I know that the dehydration of cobalt(II) chloride hexahydrate is a reversible reaction, is it a dynamic equilibrium?

$$ \begin{align} \ce{CoCl2·6H2O &<=> CoCl2·2H2O + 4 H2O}\tag{R1}\\ \ce{CoCl2·2H2O &<=> CoCl2 + 2 H2O}\tag{R2} \end{align} $$

Some sources say that it is, but others say that it isn't because the forward and reverse reactions don't occur spontaneously. If it is not a dynamic equilibrium, is it considered a chemical equilibrium? Would be great if someone could please clarify.

  • $\begingroup$ I assume you mean dehydration as a state, not a process. For given T and partial water vapor pressure, there is an equilibrium state of a particular hydration degree. Higher T or lower vapor pressure lead to higher dehydration and vice verse. $\endgroup$
    – Poutnik
    Sep 27, 2021 at 9:14
  • 1
    $\begingroup$ Out of curiosity, is there a particular reason you are asking about this very compound? Otherwise I suppose the question could be extended to pretty much all inorganic crystallohydrates. $\endgroup$
    – andselisk
    Sep 27, 2021 at 9:25
  • 1
    $\begingroup$ This was just a common example I saw in my textbooks, but I assume that you should be right to say that this question could apply to other inorganic crystallohydrates. $\endgroup$
    – Lil_Phil
    Sep 27, 2021 at 9:27
  • 4
    $\begingroup$ This compound is used to show a chemical equilibrium in classroom. Dissolving $\ce{CoCl2·6H2O}$ in ethanol produces a blue solution, due to $$\ce{2 [Co(H2O)6]Cl2 -> [Co(H2O)6]^{2+} + [CoCl4]^{2+} + 6 H2O}$$ The hydrated ion $\ce{Co(H2O)6]^{2+}}$ is pale red. As the complex $\ce{[CoCl4]^{2-}}$ has a dark blue color, the ethanol solution of $\ce{CoCl2·6H2O}$ is blue. But adding a few drops water, the solution turns red, due to $$\ce{[CoCl4]^{2-} + 6 H2O <=> [Co(H2O)6]^{2+} + 4 Cl-}$$. Now heating the solution turns it blue again, by reversing the last equation. And red again by cooling. $\endgroup$
    – Maurice
    Sep 27, 2021 at 9:57
  • $\begingroup$ And CoCl2 on blotting paper is a classical humidity indicator in air, showing the equilibrium does shift with a change in environment. $\endgroup$ Sep 27, 2021 at 19:28


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.