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STD 12 - 5. Co-ordination chemistry — JEE STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceJEESTD 12 - 5. Co-ordination chemistry4 Marks
MCQ
When concentrated $HCl$ is added to an aqueous solution of $CoCl_2$, its colour changes from reddish pink to deep blue . Which complex ion gives blue colour in this reaction?
  • ✓
    ${\left[ {CoC{l_4}} \right]^{2 - }}$
  • B
    ${\left[ {CoC{l_6}} \right]^{3 - }}$
  • C
    ${\left[ {CoC{l_6}} \right]^{4 - }}$
  • D
    ${\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }}$
✓

Answer

Correct option: A.
${\left[ {CoC{l_4}} \right]^{2 - }}$
a
Aqueous of solution of $CoCl_2$ contains $[Co(H_2O)_6]^{2+}$ which is pinkish in colour so option $(d)$ is incorrect. Reduction potential of $C{o^{3 \oplus }} \to C{o^{2 \oplus }}$ is high so option $(b)$ is incorrect. $Co^{2+}$ does not oxidises easily to $Co^{3+}$. It is general case that symmetrical substituted octahedral complexes are less deeper in colour than tetrahedral complexes. So $[CoCl_4]^{-2}$ is deep blue in colour.

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$\begin{array}{*{20}{c}}
  {\begin{array}{*{20}{c}}
  {C{H_3\,\,}} \\ 
  {\,|\,\,\,\,\,\,\,\,\,\,} 
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