nature. I didn't find much data on it and what I did find went over my head. Any help would be appreciated. Thanks!

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    $\begingroup$ Closely related: chemistry.stackexchange.com/questions/124974/… $\endgroup$
    – Poutnik
    Oct 25, 2022 at 8:21
  • $\begingroup$ The short answer is that the magnesium compound does not directly form the oxide, but forms a basic salt that decomposes further with additional heating. Moreover, calcium chloride can also be converted to the oxide by this method. See answers to the related question noted above. $\endgroup$
    – Oscar Lanzi
    Oct 26, 2022 at 10:00
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    $\begingroup$ So that means if for given appropriate conditions, same test is done all alkaline earth metal. They will all show similar behaviour. And there won't be any anomaly. $\endgroup$
    – Ayush Aggarwal
    Oct 26, 2022 at 12:35

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The answers to this related question address the thermohydrolysis of not only magnesium chloride but also calcium chloride. Details are in andeselisk's and my answers to that question; here I give a summary.

  • Magnesium chloride is more prone to thermohydrolysis than heavier AEM chloride because of the small size of its cation, but calcium chloride can be made to react also.

  • Magnesium chloride gives the normal salt upon dehydration in an HCl atmosphere, otherwise it gives a basic salt. The basic salt then requires further heating to decompose giving the oxide.

  • Calcium chloride gives the normal salt upon dehydration alone, but above the melting point it reacts with water or oxygen to give the oxide. Since the reaction occurs with the chloride molten, any intermediate basic salts would likely be dissolved in the melt rather than be observed as a separate product; in this case only the more refractory calcium oxide is so observed.

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  • $\begingroup$ What you say is correct. But it does not answer the question : Why ? Why is it so ? $\endgroup$
    – Maurice
    Nov 5, 2022 at 20:30
  • $\begingroup$ Refer to the related question for details. And I do refer here to the small size of the magnesium ion. $\endgroup$
    – Oscar Lanzi
    Nov 5, 2022 at 23:12
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    $\begingroup$ "thermohydrolysis" fancy ;) $\endgroup$
    – Nilay Ghosh
    Nov 6, 2022 at 3:05
  • $\begingroup$ @Oscar Lanzi In your answer to the related question you wrote "The driving force for the hydrolysis is the stronger bonding of alkaline earth cations with oxide or hydroxide ions than with chloride ions". Does the same reasoning goes for carbonate ions also. As magnesium bicarbonate decomposes to give magnesium hydroxide unlike calcium bicarbonate which decomposes simply to calcium carbonate? $\endgroup$
    – Ayush Aggarwal
    Nov 6, 2022 at 14:58
  • $\begingroup$ @AyushAggarwall this is not a good comparison. No alkaline earth bicarbonate exists as a solid. Solubility trends in carbonates versus hydrixides appear to correlate with magnesium bicarbonate solution forming the hydroxide. $\endgroup$
    – Oscar Lanzi
    Nov 6, 2022 at 15:39

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