Problem 79 - Chemistry: The Central Science

Question

In the course of various qualitative analysis procedures, the following mixtures are encountered: (a) \(\mathrm{Zn}^{2+}\) and \(\mathrm{Cd}^{2+}\), (b) \(\mathrm{Cr}(\mathrm{OH})_{2}\) and \(\mathrm{Fe}(\mathrm{OH})_{3}\), (c) \(\mathrm{Mg}^{2+}\) and \(\mathrm{K}^{4}\), (d) \(\mathrm{Ag}^{*}\) and \(\mathrm{Mn}^{2+}\), Suggest how each mixture might be separated.

Step-by-Step Solution

Answer

To separate the given mixtures, use the following methods: a) Add excess H₂S to the acidic solution containing Zn²⁺ and Cd²⁺ ions, which will precipitate CdS. Filter the mixture to obtain separated ions. b) Add an acidic solution to the mixture of Cr(OH)₂ and Fe(OH)₃. Cr(OH)₂ will dissolve, forming Cr³⁺ ions, while Fe(OH)₃ remains undissolved. Filter the mixture to obtain separated components. c) Add sodium oxalate (Na₂C₂O₄) to the solution containing Mg²⁺ and K⁴ ions. The insoluble magnesium oxalate (MgC₂O₄) will precipitate, while potassium oxalate remains soluble. Filter the mixture to separate the ions. d) Add a solution of NaCl to the mixture of Ag⁺ and Mn²⁺ ions. The Ag⁺ ions will form a white precipitate of AgCl, while Mn²⁺ ions remain in solution as soluble MnCl₂. Filter the mixture to obtain separated ions.

Step 1: Mixture (a): Zn2+ and Cd2+ ions

To separate Zn^2+ and Cd^2+ ions, we will use the difference in their solubilities in H₂S solutions. When excess H₂S is added to the acidic solution containing both ions, Zn ions do not precipitate as ZnS, but Cd^2+ ions will precipitate as CdS thanks to its lower solubility. Thus, we can separate the two ions by filtration.

Step 2: Mixture (b): Cr(OH)2 and Fe(OH)3

To separate Cr(OH)₂ and Fe(OH)₃, we can use the difference in their solubilities in acidic solutions. When an acidic solution is added to the mixture, Cr(OH)₂ will dissolve in acid, forming Cr³⁺ ions, while Fe(OH)₃ remains undissolved due to its lower solubility in acidic solutions. We can then filter the mixture to separate insoluble Fe(OH)3 from the Cr³⁺ ions in the solution.

Step 3: Mixture (c): Mg2+ and K4 ions

To separate Mg²⁺ and K⁴ ions in the mixture, we can take advantage of the difference in solubilities of their respective salts. If we add sodium oxalate (Na₂C₂O₄) to the solution, the virtually insoluble magnesium oxalate (MgC₂O₄) will form a white precipitate, while the potassium oxalate remains soluble. We can separate the precipitated MgC₂O₄ from the potassium ions in the solution by filtration.

Step 4: Mixture (d): Ag+ and Mn2+ ions

To separate Ag⁺ and Mn²⁺ ions, we can utilize the difference in solubilities of their chlorides. When a solution of NaCl is added to the mixture, the Ag⁺ ions will form a white precipitate of AgCl since it is insoluble in water, while Mn²⁺ ions will remain in solution as soluble MnCl₂. We can separate the precipitated AgCl from Mn²⁺ ions in the solution using filtration.

Key Concepts

Separation of Ions

The separation of ions in a mixture is a fundamental process in qualitative analytical chemistry. It's used to identify and quantify the components within a sample. To achieve separation, chemists often exploit the unique characteristics of each ion, such as their charge, size, or their specific chemical reactivity.

For instance, when faced with a mixture of zinc and cadmium ions, a selective precipitation method can be employed. By adding hydrogen sulfide to an acidic solution containing these ions, only cadmium ions react to form an insoluble compound, cadmium sulfide (CdS), while zinc ions remain in solution. After this, a simple filtration can separate the solid precipitate from the liquid phase, effectively isolating one ion from the other.

Solubility Differences

Ions and compounds have differing solubilities in various solvents, which is a key principle used in separation techniques. Solubility differences are particularly useful in processes such as dissolving, precipitating, and selectively crystallizing substances.

Depending on whether a substance is more soluble in an acidic or basic environment, chemists can adjust the pH of a solution to encourage the dissolution of one component while another remains unaltered. For instance, chromium hydroxide dissolves in acidic solutions, transforming into soluble chromium ions, while iron hydroxide does not, allowing the two to be easily separated by filtration.

Precipitation Reactions

Precipitation reactions are at the heart of many separation techniques. These reactions occur when two soluble salts react in a solution to form an insoluble solid, known as a precipitate. The precipitate can then be removed by filtration.

By adding a reagent that specifically reacts with one ion in a solution, chemists can form a solid precipitate. This is seen with magnesium and potassium ions — when sodium oxalate is added to their mixture, only magnesium forms an insoluble oxalate, which precipitates out and can be filtered, while potassium remains in solution.

Filtration Technique

Filtration is a mechanical or physical operation used to separate solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. The solid particles are trapped by the filter medium, also known as the filter.

In our previous examples, filtration is the final step after a precipitate has been formed. For instance, after forming a precipitate of silver chloride in the presence of sodium chloride, the insoluble AgCl can be separated from the remaining solution containing soluble MnCl₂ through the use of filtration. This simple technique is instrumental in a wide range of chemical analyses and synthesis applications.