Problem 30 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

In the final mixed solution, the following solids will precipitate: lead(II) sulfide (PbS), calcium sulfide (CaS), and lead(II) chloride (PbCl2).

Step 1: Identify the Possible Cations and Anions in the Solution

In the final solution, there are three possible cations and three possible anions. The cations are Pb2+ from Pb(CH3COO)2, Na+ from Na2S, and Ca2+ from CaCl2. The anions are CH3COO- from Pb(CH3COO)2, S2- from Na2S, and Cl- from CaCl2.

Step 2: Write All Possible Precipitation Reactions

To analyze whether any solid will precipitate, we will write all the possible precipitation reactions by combining each cation with each anion (except for the original combinations): - Pb2+ with S2-: \[ Pb^{2+}(aq) + S^{2-}(aq) \rightarrow PbS(s) \] - Pb2+ with Cl-: \[ Pb^{2+}(aq) + 2Cl^{-}(aq) \rightarrow PbCl_2(s) \] - Ca2+ with CH3COO-: \[ Ca^{2+}(aq) + 2CH_3COO^{-}(aq) \rightarrow Ca(CH_3COO)_2(s) \] - Ca2+ with S2-: \[ Ca^{2+}(aq) + S^{2-}(aq) \rightarrow CaS(s) \] - Na+ with CH3COO-: \[ Na^{+}(aq) + CH_3COO^{-}(aq) \rightarrow NaCH_3COO(s) \] - Na+ with Cl-: \[ Na^{+}(aq) + Cl^{-}(aq) \rightarrow NaCl(s) \]

Step 3: Check Solubility Rules for Each Reaction

Now we will use the solubility rules to check if any of the products formed in these reactions are insoluble solids: 1. According to solubility rules, most sulfide salts are insoluble, except those with alkali metals (Group 1) or ammonium (NH4+). So, PbS and CaS will precipitate: \[ Pb^{2+}(aq) + S^{2-}(aq) \rightarrow PbS(s) \] \[ Ca^{2+}(aq) + S^{2-}(aq) \rightarrow CaS(s) \] 2. Most chloride salts are soluble, except those of silver (Ag+), lead (Pb2+), and mercury (Hg2+). So, PbCl2 will precipitate: \[ Pb^{2+}(aq) + 2Cl^{-}(aq) \rightarrow PbCl_2(s) \] 3. The acetate anion (CH3COO-) forms soluble salts, so Ca(CH3COO)2 and NaCH3COO will not precipitate. 4. Sodium salts are soluble, so NaCl will not precipitate.

Step 4: Conclusion

In the final mixed solution, the following solids will precipitate: - Lead(II) sulfide (PbS) - Calcium sulfide (CaS) - Lead(II) chloride (PbCl2)

Key Concepts

Solubility Rules

Understanding solubility rules is crucial in predicting whether a chemical reaction will result in a solid precipitate. Solubility rules are guidelines that help chemists determine the solubility of various ionic compounds in water. They indicate which compounds are typically soluble and which are likely to form precipitates based on their constituent ions.

These rules are primarily focused on the combinations of cations (positively charged ions) and anions (negatively charged ions) in water. Let's go over a few fundamental solubility rules:

• Most nitrate ( itrate ions (NO₃⁻)) and acetate ( acetate ions (CH₃COO⁻)) salts are soluble in water.
• Alkali metal ions (such as sodium (Na⁺), potassium (K⁺)) and ammonium ( ammonium ions (NH₄⁺)) salts are generally soluble.
• Sulfate salts are mostly soluble, except for those containing calcium ( (Ca²⁺), barium ( Ba²⁺), and lead ( (Pb²⁺)).
• Chloride, bromide, and iodide salts are soluble, but not those of silver ( Ag⁺), lead ( Pb²⁺), and mercury ( Hg²⁺).
• Most sulfide ( S²⁻) salts are not soluble, unless they are combined with alkali metals or ammonium.

By applying these rules, one can predict which combinations of cations and anions might result in the formation of an insoluble solid, a process known as precipitation.

Cation-Anion Reactions

The interactions between cations and anions in a solution are the basis of many chemical reactions, particularly precipitation reactions. To form a precipitate, specific cations and anions must combine to produce an insoluble compound. This occurs when the resultant product does not dissolve readily in water.

Each cation or anion is derived from the substances dissolved in the solution. For instance, in the exercise, we identify several cations and anions:

• Pb²⁺, Na⁺, and Ca²⁺ as cations.
• S²⁻, CH₃COO⁻, and Cl⁻ as anions.

The resultant combinations can form a variety of compounds. The critical step is determining whether any of these are insoluble. By matching each cation with each unique anion, we predict potential products.

For example, lead ( Pb²⁺) combined with sulfide ( S²⁻) forms lead sulfide ( PbS), a compound known for its low solubility in water, and hence, a precipitate. Similarly, calcium ( Ca²⁺) can react with sulfide ( S²⁻) forming calcium sulfide ( CaS). Understanding these basics helps in predicting the occurrence of precipitation.

Precipitation Reactions

Precipitation reactions are fascinating chemical processes where two solutions react to form an insoluble solid. This solid, known as a precipitate, may initially appear as a cloudiness in the solution before settling as it becomes more distinct over time. The ability to predict precipitation reactions is a fundamental skill in chemistry.

A precipitation reaction occurs when the product of a reaction between two aqueous ionic compounds is insoluble in water according to solubility rules. When cations and anions combine, different outcomes can occur based on their solubility characteristics. If the newly formed compound is insoluble, it will separate from the solution as a solid.

In our case, when lead (Pb²⁺) meets sulfide (S²⁻), they create lead sulfide (PbS), which is insoluble, hence precipitates out of the solution. This is due to the solubility rule regarding sulfide salts, which tend to be insoluble unless paired with alkali metals or ammonium. Similarly, calcium sulfide (CaS) and lead chloride (PbCl₂) form precipitates as per their solubility characteristics.

• Example Reactions:
  • Pb²⁺ + S²⁻ → PbS(s)
  • Ca²⁺ + S²⁻ → CaS(s)
  • Pb²⁺ + 2Cl⁻ → PbCl₂(s)

Identifying such reactions in a lab setting requires careful observation and understanding of the solubility rules. Precipitation not only reveals the process of formation of insoluble compounds but also showcases the broader implications of chemical reactions in producing substances with unique properties.