Problem 69 - General Chemistry: Principles and Modern Applications

Question

A solution is believed to contain one or more of the following ions: \(\mathrm{Cr}^{3+}, \mathrm{Zn}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Ni}^{2+} .\) When the solution is treated with excess \(\mathrm{NaOH}(\mathrm{aq}),\) a precipitate forms. The solution in contact with the precipitate is colorless. The precipitate is dissolved in \(\mathrm{HCl}(\mathrm{aq}),\) and the resulting solution is treated with \(\mathrm{NH}_{3}(\text { aq })\). No precipitation occurs. Based solely on these observations, what conclusions can you draw about the ions present in the original solution? That is, which ion(s) are likely present, which are most likely not present, and about which can we not be certain? [Hint: Refer to Appendix D for solubility product and complex-ion formation data.

Step-by-Step Solution

Answer

Based on the given clues, it can be concluded that the unknown solution likely contains Fe^3+. Neither Ni^2+ nor Zn^2+ are likely to be present. There's no clear evidence about Cr^3+.

Step 1: Examine the effect with NaOH

The reaction with NaOH results in precipitation. This suggests that the original solution likely contains ions that react with hydroxide ions to form insoluble compounds. According to the solubility rules, Zn^2+ and Fe^3+ could form precipitates with hydroxide ions. So the possible ions could be Zn^2+ and Fe^3+.

Step 2: Check the colorlessness of the solution

The colorless solution after precipitation suggests that Cr^3+ is probably not present, because Cr^3+ forms a green precipitate with hydroxide. So, we could further conclude that Fe3+ is probably present as it could result colorless solution.

Step 3: Check the reaction with NH3

Consider that Zn^2+ forms soluble complexes with ammonia. Since no precipitate forms when the solution is treated with NH3, it suggests that Zn^2+ can't be present. This further confirms the likelihood of Fe^3+ being in the original solution.

Step 4: Summarize conclusions

Considering these three steps, the Fe3+ is likely to be present, because it forms a precipitate with hydroxide, does not give the solution color, and does not form a precipitate with NH3. Ni^2+ and Zn^2+ are not likely to be present due to the clues from the reactions, and there is no clear indication about Cr^3+ with given information.

Key Concepts

Solubility Rules

Understanding solubility rules is essential in predicting how ions behave in a solution. When a solution is mixed with substances like NaOH, specific ions either dissolve or form a precipitate.
Two things can happen:

• An ion can react to form an insoluble compound resulting in a precipitate.
• An ion may stay dissolved because it forms a soluble compound.

In our example, Zn²⁺ and Fe³⁺ ions form insoluble hydroxides, leading to precipitation. However, not all reactions are predictable just by general rules, hence, looking into specific interactions is crucial.
Solubility helps determine which ions can remain in solution or form precipitates when mixed with certain reagents.

Complex Ion Formation

Complex ion formation is when certain ions form a soluble complex with ligands, like ammonia (\( \text{NH}_3 \)). This is a key aspect of qualitative analysis.
For instance, Zn²⁺ can form \([ \text{Zn(NH}_3)_4 ]^{2+}\), a complex ion with ammonia. This means it remains in solution and wouldn't precipitate when NH₃ is added.
This explains why no precipitate is observed if Zn²⁺ was actually present in ammonia. The concluding factor from this reaction suggests Zn²⁺ is not a participant in our original mixture, as a precipitate would have otherwise formed.

Ion Precipitation

Ion precipitation plays an integral role in identifying the ions present in a solution. When a precipitate forms, it indicates the presence of specific ions which react with another compound, like NaOH, that leads to an insoluble product.
Fe³⁺ and Zn²⁺ are prime examples, potentially forming a precipitate when in contact with NaOH. Yet, a subsequent reaction with HCl can result in their dissolution. When a precipitate dissolves upon acid addition, it suggests the precipitate was not from a metal hydroxide that forms a stable complex. This guides chemists in further deducing the presence or absence of particular ions.
This is critical in eliminating possibilities and identifying the solution's components.

Color Changes in Reactions

Observing color changes in chemical reactions is an insightful clue in identifying specific ions. In the context of qualitative analysis, colorless or colored solutions after a reaction can hint at the presence or absence of particular ions.
For instance, the presence of Fe³⁺ usually presents as a colored compound, unless masked by complex or additional reactions. Here, the solution turns colorless, eliminating the possibility of Cr³⁺ presence, which would normally yield a green hue.
Thus, while color changes might seem subtle, they offer significant evidence in pinpointing the distinct nature of the substances involved in solutions.