8
$\begingroup$

Since $\ce{Al^3+}$ is an acidic cation, shouldn't it react with water like this? \begin{align} \ce{AlCl3 + 6H2O &<=> [Al(H2O)6]^3+ + 3Cl^-} \tag1\\ \ce{[Al(H2O)6]^3+ +H2O &<=> [Al(H2O)5(OH)]^2+ + H3O^+} \tag2\\ \ce{[Al(H2O)5(OH)]^2+ + H2O &<=> [Al(H2O)4(OH)2]^+ + H3O^+} \tag3\\ \ce{[Al(H2O)4(OH)2]^+ +H2O &<=> [Al(H2O)3(OH)3](s) + H3O^+} \tag4\\ \end{align}

The 3rd step of the equilibrium rarely happens in water. But if we add a base, strong or not ($\ce{NaOH}$, or $\ce{Na2S}$) the equilibrium weighs to the right and $\ce{Al(OH)3}$ is formed.

But I've seen people write it as this. $$ \ce{AlCl3 + 3H2O -> Al(OH)3 + 3HCl}$$ Is this correct? I need a certain answer.

$\endgroup$
  • 4
    $\begingroup$ It's not incorrect, but certainly as it stands there, it is missing a few key details. The aluminium water complexes are very sensitive to pH change, it really depends on all conditions and on the context whether this can be seen as correct or not. $\endgroup$ – Martin - マーチン Jul 25 '17 at 11:31
  • $\begingroup$ In 25 degrees celcius when the pH in water is 7, if we add anhydrous AlCl3 into the water. Can we see any precipitation? And if we heat the system all equlibrium constants move right? And then will Al(OH)3 be formed? We have very limited lab recources here. I have no way of knowing it practically. Thats why im asking for your help. Thanks so much for everyone who contributed thus far. More help is appreciated. $\endgroup$ – Nuwan yasantha Jul 25 '17 at 14:07
  • $\begingroup$ I doubt you'll see precipitation. Aluminium is a very delicate system and you need to get the pH just right to get it. Also keep in mind that the equilibria are necessarily linked and happening at the same time. If I have the time I'll add some authoritative resource. Just give it a little time, it's a good question and I'm positive you'll find a good answer. $\endgroup$ – Martin - マーチン Jul 25 '17 at 14:13
1
$\begingroup$

Aluminum chloride is very soluble in water, according to wikipedia 44-49g/100ml water depending on the temperature, so you shouldn't expect any preciptation. The reaction is very aggresive and exothermic.
Assuming you dont have any buffers or bases in solution when you add $AlCl_3$, the reaction will lower the pH. At a pH lower than 4-ish, $Al^{3+}$ is the most stable cation. Thus, the reaction $$\ce{AlCl3 + 3H2O -> Al(OH)3 + 3HCl}$$ is pretty accurate. Aluminum hydroxides are decreasingly stable at low pH, as you can see below. Since HCl is formed upon dissolution of $AlCl_3$ you can safely assume that $Al^{3+}$ is the only Al-species present.

!1

$\endgroup$
  • $\begingroup$ You didn't read what OP wrote? such "reaction" is terrible. $\endgroup$ – Mithoron Jul 25 '17 at 14:42
  • 1
    $\begingroup$ Wow. All you guys are awesome. Thanks for devoting your precious time towards me. Its apperciated. Mithoron. Would you please elaborate? Youre saying writing that last reaction is wrong? $\endgroup$ – Nuwan yasantha Jul 25 '17 at 17:33
  • $\begingroup$ The displayed reaction equation is just plain wrong. And please cite the source of the graph you use. $\endgroup$ – Martin - マーチン Jan 10 '18 at 8:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.