Problem 3
Question
A solution that may contain \(\mathrm{Cu}^{2+}, \mathrm{Bi}^{3+}, \mathrm{Sn}^{4+},\) or \(\mathrm{Sb}^{3+}\) ions is treated with thioacetamide in an acid medium. The black precipitate that forms is partly soluble in strongly alkaline solution. The precipitate that remains is soluble in \(6 \mathrm{M} \mathrm{HNO}_{3}\) and gives only a blue solution on treatment with excess \(\mathrm{NH}_{3}\). The alkaline solution, when acidified, produces an orange precipitate. On the basis of this information, which ions are present, which are absent, and which are still in doubt? (As with Group I, evidence other than confirmatory tests may show the presence or absence of an ion.) Present ____________ Absent ____________ In doubt ____________
Step-by-Step Solution
Verified Answer
Present: \( \text{Cu}^{2+}, \text{Sn}^{4+}, \text{Sb}^{3+} \). Absent: \( \text{Bi}^{3+} \). In doubt: None.
1Step 1: Analyze the Reaction with Thioacetamide
When the solution is treated with thioacetamide in an acidic medium, thioacetamide decomposes to produce hydrogen sulfide (H₂S), which reacts with metal ions to form their respective sulfides. The ions mentioned can form sulfides: CuS, Bi₂S₃, SnS₂, and Sb₂S₃. All of these are typically black precipitates.
2Step 2: Consider Solubility in Strongly Alkaline Solution
The black precipitate formed is partially soluble in a strongly alkaline solution. Among the ions given, SnS₂ is known to be soluble in strong alkaline solutions forming stannate ion \( \text{Sn(OH)₆}^{2-} \). This indicates the presence of \( \text{Sn}^{4+} \). Sb₂S₃ can also dissolve to some extent under these conditions.
3Step 3: Examine Solubility in 6 M HNO₃
The precipitate remaining after alkaline treatment is soluble in 6 M HNO₃, and yields a blue solution with excess NH₃. This characteristic reaction is typical of \( \text{Cu}^{2+} \), suggesting that \( \text{Cu}^{2+} \) is present.
4Step 4: Evaluate the Orange Precipitate from Alkaline Solution
Upon acidifying the alkaline solution, an orange precipitate is formed. Among the ions, Sb₂S₃ would give an orange precipitate of Sb₂O₃ (antimony oxide) or its equivalent under these conditions, indicating the presence of \( \text{Sb}^{3+} \).
5Step 5: Conclude Which Ions are Absent
\( \text{Bi(OH)₃} \) would not dissolve in alkaline solution and would not form a distinctive orange precipitate in the subsequent reactions described, suggesting \( \text{Bi}^{3+} \) is absent.
Key Concepts
Chemical ReactionsTransition Metal IonsPrecipitation Reactions
Chemical Reactions
Chemical reactions involve the transformation of substances through the making and breaking of chemical bonds, resulting in new substances. In the context of qualitative analysis, chemical reactions help identify the presence of ions in a solution. In the exercise, the main reaction involves thioacetamide decomposing to form \( \text{H}_2\text{S} \), hydrogen sulfide. This gas, when introduced to the solution, causes the metal ions like \( \text{Cu}^{2+}, \text{Bi}^{3+}, \text{Sn}^{4+}, \text{and} \text{Sb}^{3+} \) to form insoluble sulfides. These sulfides manifest as black precipitates, key indicators in determining which ions are present.
In addition to precipitation, reactions involving solubility changes, such as dissolving in strong acids and bases, provide further identification clues. For example, dissolved metal sulfides in acid or base solutions indicate a specific ion presence. Describing reactions in terms of color changes, precipitate formation, and solubility shifts helps unravel complex ion presence.
In addition to precipitation, reactions involving solubility changes, such as dissolving in strong acids and bases, provide further identification clues. For example, dissolved metal sulfides in acid or base solutions indicate a specific ion presence. Describing reactions in terms of color changes, precipitate formation, and solubility shifts helps unravel complex ion presence.
Transition Metal Ions
Transition metal ions are a specific group of elements characterized by partially filled d-orbitals. Among the ions explored in the solution \( \text{Cu}^{2+} \) is a classic transition metal ion and plays a distinct role in the exercise's observations. Transition metals like copper often engage in complex formation and exhibit unique color changes, which assist in qualitative analysis.
For instance, \( \text{Cu}^{2+} \) forms a distinctive blue complex when reacted with excess ammonia \( \text{NH}_3 \). This property is an essential clue in the solution analysis, allowing us to confirm the presence of \( \text{Cu}^{2+} \). Not all ions in the investigation belong to the transition metal category, but the deep understanding of how transition metal ions behave, such as changes in oxidation states or color reactions, is crucial for analysis.
For instance, \( \text{Cu}^{2+} \) forms a distinctive blue complex when reacted with excess ammonia \( \text{NH}_3 \). This property is an essential clue in the solution analysis, allowing us to confirm the presence of \( \text{Cu}^{2+} \). Not all ions in the investigation belong to the transition metal category, but the deep understanding of how transition metal ions behave, such as changes in oxidation states or color reactions, is crucial for analysis.
Precipitation Reactions
Precipitation reactions occur when soluble ions in separate solutions interact to form an insoluble compound. This process results in a solid precipitate settling out of the solution. In the exercise, when the metal ions are treated with thioacetamide, black precipitates of sulfide ions like \( \text{CuS}, \text{Bi}_2\text{S}_3, \text{SnS}_2, \text{and} \text{Sb}_2\text{S}_3 \) form. These precipitates are crucial markers for detecting the specific ions present.
The solubility of these sulfide compounds in different mediums provides additional analytical insights. Some sulfides, such as \( \text{SnS}_2 \), are soluble in alkaline solutions, while others might dissolve in acidic solutions like \( \text{6 M} \text{HNO}_3 \). The formation, dissolution, and color change of precipitates, like the orange precipitate when alkaline solutions are acidified, indicate the presence or absence of particular ions, showcasing the precision and importance of precipitation reactions in qualitative analysis.
The solubility of these sulfide compounds in different mediums provides additional analytical insights. Some sulfides, such as \( \text{SnS}_2 \), are soluble in alkaline solutions, while others might dissolve in acidic solutions like \( \text{6 M} \text{HNO}_3 \). The formation, dissolution, and color change of precipitates, like the orange precipitate when alkaline solutions are acidified, indicate the presence or absence of particular ions, showcasing the precision and importance of precipitation reactions in qualitative analysis.