Question

Suggest how the cations in each of the following solution mixtures can be separated: (a) \(\mathrm{Na}^{+}\) and \(\mathrm{Cd}^{2+}\) (b) \(\mathrm{Cu}^{2+}\) and \(\mathrm{Mg}^{2+}\), (c) \(\mathrm{Pb}^{2+}\) and \(\mathrm{Al}^{3+}\) (d) \(\mathrm{Ag}^{+}\) and \(\mathrm{Hg}^{2+}\).

Short answer

(a) Add sodium sulfide (Na2S) solution to the mixture, forming cadmium sulfide (CdS) precipitate: \(Cd^{2+} + S^{2-} \rightarrow CdS \downarrow\). Filter to separate CdS and Na+ ions. (b) Add dilute sodium hydroxide (NaOH) solution, forming copper (II) hydroxide (Cu(OH)2) precipitate: \(Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2 \downarrow\). Filter to separate Cu(OH)2 and Mg2+ ions. (c) Add concentrated sodium hydroxide (NaOH) solution, forming lead (II) hydroxide (Pb(OH)2) precipitate: \(Pb^{2+} + 2OH^- \rightarrow Pb(OH)_2 \downarrow\). Filter to separate Pb(OH)2 and Al3+ ions. (d) Add sodium chloride (NaCl) solution, forming silver chloride (AgCl) precipitate: \(Ag^+ + Cl^- \rightarrow AgCl \downarrow\). Filter to separate AgCl and Hg2+ ions.

Step-by-step solution

Determine the properties of Na+ and Cd2+

Sodium (Na+) ion is more soluble in water, and it doesn't form an insoluble precipitate with sulfide ions. Cadmium (Cd2+) can form an insoluble precipitate with sulfide ions.

Separation method using Sodium Sulfide (Na2S)

Add an aqueous solution of sodium sulfide (Na2S) to the mixture. The Cd2+ ions will react with the S2- ions to form a precipitate of cadmium sulfide (CdS), while the Na+ ions remain in solution. The reaction can be represented as: \[Cd^{2+} + S^{2-} \rightarrow CdS \downarrow\]

Filtering the precipitate

Filter the mixture to separate the solid cadmium sulfide (CdS) from the solution containing the sodium ions (Na+). The filtrate will contain Na+ ions, and the leftover residue will be the cadmium sulfide (CdS) precipitate. #b) \(\mathrm{Cu}^{2+}\) and \(\mathrm{Mg}^{2+}\)

Determine the properties of Cu2+ and Mg2+

Copper (Cu2+) ions can form a solid precipitate with hydroxide ions, while magnesium (Mg2+) ions cannot.

Separation method using Sodium Hydroxide (NaOH)

Add a dilute aqueous solution of sodium hydroxide (NaOH) to the mixture. The Cu2+ ions will react with the OH- ions to form a precipitate of copper (II) hydroxide (Cu(OH)2), while the Mg2+ ions remain in solution. The reaction can be represented as: \[Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2 \downarrow\]

Filtering the precipitate

Filter the mixture to separate the solid copper (II) hydroxide (Cu(OH)2) from the solution containing the magnesium ions (Mg2+). The filtrate will contain Mg2+ ions, and the leftover residue will be the copper (II) hydroxide (Cu(OH)2) precipitate. #c) \(\mathrm{Pb}^{2+}\) and \(\mathrm{Al}^{3+}\)

Determine the properties of Pb2+ and Al3+

Lead (Pb2+) ions are less soluble in water than aluminum (Al3+) ions at high pH values.

