Question

A solution may contain any of the Group III cations. Treatment of the solution with OCI- in alkaline medium yiclds a ycllow solution and a colored precipitate. The acidificd solution is unaffected by treatment with \(\mathrm{NH}_{3}\). The colored precipitate dissolves in nitric acid; addition of excess \(\mathrm{NH}_{3}\) to this acid solution produces only a blue solution. On the basis of this information, which Group III cations are present, abscnt, or still in doubt? Present __________ Absent __________ In doubt __________

Short answer

Present: Cr^3+, Fe^3+ Absent: Al^3+ In doubt: None

Step-by-step solution

Cations Present

Cr^3+, Fe^3+

Cations Absent

Al^3+

Cations In Doubt

None, as the information provided allows us to confidently identify the presence and absence of the common Group III cations.

Key concepts

Qualitative Analysis in Chemistry

Qualitative analysis in chemistry is a process used to determine the presence of certain ions or elements in a substance without necessarily measuring their quantity. It's a fundamental tool for understanding the composition of chemical samples, particularly useful in fields like environmental science, pharmaceuticals and material science.

In the context of the exercise we're discussing, qualitative analysis is applied to identify which Group III cations are present in a given solution. Group III cations—typically including ions like Al³⁺, Cr³⁺, and Fe³⁺—can be distinguished by their specific chemical reactions when subjected to various reagents.

For example, if a yellow solution and a colored precipitate form upon treatment with OCI^- in alkaline medium and the precipitate dissolves in nitric acid to give a blue solution with excess ammonia (NH₃), these characteristic reactions can help us ascertain the presence of certain cations while ruling out others.

Cation Precipitation Reactions

Cation precipitation reactions are involved when a reaction between ionic compounds in solution results in an insoluble product, known as a precipitate. This is often due to the formation of a compound that does not dissolve in water.

The process is crucial in the separation and identification of cations in a mixture. A classic qualitative analysis protocol involves adding specific reagents that react with particular ions to produce precipitates. The color, texture, and other qualities of the precipitate can provide clues to the identity of the ions present.

In our exercise, the formation of a colored precipitate in the presence of OCI^- in an alkaline medium and the solubility of the precipitate in nitric acid can be indicative of certain transition metals. The blue solution formed upon adding excess NH₃ indicates a complex ion formation, a typical behavior for certain transition metal ions, which helps us in pinpointing the specific cations present.

Chemical Properties of Transition Metals

Transition metals, occupying the central block of the periodic table, exhibit a remarkable set of chemical properties due to their unique electron configurations. These properties include the ability to form colored compounds, variable oxidation states, and a tendency to form complex ions.

For instance, Chromium (Cr³⁺) and Iron (Fe³⁺), which belong to Group III cations, often form colored compounds such as the yellow solution observed in the reaction from the exercise. These colors arise from the d-orbitals' ability to absorb and emit certain wavelengths of light.

The formation of colored complexes is also a notable trait of transition metals. In the given problem, the metal ion reacts to form a blue complex after the addition of ammonia, indicative of a transition metal like Chromium (Cr³⁺) ions. Aluminum (Al³⁺), however, does not typically form colored complexes under similar conditions, suggesting its absence in the solution. This rich chemistry of transition metals enables the analytical procedures to determine their presence or absence in a mixture, as illustrated by the exercise.