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As observed in the lab, when concentrated $\ce{HCl}$ is added to a solution of deep blue $\ce{[Cu(NH3)4]^2+}$ a green color appears indicating the partial formation of the yellow colored $\ce{[CuCl4]^2-}$. However, when diluted $\ce{HCl}$ is added, a light blue color appears because the tetrachlorocuprate complex doesn't form. Instead, $\ce{[Cu(NH3)4]^2+}$ reacts with $\ce{H+}$ to form free $\ce{Cu^2+}$ ions in water.

I wanted to prove this by calculations, so I wrote the reactions happening.

$$ \ce{[Cu(NH3)4(H2O)2]^{2+} + 4H2O <=> [Cu(H2O)6]^2+ + 4NH3} \tag{1}\\ K_1 = 10^{-13}\\\\$$ $$4×(\ce{H3O+ + NH3 -> NH4+ + H2O}) \tag{2}\\ K_2 = 10^{9.25}\\\\$$ $$\ce{[Cu(H2O)6]^{2+} + 4Cl- -> [CuCl4(H2O)2]^2- + 4H2O}b\tag{3}\\ K_3 = 4×10^5$$

Overall: $$\ce{[Cu(NH3)4(H2O)2]^{2+} + 4H3O+ + 4Cl- -> 4NH4+ +4H2O + [CuCl4(H2O)2]^2-}\\ K= (10^{-13}) \cdot (10^{9.25})^4 \cdot (4×10^5) = 4×10^{29}$$

But as I obtained, $\ce{[CuCl4]^2-}$ must form regardless whether the $\ce{HCl}$ is concentrated or diluted, because the constant of the overall reaction is very large. I know this is wrong but I don't know where have I mistaken.

I speculated that the stability constant of $\ce{[CuCl4]^2-}$ is indeed not $4×10^{5}$ but a number much, much smaller. In this case, the constant of the overall reaction will also be very small, and $\ce{HCl}$ has to be concentrated in order for the equilibrium to be shifted forward.

But actually I searched for long on the internet and that was the only stabilty constant that I found on a site that I don't know whether is credible or not.

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    $\begingroup$ You can find useful Notation basics and How can I format math/chemistry expressions on Chemistry SE. See also upright vs italic and Math SE MathJax tutorial. $\endgroup$
    – Poutnik
    Commented Jun 4, 2021 at 7:47
  • $\begingroup$ Using photos/screenshots of text ( especially if handwritten ) instead of typing text itself is highly discouraged. The image text content cannot be indexed nor searched for, cannot be reused nor referred in answers. Additionally, it can be challenge to decipher. Consider copy/pasting or rewriting of the essential parts and using of MathJax as written above. $\endgroup$
    – Poutnik
    Commented Jun 4, 2021 at 7:48
  • $\begingroup$ Have you considered lower complexes with 1-3 Cl- , which may be preferred at low [Cl-] ? $\endgroup$
    – Poutnik
    Commented Jun 4, 2021 at 10:13
  • $\begingroup$ @Poutnik you mean that the stability constant of tetrachlorocuprate II depends on the concentration of Cl- ? Or why should these complexes be preferred at low concentrations? $\endgroup$
    – user208973
    Commented Jun 4, 2021 at 10:42
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – Poutnik
    Commented Jun 4, 2021 at 10:55

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