CHM107: Qualitative Analysis and Solubility of Salts Study Notes

Qualitative Analysis of Inorganic Radicals: Precipitation with Hydrogen Sulfide ($H_2 S$)

• Inorganic Qualitative Analysis Overview: This field involves the systematic identification of cations and anions (radicals) in a sample. Cations are traditionally divided into five or six analytical groups based on their reaction with specific group reagents.
• CHM107 Question 26 Analysis: The transcript asks which of the following radicals will not be precipitated by passing $H_2 S$ gas through the solution. The options provided are:   - Option A: Antimony ($Sb$): Specifically Antimony(III) ($Sb^{3+}$) or Antimony(V) ($Sb^{5+}$).   - Option B: Copper ($Cu$): Specifically Copper(II) ($Cu^{2+}$).   - Option C: Cadmium ($Cd$): Specifically Cadmium(II) ($Cd^{2+}$).   - Option D: Arsenic ($As$): Specifically Arsenic(III) ($As^{3+}$) or Arsenic(V) ($As^{5+}$).
• The Role of Hydrogen Sulfide ($H_2 S$): $H_2 S$ serves as the source of sulfide ions ($S^{2-}$) for precipitation. It is a weak diprotic acid that ionizes in water according to the following equilibrium:   - H_2 S ightleftharpoons 2H^+ + S^{2-}
• Group II vs. Group IV Cations:   - Group II (The Acidic Sulfide Group): Includes $Hg^{2+}$, $Pb^{2+}$, $Bi^{3+}$, $Cu^{2+}$, $Cd^{2+}$, $As^{3+}$, $Sb^{3+}$, and $Sn^{2+}$. These cations are precipitated as sulfides in an acidic medium (usually $0.2$ to $0.3M$ dilute $HCl$). The presence of $HCl$ provides a common ion ($H^+$) that shifts the equilibrium of $H_2 S$ to the left, significantly lowering the concentration of $S^{2-}$ ions. Only sulfides with extremely low solubility product constants ($K_{sp}$) will precipitate under these conditions.   - Group IV (The Basic Sulfide Group): Includes $Zn^{2+}$, $Mn^{2+}$, $Ni^{2+}$, and $Co^{2+}$. These are precipitated as sulfides in an alkaline medium (using $NH_4 OH$ and $NH_4 Cl$). The alkaline medium removes $H^+$ ions, shifting the equilibrium to the right and increasing $S^{2-}$ concentration, allowing more soluble sulfides to precipitate.
• Specific Cation Behaviors:   - All four options listed (Antimony, Copper, Cadmium, and Arsenic) are traditionally classified as Group II radicals. Under standard acidic conditions ($HCl$ provided), all four should precipitate as sulfides:     - 2Sb^{3+} + 3H_2 S ightarrow Sb_2 S_{3(s)} + 6H^+ (Orange precipitate)     - Cu^{2+} + H_2 S ightarrow CuS_{(s)} + 2H^+ (Black precipitate)     - Cd^{2+} + H_2 S ightarrow CdS_{(s)} + 2H^+ (Yellow precipitate)     - 2As^{3+} + 3H_2 S ightarrow As_2 S_{3(s)} + 6H^+ (Yellow precipitate)
• Anomalies and Exceptions: If the question implies that one of these might not precipitate, it typically refers to Cadmium ($Cd^{2+}$). $CdS$ has the highest $K_{sp}$ among Group II sulfides. If the concentration of $HCl$ is too high (exceeding $0.3M$), the concentration of $S^{2-}$ may drop so low that the ionic product ($IP$) of $CdS$ does not exceed its $K_{sp}$, preventing precipitation.

Solubility Principles of Inorganic Salts in Water

• CHM107 Question 21 Analysis: The question requires identifying which of the listed salts is insoluble in water.
• Option A: $(CH_3 COO)_2 Pb$ (Lead(II) Acetate):   - Chemical Rule: Almost all salts containing the acetate ion ($CH_3 COO^-$) are soluble in water.   - Specific Detail: Lead(II) acetate, also known as sugar of lead, is a white crystalline substance that is readily soluble in water (solubility approximately $44.3g/100g$ of water at $20^℃$).
• Option B: $AgCl$ (Silver Chloride):   - Chemical Rule: While most chlorides ($Cl^-$) are soluble, there are three major exceptions: Silver ($Ag^+$), Mercury(I) ($Hg_2^{2+}$), and Lead(II) ($Pb^{2+}$).   - Specific Detail: $AgCl$ is a well-known white, curdy precipitate that is highly insoluble in water ($K_{sp} hickapprox 1.8 imes 10^{-10}$). It is, however, soluble in aqueous ammonia due to the formation of the complex ion $[Ag(NH_3)_2]^+$.
• Option C: $NaCl$ (Sodium Chloride):   - Chemical Rule: All salts of alkali metals (Group 1), including Sodium ($Na^+$), are soluble in water.   - Specific Detail: Table salt is highly soluble in water, reaching a saturation point around $36g/100g$ of water at room temperature.
• Option D: $NH_4 NO_3$ (Ammonium Nitrate):   - Chemical Rule: All ammonium ($NH_4^+$) salts and all nitrate ($NO_3^-$) salts are universally soluble in water.   - Specific Detail: Ammonium nitrate is exceptionally soluble ($118g/100ml$ at $0^℃$), often used in fertilizers and explosives because it dissolves so completely.
• Conclusion: Among the options provided in Question 21, $AgCl$ is the only salt that is insoluble in water.

Questions & Discussion

• Question CHM107:26: Which of the following radicals will not be precipitated by passing $H_2 S$ in [conditions not fully visible]?   - Response: All options listed (Antimony, Copper, Cadmium, Arsenic) are Group II cations that typically precipitate in acidic $H_2 S$. However, Cadmium ($Cd$) is the most likely candidate for "not precipitating" if the acidity is excessively high.
• Question CHM107:21: The following salts are soluble in water except:   - Options: $(CH_3 COO)_2 Pb$, $AgCl$, $NaCl$, $NH_4 NO_3$.   - Response: $AgCl$ is the correct choice as it is an insoluble chloride, whereas acetates, sodium salts, and nitrates are generally soluble.