Question

An aqueous solution of ${\text{KI}}$ dissolves ${{\text{I}}_2}$ to form ${\text{K}}{{\text{I}}_3}$ . Which of the following statements is/are true?
a.) The solution contains ${{\text{K}}^{2 + }}$ and ${{\text{I}}^ - }$
b.) The solution contains ${{\text{K}}^ + }$ and ${{\text{I}}_3}^ -$
c.) The solution is violet in color
d.) In ${\text{K}}{{\text{I}}_3}$ , ${\text{K}}{{\text{I}}_3}$ behaves as Lewis base and ${{\text{I}}^ - }$ as Lewis acid

Answer 1

Potassium iodide is an ionic compound, composed of potassium and iodide. Potassium iodide behaves as a simple ionic salt ${{\text{K}}^ + }{{\text{I}}^ - }$ . Where the iodide ion is a mild reducing agent. Under acidic conditions, KI is oxidized even more easily, due to the formation of hydroiodic acid, which is a powerful reducing agent.

Complete step by step answer:
When an aqueous solution of ${\text{KI}}$ dissolves ${{\text{I}}_2}$ to form ${\text{K}}{{\text{I}}_3}$
${\text{KI }}\left( {aq} \right){\text{ + }}{{\text{I}}_2}{\text{ }} \to {\text{ K}}{{\text{I}}_3}\left( {aq} \right)$
${\text{KI}}$ forms ${{\text{I}}_3}^ -$ when combined with elemental iodine. Unlike ${{\text{I}}_2}$, ${{\text{I}}_3}^ -$ salts can be highly water soluble. So here iodide ion reacts with iodine in aqueous solution to form the ${{\text{I}}_3}^ -$ or triiodide ion.
${{\text{I}}^ - }\left( {aq} \right){\text{ + }}{{\text{I}}_2}\left( {aq} \right){\text{ }} \to {\text{ }}{{\text{I}}_3}^ - \left( {aq} \right)$
Here ${{\text{I}}^ - }$ giving an electron pair to the ${\text{K}}{{\text{I}}_3}$ molecule to form ${{\text{I}}_3}^ -$ . so that ${{\text{I}}^ - }$ act as a Lewis base and ${\text{K}}{{\text{I}}_3}$ act as Lewis acid.
${\text{KI}}$ contains ${{\text{K}}^ + }$and ${{\text{I}}^ - }$ where as ${\text{K}}{{\text{I}}_3}$ contains ${{\text{K}}^ + }$ and ${{\text{I}}_3}^ -$ ions. These salts are more soluble in polar solvents such as water than in non polar solvents. So that ${\text{KI}}$ and ${\text{K}}{{\text{I}}_3}$ would therefore be more soluble in water. Whereas ${\text{K}}{{\text{I}}_3}$ being a nonpolar molecule , it is more soluble in non polar solvent. Due to this reason iodine will not dissolve in water but will dissolve in a solution of potassium iodide and to form ${{\text{I}}_3}^ -$ ions. Unlike ${\text{K}}{{\text{I}}_3}$ , ${{\text{I}}_3}^ -$ salt is highly soluble in water. This polyiodide is the simplest one. In solution it appears as yellow in low concentration and as brown colour at high concentrations.
So here the statement which is true is ${\text{K}}{{\text{I}}_3}$ contains ${{\text{K}}^ + }$ and ${{\text{I}}_3}^ -$ ions. The correct answer is option “B” .

Note: Since the iodide ion is a mild reducing agents , ${{\text{I}}^ - }$ is easily oxidised to ${\text{K}}{{\text{I}}_3}$ by powerful oxidising agents such as chlorine. Even air will oxidize iodine; it is evidenced by the observation of a purple extract when ${\text{KI}}$ rinsed with dichloromethane.
${\text{K}}{{\text{I}}_3}$ and ${{\text{I}}_3}^ -$ have identical redox potential ,that is they both are mild oxidant relative to ${{\text{H}}_2}$