Separation method using Sodium Hydroxide (NaOH)

Add a concentrated aqueous solution of sodium hydroxide (NaOH) to the mixture. The Pb2+ ions will react with the OH- ions to form a precipitate of lead (II) hydroxide (Pb(OH)2), while the Al3+ ions remain in solution. The reaction can be represented as: \[Pb^{2+} + 2OH^- \rightarrow Pb(OH)_2 \downarrow\]

Filtering the precipitate

Filter the mixture to separate the solid lead (II) hydroxide (Pb(OH)2) from the solution containing the aluminum ions (Al3+). The filtrate will contain Al3+ ions, and the leftover residue will be the lead (II) hydroxide (Pb(OH)2) precipitate. #d) \(\mathrm{Ag}^{+}\) and \(\mathrm{Hg}^{2+}\)

Determine the properties of Ag+ and Hg2+

Silver (Ag+) ions can form a solid precipitate with chloride ions, while mercury (II) (Hg2+) ions do not form a precipitate with chloride ions in high concentrations.

Separation method using Sodium Chloride (NaCl)

Add an aqueous solution of sodium chloride (NaCl) to the mixture. The Ag+ ions will react with the Cl- ions to form a precipitate of silver chloride (AgCl), while the Hg2+ ions remain in solution. The reaction can be represented as: \[Ag^+ + Cl^- \rightarrow AgCl \downarrow\]

Filtering the precipitate

Filter the mixture to separate the solid silver chloride (AgCl) from the solution containing the mercury (II) ions (Hg2+). The filtrate will contain Hg2+ ions, and the leftover residue will be the silver chloride (AgCl) precipitate.

Key concepts

Solubility Properties

Solubility properties refer to how well a substance dissolves in a particular solvent, usually water. Different ions have varying solubility levels, which means they dissolve either poorly or well when mixed with water. Here's why this matters:

• Identifying Differences: By looking at how soluble certain ions are with different compounds, we can choose the best conditions for separating them.
• Selective Dissolution: Some ions remain in solution, while others form solid precipitates. This trait is crucial for chemical separation.
• Cation Examples: For instance, in the case of Na⁺ and Cd²⁺, sodium remains soluble, whereas cadmium forms insoluble precipitates under specific conditions.

Understanding solubility is key to choosing the right chemicals for separating different ions efficiently.

Precipitation Reactions

Precipitation reactions occur when soluble ions in different solutions interact to form an insoluble compound, called a precipitate. This is like a dance where specific partners only align under certain conditions:

• Reactants: The ions involved must be conducive to forming an insoluble compound upon mixing.
• Environment Manipulation: By changing environmental factors like pH or by adding specific reagents, some ions will turn into a solid form.
• Example Reactions: When Cu²⁺ ions meet OH^- ions, they react to form copper(II) hydroxide, Cu(OH)₂, a solid precipitate.

By utilizing precipitation reactions, we can trigger the formation of solid compounds that can be easily extracted from a solution.

Chemical Separation Techniques

Chemical separation techniques are essential in isolating different components in a mixture using their unique chemical properties. This includes solubility and reactivity differences:

• Cation-specific Reactions: Certain chemicals react only with specific cations leading to their segregation.
• Sequential Application: Techniques like precipitation can be applied in a sequence to isolate multiple ions.
• Practical Examples: By adding NaOH to a mix containing Pb²⁺ and Al³⁺, lead forms a precipitate allowing its separation before attending to aluminum.

Choosing the correct separation technique for each ion type allows for efficient extraction and purification, critical in both laboratory and industrial settings.

Filtration Process

The filtration process offers a practical solution to separate insoluble solids from the liquid phase. After a precipitation reaction forms a solid precipitate, filtration is the next logical step to isolate the desired components:

• Physical Separation: Filters physically separate solid particles from liquids by trapping the solid on a filtering medium.
• Simple to Complex: While some filtrations are simple gravity processes, others might need vacuum technology for more efficient separation.
• Use Case: After precipitating Ag⁺ from a mix with Hg²⁺ as AgCl, filtration helps separate the solid silver chloride from remaining liquid content.

Understanding how to effectively utilize filtration in conjunction with precipitation reactions ensures the complete separation of ions and perfect results in complex chemical mixture handling.