Question

Give a method for separating the following pairs of ions by the addition of a single reagent. Include formulas for the major products of the reactions. (a) \(\mathrm{K}^{+}\) and \(\mathrm{Cu}^{2+}\) (b) \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cr}^{3+}\) (c) \(\mathrm{Fe}^{3+}\) and \(\mathrm{Al}^{3+}\)

Short answer

(a) Use \( \mathrm{H_2S} \) to precipitate \( \mathrm{CuS} \); (b) Use \( \mathrm{NH_3} \) to form \( \mathrm{Cu(NH_3)_4^{2+}} \); (c) Use \( \mathrm{SCN}^- \) to form \( \mathrm{Fe(SCN)_3} \).

Step-by-step solution

Analyzing the Ions in Pair (a)

For the pair \( \mathrm{K}^{+} \) and \( \mathrm{Cu}^{2+} \), we need to find a reagent that reacts with one ion but not the other. \( \mathrm{Cu}^{2+} \) ions can precipitate with sulfide ions from \( \mathrm{H_2S} \) to form \( \mathrm{CuS} \). \( \mathrm{K}^{+} \) does not form a precipitate with sulfide ions.

Writing Reaction for Pair (a)

The reaction with \( \mathrm{H_2S} \) will be: \[ \mathrm{Cu}^{2+} + \mathrm{H_2S} \rightarrow \mathrm{CuS} (\text{solid}) + 2\mathrm{H}^+ \]\( \mathrm{K}^{+} \) remains in solution as it does not react.

Analyzing the Ions in Pair (b)

For \( \mathrm{Cu}^{2+} \) and \( \mathrm{Cr}^{3+} \) ions, \( \mathrm{Cu}^{2+} \) can be precipitated with an excess of ammonia (\( \mathrm{NH_3} \)) as \( \mathrm{Cu(NH_3)_4}^{2+} \), while \( \mathrm{Cr}^{3+} \) doesn't form a similar complex and remains in solution.

Writing Reaction for Pair (b)

The reaction with \( \mathrm{NH_3} \) will be: \[ \mathrm{Cu}^{2+} + 4\mathrm{NH_3} \rightarrow \mathrm{Cu(NH_3)_4}^{2+} \]\( \mathrm{Cr}^{3+} \) stays in solution as it doesn't form a similar complex.

Analyzing the Ions in Pair (c)

For the pair \( \mathrm{Fe}^{3+} \) and \( \mathrm{Al}^{3+} \), \( \mathrm{Fe}^{3+} \) can react with \( \mathrm{SCN}^- \) ions to form a colored complex \( \mathrm{Fe(SCN)^{3+}} \). \( \mathrm{Al}^{3+} \) does not form such a complex.

Writing Reaction for Pair (c)

The reaction with \( \mathrm{SCN}^- \) will be: \[ \mathrm{Fe}^{3+} + \mathrm{3SCN}^- \rightarrow \mathrm{Fe(SCN)_3} (\text{complex}) \]\( \mathrm{Al}^{3+} \) does not react with \( \mathrm{SCN}^- \) and stays in solution as a free ion.

Key concepts

Ion Precipitation

Ion precipitation is a selective technique where specific ions in a solution form solid particles, known as precipitates, when certain reagents are introduced. These precipitates can be separated from the solution by methods like filtration. This process is useful where you want to isolate specific ions from a mixture. Let’s delve into how this works with an example: when

• Potassium (\(\mathrm{K}^{+}\) ions) is mixed with \(\mathrm{Cu}^{2+}\), adding sulfide ions from \(\mathrm{H_2S}\) results in the formation of copper sulfide (\(\mathrm{CuS}\)) precipitate.
• This reaction occurs because \(\mathrm{Cu}^{2+}\) ions have an affinity for forming solid \(\mathrm{CuS}\), whereas \(\mathrm{K}^{+}\) ions do not react with sulfide anions, allowing them to remain in aqueous form.

The separation is achieved simply by the fact that one ion will create an insoluble compound with the added reagent, while the other does not engage in the reaction. This technique highlights the strategic use of certain reagents to trigger precipitation, helping in cleanly identifying and separating ions from mixtures.

Chemical Reactions

Chemical reactions involve the transformation of one or more substances into different substances. In the context of ion separation, they are used to either form precipitates or complexes that facilitate the separation process.
A fascinating example is when

• \(\mathrm{Cu}^{2+}\) ions are treated with ammonia (\(\mathrm{NH_3}\)) to form a complex ion - \(\mathrm{Cu(NH_3)_4}^{2+}\). This doesn’t happen with \(\mathrm{Cr}^{3+}\) ions, which helps in their separation.
• The reaction: \(\mathrm{Cu}^{2+} + 4\mathrm{NH_3} \rightarrow \mathrm{Cu(NH_3)_4}^{2+}\), results in a clear differentiation between the two ions.

Chemical reactions not only change the state of the ions but can also lead to significant visible changes, making them effective for analytical purposes. These reactions are essential in many fields, from analytical chemistry to industrial processes, demonstrating both specificity and efficiency.

Complex Ion Formation

Complex ion formation involves ions combining with neutral molecules or ions to form complex structures. These complexes can often be visually identified by a change in color or solubility. Such formations can be particularly useful in differentiating ions in solutions.
For instance, the reaction of

• \(\mathrm{Fe}^{3+}\) ions with thiocyanate ions (\(\mathrm{SCN}^-\)) forms a red-colored complex (\(\mathrm{Fe(SCN)_3}\)), indicating that a new complex ion has been formed.
• The distinct coloration is not achieved with \(\mathrm{Al}^{3+}\) ions, which makes it easier to identify and separate iron ions in the solution effectively.

Such interactions, which result in complex ion formation, have diverse applications in various areas of chemistry and environmental science, showcasing the diversity and utility of chemical bonding and reactions in practical applications.