A Jamaican chemistry student added a few drops of concentrated hydrochloric acid to a pink cobalt(II) chloride solution. The solution turned blue. The equilibrium is: [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l). Which species is responsible for the blue colour?

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Question

A Jamaican chemistry student added a few drops of concentrated hydrochloric acid to a pink cobalt(II) chloride solution. The solution turned blue. The equilibrium is: [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l). Which species is responsible for the blue colour?

Accepted Answer

[CoCl₄]²⁻ — Award 1 mark for identifying [CoCl₄]²⁻ as the blue species. Adding HCl increases Cl⁻ concentration, shifting equilibrium right to form more [CoCl₄]²⁻. A is incorrect — [Co(H₂O)₆]²⁺ is the pink species. B is incorrect — water is colourless. D is incorrect — chloride ions are colourless.

Answer

[CoCl₄]²⁻

Answer

Cl⁻

Answer

H₂O

Answer

[Co(H₂O)₆]²⁺

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