1. $\ce{HgCl2}$ and $\ce{I2}$ both when dissolved in water containing $\ce{I-}$ ions the pair of species formed is

A. $\ce{HgI2, I3-}$

B. $\ce{HgI2, I-}$

C. $\ce{HgI4^2-, I3-}$

D. $\ce{Hg2I2, I-}$

Answer: option (c)

In this question, I was able to predict the formation of $\ce{I3-}$. So, I could eliminate option (b) and (d). But, between option (a) and (d), I could not distinguish. Is there any specific way to guess answers for these type of questions in exam?

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    $\begingroup$ There should be a redox reaction taken place here: $$\ce{I3- + 2 e <=> 3I-}$$ and $$\ce{Hg^2+ + 2 e <=> Hg2^2+}$$ $\endgroup$ – Mathew Mahindaratne Aug 16 '20 at 7:34

You are right about the formation of $\ce{I3-}$ ion (from the reaction of $\ce{I2}$ and solution containing iodide ion, let's say $\ce{KI}$ solution). Reaction between $\ce{HgCl2}$ and $\ce{KI}$ solution at $\pu{70 ^\circ C}$ produces mercury(II) iodide ($\ce{HgI2}$):

$$\ce{HgCl2 + 2 KI → HgI2 + KCl}$$

But the story doesn't end here. This mercury(II) iodide reacts with $\ce{KI}$ solution to form potassium tetraiodomercurate(II)($\ce{K2HgI4}$):

Crystallizing from a concentrated aqueous solution of mercuric iodide with potassium iodide is the monohydrate $\ce{KHgI3.H2O}$, which is pale orange. In aqueous solution, this triodido complex adds iodide to give the tetrahedral tetraiodo dianion.

So, the answer is C. but there is a typo. It should be $\ce{[HgI4]^2-}$. The overall reaction is:

$$\ce{HgCl2 + 5KI + I2 -> K2HgI4 + KI3 + 2KCl}$$

  • $\begingroup$ Thank you so much. :) $\endgroup$ – Hydroquinone Aug 16 '20 at 7:02
  • $\begingroup$ Yes there is a typo . The option is HgI4 2-. $\endgroup$ – Hydroquinone Aug 16 '20 at 7:04
  • $\begingroup$ Is there any way to guess such questions. The info is not given in ncert too. $\endgroup$ – Hydroquinone Aug 16 '20 at 7:06
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    $\begingroup$ @Hydroquinone You don't guess chemical equations. You solve it logically. $\endgroup$ – Nilay Ghosh Aug 16 '20 at 7:20
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    $\begingroup$ Some behaviour patterns can be derived from basic or general principles, some just guessed as theory/empirical knowledge interpolation, but the rest is to be learned as the fact. $\endgroup$ – Poutnik Aug 16 '20 at 13:39

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