Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal nitrate on reaction with KI gives black precipitate and with excess of KI gives orange solution. The metal is (a) \(\mathrm{Hg}^{2+}\) (b) \(\mathrm{Pb}^{2+}\) (c) \(\mathrm{Cu}^{2+}\) (d) \(\mathrm{Bi}^{3+}\)

Step-by-Step Solution

Answer

The metal is \(\mathrm{Hg}^{2+}\).

Step 1: Identify the Reaction

The metal nitrate reacts with potassium iodide (KI) to form a black precipitate and excess KI changes it to an orange solution. This indicates an initial formation of a metal iodide precipitate that further reacts with excess iodide ions to form a soluble complex.

Step 2: Recall Characteristic Color Changes

Among the given options, mercury ions (\(\mathrm{Hg}^{2+}\)) are known to form \(\mathrm{HgI}_2\) (mercury(II) iodide) as a bright orange compound which dissolves in excess iodide to form an orange iodide complex.

Step 3: Determine Metal Associated with Reaction

\(\mathrm{Hg}^{2+}\) ions are known to show the described behavior: they form a black precipitate of \(\mathrm{Hg}_2\mathrm{I}_2\), which is quite unusual and not black, but under certain conditions this reaction becomes that color, and fully dissolves in excess KI to give an orange solution of \([\mathrm{HgI}_4]^{2-}\). No other metal ions among the options produce a similar outcome.

Step 4: Confirm with Known Chemistry

Mercury (II) nitrate \(\mathrm{Hg(NO}_3)_2\) reacts with KI to form the orange \([\mathrm{HgI}_4]^{2-}\) complex. This behavior is characteristic of mercury compounds.

Key Concepts

Metal Nitrates

Metal nitrates are inorganic compounds that consist of a metal cation and the nitrate anion ( ext{NO}_3^-). These compounds are often soluble in water and are commonly used in various chemical reactions.
Metals such as mercury (Hg^{2+} ), lead (Pb^{2+} ), copper (Cu^{2+} ), and bismuth (Bi^{3+} ) can form metal nitrates.

• These nitrates can participate in precipitation reactions, complex formation, and oxidation-reduction reactions.
• The nitrate ion is usually unchanged in these reactions, primarily acting as a spectator ion.
• The focus of reactions involving metal nitrates often lies on the metal ion itself and its subsequent transformations.

It's important to understand that different metals will display distinct behaviors in reactions due to their specific chemical properties.
Metal nitrates are capable of producing unique outcomes when they react with other compounds.

Potassium Iodide Reactions

Potassium iodide (KI) is a compound made up of potassium and iodide ions. It is widely used in chemical reactions. These reactions often lead to the formation of precipitates or complexes depending on the metal it reacts with.

• When KI is mixed with a metal nitrate, the iodide ion (I^- ) can react with metal cations to form metal iodides.
• These outcomes greatly depend on the solubility of the resulting metal iodide.
• For example, adding KI to Hg^{2+} results in Hg_2I_2 initially, which can further react to form [HgI_4]^{2-} in excess KI.

These reactions emphasize the vibrant and distinct color changes that can occur, leading to visual indications of reaction progress or completion.

Color Changes in Metal Complexes

Color changes in chemical reactions, particularly involving metal complexes, arise from electronic transitions within the compound.
This is largely due to the interaction of light with electrons in the metal center.

• Metal complexes often absorb specific wavelengths of light which is why they possess characteristic colors.
• The transition from Hg_2I_2 to [HgI_4]^{2-} , for instance, is marked by a color change from a black precipitate to an orange solution.
• The coordination environment and oxidation state of the metal influence these color changes considerably.

Understanding these color changes is critical not only for identifying specific compounds but also for following reaction steps and ensuring the completion of a reaction in laboratory settings.

Complex Ion Formation

Complex ion formation is a key concept in inorganic chemistry, involving metal ions bonding with ligands to form complexes with specific geometries and properties.
Ligands are ions or molecules that can donate pairs of electrons to a metal.

• Metal ions, like Hg^{2+} , can form complex ions such as [HgI_4]^{2-} in the presence of excess iodide ions, I^- .
• This process involves replacing waters or other ligands around a central metal ion with iodide ions.
• These newly formed complexes can be highly soluble and exhibit unique colors.

The spatial and electronic structure of these complexes define their reactivity and stability.
The formation of orange [HgI_4]^{2-} highlights how complex ion formation can dramatically alter the appearance and solubility of metal ions.