Selective Precipitation: Videos & Practice Problems

Selective precipitation is a method used to separate ions from a solution by forming a precipitate with a reagent that binds to the desired ions. The success of this process hinges on the solubility product constant ($K$_sp) of the salt formed. When the reaction quotient ($Q$) exceeds the $K$_sp value, precipitation occurs as the system shifts to restore equilibrium, leading to the formation of a solid. This principle allows for the separation of specific ions from a mixture by strategically choosing reagents that will precipitate the target ion first, based on differing $K$_sp values. Understanding this concept is crucial for executing selective precipitation effectively in separating ionic compounds.

Selective Precipitation

Selective precipitation is a method used to separate ions from a solution by introducing reagents that react with specific ions to form a solid precipitate. A reagent is essentially another ion that interacts with the dissolved ion, leading to the formation of a solid. The effectiveness of this process hinges on the solubility of the resulting salt, which is quantified by its solubility product constant, denoted as K_sp.

In this context, the reaction quotient, represented as q, plays a crucial role. When q is greater than K_sp, precipitation occurs successfully. To illustrate, consider a hypothetical ionic solid that dissociates into its constituent ions, A⁺ and B^-. The behavior of q in relation to K_sp can be summarized as follows:

• If q < K_sp, the system shifts in the forward direction, favoring the formation of more ions from the solid, resulting in no precipitate.
• If q = K_sp, the system is at equilibrium, and again, no precipitate forms.
• Only when q > K_sp does the system shift in the reverse direction, leading to the formation of a solid precipitate.

This principle of selective precipitation allows for the separation of ions based on their differing K_sp values. By analyzing a mixture of ions, one can determine which precipitate will form first by comparing the K_sp values of the potential solids. This approach is essential for effectively isolating specific ions from a complex mixture.

Selective Precipitation Example

In a solution containing 0.405 molar of chromate ions and 0.628 molar of sulfide ions, the precipitation of these ions can be influenced by the addition of lead(II) fluoride. The solubility product constants (K_sp) for the relevant compounds are crucial in determining which ion will precipitate first. The K_sp for lead(II) chromate is 2.0 × 10⁻¹⁶, while the K_sp for lead(II) sulfide is significantly lower at 7.0 × 10⁻²⁹.

Since the K_sp value indicates the solubility of a compound, a lower K_sp means that the compound is less soluble and will precipitate out of solution first. Therefore, lead(II) sulfide, with its lower K_sp, will precipitate before lead(II) chromate.

To determine the concentration at which lead(II) sulfide begins to precipitate, we can use the K_sp expression for lead(II) sulfide:

K_sp = [Pb²⁺][S²⁻]

Letting [Pb²⁺] = x and knowing that the initial concentration of sulfide ions is 0.628 M, we can set up the equation:

7.0 × 10⁻²⁹ = (x)(0.628 + x)

Since x is expected to be very small compared to 0.628, we can simplify the equation to:

7.0 × 10⁻²⁹ ≈ x(0.628)

Solving for x gives:

x = \(\frac{7.0 \times 10^{-29}}{0.628} \approx 1.11 \times 10^{-28}\) M

This concentration represents the point at which lead(II) sulfide will begin to precipitate from the solution. Thus, lead(II) sulfide will precipitate first at a concentration of approximately 1.11 × 10⁻²⁸ M.

Solution contains [Cu²⁺] = 0.035 M, [Sr²⁺] = 0.054 M, [Al³⁺] = 0.23 M. Cu²⁺ can be separated by selective precipitation using NaOH. What is the minimum concentration of NaOH needed to start precipitation of Cu²⁺? (K_sp = 2.2 × 10⁻²⁰ of Cu(OH)₂).

• A. 1.57× 10⁻¹⁹ M
• B. 7.93 × 10⁻¹⁰ M
• C. 3.96 × 10⁻¹⁰ M
• D. 1.19 × 10⁻⁹ M

Answer: (B)

Here’s what students ask on this topic:

Which of the statements correctly describe selective precipitation? Can you select all that apply?

Selective precipitation is a process used in chemistry to separate ions from a solution through the addition of a precipitating agent, which forms an insoluble compound with one of the ions. Here are the statements that correctly describe selective precipitation:

1. It involves the addition of a reagent that reacts with a specific ion to form an insoluble precipitate.
2. The precipitate that forms can be removed by filtration, leaving other ions in solution.
3. The choice of precipitating agent is crucial, as it must selectively react with one ion while not affecting others.
4. The process is governed by the solubility product constant (K_sp) of the precipitate; a lower K_sp indicates a less soluble compound, which is more likely to precipitate.
5. Selective precipitation is often used in qualitative analysis to identify the presence of specific ions in a mixture.
6. It can also be used in quantitative analysis by measuring the mass of the precipitate to determine the concentration of the ion in the original solution.

These statements encapsulate the concept and application of selective precipitation in chemical analysis and separation techniques.

What is selective precipitation?

Selective precipitation is a chemical process used to separate different ions from a solution through the addition of a reagent that precipitates one ion while leaving the others in solution. This is possible because different ions have different solubilities in a given solvent. By carefully choosing a reagent and controlling the conditions (such as concentration, pH, and temperature), you can selectively precipitate out a specific ion.

For example, if you have a solution containing both silver ions (Ag⁺) and sodium ions (Na⁺), and you add a reagent like hydrochloric acid (HCl), the silver ions will react with the chloride ions (Cl^-) from the HCl to form silver chloride (AgCl), which is insoluble in water and will precipitate out. The sodium ions, however, will remain in solution because sodium chloride (NaCl) is soluble in water. This technique is widely used in qualitative analysis and in purifying compounds in chemistry.

What is the name of the principle of selective precipitation used to identify the types of ions present in a solution?

The principle you're referring to is called qualitative analysis by selective precipitation. This method is used in chemistry to identify the types of ions present in a solution. It involves adding specific reagents that will react with certain ions to form a precipitate, which is a solid that separates from the solution. Each precipitate is characteristic of particular ions, and by observing the formation of these solids, you can deduce which ions are present. The process typically follows a series of steps, starting with the most insoluble compounds. By carefully choosing the sequence of reagents and conditions, chemists can methodically separate and identify the ions in a mixture. This technique is particularly useful in inorganic chemistry for analyzing the composition of minerals and other chemical samples.

When does selective precipitation occur?

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Selective precipitation occurs in the context of chemical solutions when a particular substance is caused to form a solid precipitate while leaving other substances in solution. This typically happens when the solubility limits of different ions in a solution are exploited. By adjusting the concentration of certain ions or changing the pH of the solution, you can reach a point where one ion exceeds its solubility product and precipitates out of the solution without affecting the others.

For example, if you have a solution containing both silver ions (<sup>Ag+</sup>) and lead ions (<sup>Pb2+</sup>), you can add chloride ions (<sup>Cl-</sup>) to the solution. Silver chloride (<sub>AgCl</sub>) has a much lower solubility than lead chloride (<sub>PbCl2</sub>), so the silver ions will precipitate as <sub>AgCl</sub> when a sufficient amount of chloride is added, while the lead ions remain in solution. This is a common technique in analytical chemistry for separating and identifying ions in a mixture.

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