Question

In the course of various qualitative analysis procedures, the following mixtures are encountered: (a) \(\mathrm{Zn}^{2+}\) and \(\mathrm{Cd}^{2+}\) (b) \(\mathrm{Cr}(\mathrm{OH})_{3}\) and \(\mathrm{Fe}(\mathrm{OH})_{3}\), (c) \(\mathrm{Mg}^{2+}\) and \(\mathrm{K}^{+}\), (d) \(\mathrm{Ag}^{+}\) and \(\mathrm{Mn}^{2+} .\) Suggest how each mixture might be separated.

Short answer

To separate the given mixtures, the following techniques can be used: (a) Add hydrogen sulfide (H₂S) to the solution containing Zn²⁺ and Cd²⁺ ions. This causes ZnS to precipitate while CdS stays in solution. (b) Dissolve both Cr(OH)₃ and Fe(OH)₃ in an acidic solution, then add excess NaOH to the mixture. Cr(OH)₃ will re-precipitate and can be separated by filtration. (c) Add NH₄Cl to the mixture containing Mg²⁺ and K⁺ ions. Mg²⁺ ions will form a complex with NH₄Cl and precipitate as magnesium chloride (MgCl₂.6H₂O), while K⁺ ions will stay in the solution. (d) Add an excess of chloride ions (Cl⁻) to the solution containing Ag⁺ and Mn²⁺ ions. Ag⁺ ions will precipitate as silver chloride (AgCl), while Mn²⁺ ions remain in the solution.

Step-by-step solution

a) Separation of Zn²⁺ and Cd²⁺ ions

To separate these two ions, we could add a sulfide source, such as hydrogen sulfide (H₂S) gas, to the solution containing Zn²⁺ and Cd²⁺. Due to the difference in solubility of their sulfides, ZnS will precipitate out while CdS will stay in solution: \(6 \,\mathrm{Zn}^{2+}(aq) + 3 \,\mathrm{H}_{2}\mathrm{S}(g) \rightarrow 2 \,\mathrm{Zn}_{3}\mathrm{S}_{2}(s) + 6 \mathrm{H}^{+}\, (aq)\) After precipitating ZnS, filtration can be used to separate the solid from the remaining Cd²⁺ ions in the solution.

b) Separation of Cr(OH)₃ and Fe(OH)₃

Both Cr(OH)₃ and Fe(OH)₃ are insoluble in water and have a similar appearance. However, a difference in their chemical properties can be utilized for their separation. We can start by adding an acidic solution to dissolve both hydroxides, forming Cr³⁺ and Fe³⁺ ions. Next, we can add an excess of a strong, alkaline solution such as NaOH; only Cr(OH)₃ will re-precipitate due to its lower solubility product than Fe(OH)₃: \(\mathrm{Cr}^{3+}(aq) + 3 \text{OH}^-(aq) \rightarrow \mathrm{Cr(OH)}_{3}(s)\) Now, we can separate the Cr(OH)₃ precipitate from the solution, which still contains the Fe³⁺ ions, by filtration.

c) Separation of Mg²⁺ and K⁺ ions

Mg²⁺ and K⁺ ions can be separated based on the difference in their solubility in ammonium chloride (NH₄Cl) solution. We can add NH₄Cl to the mixture containing Mg²⁺ and K⁺ ions. The Mg²⁺ ions will form a complex with NH₄Cl and precipitate as magnesium chloride (MgCl₂.6H₂O), while K⁺ ions will stay in the solution: \(\mathrm{Mg}^{2+}(aq) + 2\, \mathrm{Cl}^{-}(aq) + 6\, \mathrm{H}_{2}\mathrm{O}(l) \rightarrow \mathrm{MgCl}_{2}\cdot 6\, \mathrm{H}_{2}\mathrm{O}(s)\) We can then use filtration to separate the solid magnesium chloride from the K⁺ ions that remain in the solution.

d) Separation of Ag⁺ and Mn²⁺ ions

To separate the Ag⁺ and Mn²⁺ ions, we can use a selective precipitation method by adding an excess of chloride ions (Cl⁻). Due to the very low solubility product of AgCl, only Ag⁺ ions will precipitate, forming silver chloride (AgCl), while Mn²⁺ ions remain in the solution: \(\mathrm{Ag}^+ (aq) + \mathrm{Cl}^- (aq) \rightarrow \mathrm{AgCl}(s)\) We can then separate the solid AgCl from the remaining Mn²⁺ ions in the solution using filtration.

Key concepts

Separation Techniques

Qualitative analysis often involves separating ions from a mixture, which is where separation techniques come into play. These methods are crucial in determining the composition of a mixture by selectively isolating its components. Common techniques include filtration, which is used to separate a precipitated solid from a solution, and precipitation, where we induce a solid’s formation by adding a reagent that causes certain ions to become insoluble. Separation strategies often rely on differences in solubility, chemical reactivity, or physical properties of the components. For instance, in our exercise, using hydrogen sulfide gas allows us to precipitate zinc sulfide while cadmium sulfide remains in solution, showcasing a technique dependent on differential solubilities.

Precipitation Reactions

Precipitation reactions are a cornerstone of qualitative analysis, designed to create an insoluble product from a solution that drives the separation process. The general principle is to add a reagent that reacts with specific ions to form a solid, allowing it to be filtered out. In our examples, hydrogen sulfide gas is used to react with zinc ions to form zinc sulfide, a solid, while cadmium ions remain in solution. This method is sensitive to the concentrations of the reactants and the nature of the precipitating agent, playing a critical role in isolating particular ions from mixtures.

Ionic Solubility

Ionic solubility refers to the extent to which ions dissolve in a solvent, usually water. It is an essential concept when deciding if a precipitation reaction will occur. In practical scenarios, like our exercise, solubility differences make it possible to selectively precipitate ions. For instance, when adding chloride ions to a mixture of silver and manganese ions, only silver ions form an insoluble compound, silver chloride, due to its low solubility compared to manganese compounds. Understanding solubility guides the choice of reagents in qualitative analysis, enabling effective separation of ions based on solubility discrepancies.

Solubility Product

The solubility product, denoted as K ext{sp}, is a numerical value representing a compound's solubility in water. It is determined by the product of the concentrations of its constituent ions at saturation. In separation processes, the K ext{sp} helps predict whether a salt will precipitate under given conditions. For example, knowing that silver chloride has a very low K ext{sp}, we can anticipate its precipitation in the presence of chloride ions. Manipulating conditions like ion concentration can shift the equilibrium, influencing precipitation and aiding in the separation of closely related ions, as seen when distinguishing between chromium and iron hydroxides based on their differing K ext{sp} values.

Chemical Properties of Ions

The chemical properties of ions, including their reactivity and preferred compounds, substantially influence separation techniques. For instance, chromium and iron hydroxides both dissolve in acidic solutions, forming Cr³⁺ and Fe³⁺ ions. However, when an alkaline solution is added, only chromium hydroxide re-precipitates due to its chemical properties, specifically its solubility in basic conditions compared to iron hydroxide. This selectivity is pivotal, allowing chemists to determine suitable conditions for selective precipitation. Understanding the unique chemical behaviors of ions ensures effective design of qualitative analysis protocols and accurate identification of components within a complex mixture.