Group 1 to 7
Understanding Group 1 to 7 in Qualitative Analysis
Qualitative analysis in chemistry refers to the process of determining the presence of particular elements or ions in a sample. The analysis is often organized into groups based on the chemistry of the ions. Groups 1 to 7 are a classification system used to systematically identify the cations present in a mixture. Each group has specific reagents that precipitate the ions of that group, while not affecting the ions in other groups.
Group 1: The Silver Group
Group 1 cations include Ag⁺, Pb²⁺, and Hg₂²⁺. These ions form insoluble chlorides when treated with dilute hydrochloric acid (HCl).
Reagent: Dilute HCl
Reaction Examples:
$$ \text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl} \downarrow $$ $$ \text{Pb}^{2+} + 2\text{Cl}^- \rightarrow \text{PbCl}_2 \downarrow $$ $$ \text{Hg}_2^{2+} + 2\text{Cl}^- \rightarrow \text{Hg}_2\text{Cl}_2 \downarrow $$
Group 2: The Copper Group
Group 2 cations include ions like Hg²⁺, Pb²⁺, Bi³⁺, Cu²⁺, and Cd²⁺. These ions form insoluble sulfides in the presence of hydrogen sulfide (H₂S) in acidic medium.
Reagent: H₂S gas in the presence of dilute HCl
Reaction Examples:
$$ \text{Hg}^{2+} + \text{H}_2\text{S} \rightarrow \text{HgS} \downarrow + 2\text{H}^+ $$ $$ \text{Pb}^{2+} + \text{H}_2\text{S} \rightarrow \text{PbS} \downarrow + 2\text{H}^+ $$
Group 3: The Aluminum Group
Group 3 cations include Al³⁺, Fe³⁺, Cr³⁺, and other ions that form hydroxides which are insoluble in water but soluble in excess NaOH or NH₄OH.
Reagent: Ammonium chloride (NH₄Cl), ammonia (NH₃), and ammonium hydroxide (NH₄OH)
Reaction Examples:
$$ \text{Al}^{3+} + 3\text{OH}^- \rightarrow \text{Al(OH)}_3 \downarrow $$ $$ \text{Fe}^{3+} + 3\text{OH}^- \rightarrow \text{Fe(OH)}_3 \downarrow $$
Group 4: The Zinc Group
Group 4 cations include Zn²⁺, Mn²⁺, Ni²⁺, and Co²⁺. These ions form sulfides that are insoluble in water but soluble in dilute acids.
Reagent: H₂S gas in the presence of ammonium chloride and ammonium hydroxide
Reaction Examples:
$$ \text{Zn}^{2+} + \text{H}_2\text{S} \rightarrow \text{ZnS} \downarrow + 2\text{H}^+ $$ $$ \text{Mn}^{2+} + \text{H}_2\text{S} \rightarrow \text{MnS} \downarrow + 2\text{H}^+ $$
Group 5: The Barium Group
Group 5 cations include Ba²⁺, Sr²⁺, and Ca²⁺. These ions form carbonates that are insoluble in water.
Reagent: Ammonium carbonate ((NH₄)₂CO₃) in the presence of ammonium chloride and ammonium hydroxide
Reaction Examples:
$$ \text{Ba}^{2+} + \text{CO}_3^{2-} \rightarrow \text{BaCO}_3 \downarrow $$ $$ \text{Sr}^{2+} + \text{CO}_3^{2-} \rightarrow \text{SrCO}_3 \downarrow $$
Group 6: The Magnesium Group
Group 6 cations include Mg²⁺, along with other alkaline earth metals. These ions form phosphates that are insoluble in water.
Reagent: Sodium phosphate (Na₃PO₄)
Reaction Examples:
$$ \text{Mg}^{2+} + \text{PO}_4^{3-} \rightarrow \text{Mg}_3(\text{PO}_4)_2 \downarrow $$
Group 7: The Soluble Group
Group 7 includes cations that do not precipitate with the reagents used for Groups 1 to 6. These ions are generally alkali metals like Na⁺, K⁺, and NH₄⁺.
Reagent: No specific reagent, as these cations remain in solution.
Reaction Examples:
No specific reactions, as these ions do not form precipitates under the conditions used for Groups 1 to 6.
Summary Table
| Group | Ions | Reagent | Precipitate Formed |
|---|---|---|---|
| 1 | Ag⁺, Pb²⁺, Hg₂²⁺ | Dilute HCl | Chlorides |
| 2 | Hg²⁺, Pb²⁺, Bi³⁺, Cu²⁺, Cd²⁺ | H₂S in acidic medium | Sulfides |
| 3 | Al³⁺, Fe³⁺, Cr³⁺ | NH₄Cl, NH₃, NH₄OH | Hydroxides |
| 4 | Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺ | H₂S in NH₄Cl and NH₄OH | Sulfides |
| 5 | Ba²⁺, Sr²⁺, Ca²⁺ | (NH₄)₂CO₃ in NH₄Cl and NH₄OH | Carbonates |
| 6 | Mg²⁺ | Na₃PO₄ | Phosphates |
| 7 | Na⁺, K⁺, NH₄⁺ | None | None (Soluble) |
Conclusion
Understanding the groups in qualitative analysis is crucial for systematically identifying the ions present in a mixture. Each group has specific reagents that cause the ions to precipitate, allowing for their detection and identification. By following the group separation scheme, chemists can isolate and test for individual ions, which is essential for qualitative analysis in both educational and professional laboratory settings.