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I have this reaction : $$\ce{K2SO4.Al2(SO4)3.24H2O -> K2SO4 + Al2O3 + 24H2O}$$ And I notice that while $\ce{Al2(SO4)3}$ becomes $\ce{Al2O3}$ it doesn't happen so with $\ce{K2SO4}$. Why ? I guessed that it would have to do something with the oxygen affinity of K. As $\ce{K2O}$ has less oxygen than $\ce{K2SO4}$ Am I right ?

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Aluminium sulfate is an unstable compound probably due to size difference between cation and anion and thus decomposes to aluminum oxide. (for e.g. size difference of the component ions of nitrogen triiode leads to instability resulting in an explosion).

$$\ce{2Al2(SO4)3 ->[770-860 C] 2Al2O3 + 6SO2 + 3O2}$$

Some sulfur dioxide react with oxygen to form sulfur trioxide.

On the other hand, potassium sulfate is stable due to equal size of potassium ion and sulfate ion and there is much stronger ionic attraction between the ions and thus require much heat in presence of reducing agent(carbon monoxide) to decompose it to potassium sulfide.

$$\ce{K2SO4 ->[high~temp.~,~CO] K2S}$$

Thanks to @narendrakumar who extended the reasoning of instability of aluminum sulfate. He says because of high polarization power of aluminium ion, the sulphate ion get polarized and this make it partially covalent . Compound having partial covalent character decomposes on heating and that's why aluminium sulphate decomposes while potassium sulphate remain unaffected.

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    $\begingroup$ Because of high polarization power of aluminium ion large sulphate ion get polarized and this make it partially covalent . Compound having partial covalent character decomposes on heating. That's why aluminium sulphate decomposes while potassium sulphate remain unaffected $\endgroup$ Jan 16, 2017 at 15:08

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