Problem 34 - Chemistry in 30 days

Question

A metal nitrate on reaction with \(\mathrm{KI}\) gives black precipitate and with exeess \(\mathrm{KI}\) gives orange solution. The metal ion is (a) \(\mathrm{Hg}^{2+}\) (b) \(\mathrm{Pb}^{2+}\) (c) \(\mathrm{Cu}^{2+}\) (d) \(\underline{\mathrm{B} i}^{\mathrm{M}}\)

Step-by-Step Solution

Answer

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

Step 1: Identify the Reaction Conditions

We are given that a metal nitrate reacts with KI to produce a black precipitate and an orange solution when excess KI is added.

Step 2: Analyze the Precipitate Formation

A black precipitate with KI suggests the formation of a compound like mercury(II) iodide (HgI_2), which is known to be black.

Step 3: Consider the Color Change with Excess KI

When excess KI is added, the black precipitate (HgI_2) dissolves to form a soluble orange complex ion, [HgI_4]^{2-}, which points toward mercury ions having this characteristic behavior.

Step 4: Compare with Other Metal Ions

Lead(II) iodide (PbI_2) forms a yellow precipitate, not black. Copper(II) and bismuth ions do not form black precipitates or orange solutions when they react with excess KI.

Step 5: Conclusion

Based on the reaction's color changes and precipitate, the metal ion that fits the criteria is Hg^{2+}.

Key Concepts

Metal Nitrate Reactions

When metal nitrates react with potassium iodide (KI), a fascinating array of chemical processes occur. A metal nitrate typically consists of a metal cation paired with the nitrate anion \(\text{NO}_3^-\). As it combines with KI, the iodide ion \(\text{I}^-\) tends to replace the nitrate. The resulting reaction depends significantly on the specific metal ion involved. In our case, involving mercury(II) nitrate \(\text{Hg(NO}_3)_2\), the reaction with KI sets the stage for several observable changes. Understanding how different metal nitrates behave involves recognizing their different affinities and reaction products, including precipitate formation and potential complex ion generation.

Precipitate Formation

During the reaction of mercury(II) nitrate with KI, the initial result is the formation of a black precipitate. Precipitates form as solid particles when two aqueous solutions combine to produce an insoluble compound. Here, the important player is mercury(II) iodide \(\text{HgI}_2\), which forms the characteristic black precipitate upon reaction with KI. Precipitation serves as an immediate, visually detectable sign that a chemical reaction is taking place. Not all metal ions will produce a precipitate in reaction with KI, and the color and form of the precipitate can be a helpful clue in identifying the metal involved.

Complex Ion Formation

Complex ion formation is a process where a central metal ion binds to one or more ions or molecules, resulting in a stable, complex structure. In this scenario, excess potassium iodide reacts with mercury iodide, transforming the insoluble black precipitate into a soluble orange complex ion known as \[\text{HgI}_4^{2-}\]. This process is a hallmark of the specific behavior of mercury ions, reflecting their ability to form such complexes by adding more iodide ions beyond the precipitate formation. These transformations highlight the dynamic nature of chemical reactions and the diverse capabilities of metal ions to undergo different processes under varied conditions.

Color Change in Reactions

The color changes observed in metal nitrate reactions are not just visually stunning but are crucial clues in chemical analysis. When dealing with these reactions, initial and subsequent colors can reveal important details about the ions present and their interactions. In our specific reaction, the progression from a black precipitate to an orange solution clearly indicates the initial formation of mercury iodide and its subsequent transformation into a complex ion with excess iodide ions. Such color changes serve as reliable indicators of specific reactions, helping chemists identify the substances and processes involved in a reaction. This feature is especially useful in educational settings or when performing qualitative chemical analysis.