Problem 30 - Chemistry The Central Science In Si Units

Question

Three solutions are mixed together to form a single solution; in the final solution, there are \(0.2 \mathrm{~mol} \mathrm{~Pb}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\), \(0.1 \mathrm{~mol} \mathrm{Na}_{2} \mathrm{~S}\), and \(0.1 \mathrm{~mol} \mathrm{CaCl}_{2}\) present. What solid(s) will precipitate?

Step-by-Step Solution

Answer

Lead(II) sulfide (PbS) will precipitate.

Step 1: Determine Possible Precipitation Reactions

Identify the possible precipitation reactions based on the elements and ions present in the mixed solution. These are: 1) Lead acetate \((\text{Pb(CH}_3\text{COO)}_2)\) can react with sodium sulfide \(\text{(Na}_2\text{S)}\) to form lead(II) sulfide \(\text{(PbS)}\); and 2) Calcium chloride \(\text{(CaCl}_2)\) can potentially react with lead acetate, but there is no common precipitate formed from these two based on their solubility rules.

Step 2: Check Solubility Rules

Using solubility rules: Lead(II) sulfide \(\text{(PbS)}\) is known to be insoluble in water, and therefore, it will precipitate from the solution. There are no common insoluble salts formed from calcium chloride and the other ions in the solution according to typical solubility rules.

Step 3: Identify the Precipitate

Based on the solubility rules and the potential reactions identified, the precipitate formed from the reaction of lead(II) acetate and sodium sulfide will be lead(II) sulfide \(\text{(PbS)}\). No other insoluble salt is expected to form in the solution.

Key Concepts

Precipitation Reaction

In chemistry, a precipitation reaction occurs when two solutions are mixed, and an insoluble solid, called a precipitate, forms. When solutions of ionic compounds are combined, the ions may interact in such a way that a solid forms. Precipitation reactions generally happen in aqueous solutions where two substances react to form one or more insoluble products.
A classic example is when a solution containing lead acetate \((\text{Pb(CH}_3\text{COO)}_2)\) is mixed with sodium sulfide \(\text{(Na}_2\text{S)}\). In this case, lead(II) sulfide (PbS) precipitates out of the solution due to its insolubility in water.
The reaction can be written as a balanced chemical equation: \[ \text{Pb(CH}_3\text{COO)}_2 (aq) + \text{Na}_2\text{S} (aq) \rightarrow \text{PbS} (s) + 2\text{NaCH}_3\text{COO} (aq) \] This equation shows the conversion of soluble ions into an insoluble compound that can be visually detected as a solid at the bottom of the reaction vessel.

Solubility Rules

Solubility rules are guidelines that help predict whether a compound will dissolve or form a precipitate in a given environment. They provide valuable insights when trying to determine the outcomes of mixing solutions in chemical reactions.
According to these rules, most sulfides are insoluble with exceptions for those containing alkali metals and some alkaline earth metals. Lead sulfide \(\text{PbS}\) is one compound that conforms to this rule, being largely insoluble in water. In the given exercise, applying these rules allows us to predict that lead(II) sulfide will form as a precipitate when mixed with sodium sulfide.
Key points from solubility rules:

• Sulfates (except with barium, strontium, lead, and calcium) are soluble.
• Sodium, potassium, and ammonium salts are almost always soluble.
• Most chlorides, bromides, and iodides are soluble, except those of silver, lead, and mercury.

Utilizing these rules simplifies the prediction of precipitate formation when dealing with various chemical reactions.

Insoluble Salts

Insoluble salts are those that do not dissolve significantly in water, leading to the formation of a solid precipitate in a solution. Understanding the formation of these salts is crucial when studying precipitation reactions in aqueous solutions.
For example, lead(II) sulfide \(\text{PbS}\) is an insoluble salt formed from the interaction of lead ions \(\text{Pb}^{2+}\) and sulfide ions \((\text{S}^{2-}\)). When these ions encounter one another in a solution, they form a solid due to their strong ionic bonds and limited solubility in water.
Other common insoluble salts:

• Barium sulfate \(\text{(BaSO}_4)\)
• Calcium carbonate \(\text{(CaCO}_3)\)
• Silver chloride \(\text{(AgCl)}\)

Recognizing these insoluble compounds is vital for chemists, especially when trying to purify substances or remove unwanted ions from a solution. This understanding helps in tracing the pathway of certain reactions and making accurate predictions about their end products.