Al2S3(s) + 6H2O(l) → 2Al(OH)3(s) + 3H2S(g) I found this reaction on the web.I am not able to understand why this reaction proceeds from a weaker acid(H2O) to a stronger acid(H2S);contradictory to the usual behaviour.
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1$\begingroup$ It is H2S(g), you said so yourself; do you know what that (g) means? $\endgroup$– Ivan NeretinCommented Apr 3, 2023 at 10:36
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1$\begingroup$ Salts of weak bases and weak acids are regularly hydrolyzed in large extent, especially if hydrolysis is supported by leaving gaseous products or by precipitation of solid ones. Additionally, Al has stronger chemical affinity to O than to S. $\endgroup$– PoutnikCommented Apr 3, 2023 at 11:22
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$\begingroup$ Note that $\ce{HS-}$ is very weak acid. $\endgroup$– PoutnikCommented Apr 3, 2023 at 12:12
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1 Answer
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As Ivan says, hydrogen sulfide departs as a gas, thus upsetting the proposed equilibrium between a weak acid and a stronger one. Precipitation of the aluminum hydroxide also pulls the reaction away from the equilibrium that would have been expected based on $\pu{pK_a}$ values alone.
Aluminum sulfide is not the only sulfide that does this. Most sulfide salts that are otherwise soluble in water react similarly and emit hydrogen sulfide gas when exposed to moisture, including sodium sulfide where the hydroxide would be water-soluble.
answered Apr 3, 2023 at 12:10