# Predict the species formed by the reaction of mercury(II) chloride and iodine

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

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?

• There should be a redox reaction taken place here: $$\ce{I3- + 2 e <=> 3I-}$$ and $$\ce{Hg^2+ + 2 e <=> Hg2^2+}$$ – 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:

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

• Thank you so much. :) – Hydroquinone Aug 16 '20 at 7:02
• Yes there is a typo . The option is HgI4 2-. – Hydroquinone Aug 16 '20 at 7:04
• Is there any way to guess such questions. The info is not given in ncert too. – Hydroquinone Aug 16 '20 at 7:06
• @Hydroquinone You don't guess chemical equations. You solve it logically. – Nilay Ghosh Aug 16 '20 at 7:20
• 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. – Poutnik Aug 16 '20 at 13:39