Question

Give two examples of metal ions in each group (1 through 5) in the qualitative analysis scheme.

Short answer

Group 1: Ag+, Hg2 2+, Group 2: Pb2+, Cd2+, Group 3: Fe3+, Al3+, Group 4: Ca2+, Ba2+, Group 5: K+, Na+

Step-by-step solution

Group 1: Cations Forming Chlorides

In Group 1, typical examples are Ag+ and Hg2 2+. This group includes metal ions that form insoluble chlorides. When a diluted hydrochloric acid (HCl) is added to a solution containing these ions, a white precipitate forms, indicating the presence of these ions as they react with chlorine to become AgCl and Hg2Cl2 respectively.

Group 2: Cations Forming Sulfides

For Group 2, the ions Pb2+, Cd2+ can be chosen. They are metal ions that form insoluble sulfides but soluble chlorides, reacting with hydrogen sulfide under acidic conditions to form precipitate like PbS or CdS.

Group 3: Cations Forming Hydroxides

In Group 3, metal ions that form hydroxides but not sulfides fall into this category. Representative ions include Fe3+ and Al3+. In the presence of a base like NaOH, these ions react to form Fe(OH)3 or Al(OH)3.

Group 4: Cations Forming Carbonates

In Group 4, the representative ions are Ca2+ and Ba2+, they form insoluble carbonates but soluble sulfides and hydroxides. When subjected to a carbonate solution, these ions participate in the reaction and form CaCO3 or BaCO3 as precipitate.

Group 5: Mostly Alkali Metal Ions

And finally, for Group 5, the ions K+ and Na+ are representative examples. These ions don't form precipitates and are typically found as alkali metal ions. They can be usually identified with flame tests. Sodium gives a distinctive yellow flame, while potassium gives a lilac flame.

Key concepts

Cation Precipitation

Cation precipitation is a pivotal concept in qualitative analysis, used to identify metal ions within a solution. This process involves adding reagents to form insoluble compounds, usually as a solid precipitate, which can then be separated from the solution.

The insolubility of these compounds is key. By knowing the solubility rules and combining them with appropriate reagents, certain ions can be selectively precipitated out.

• For example, ions forming chlorides, like Ag⁺ and Hg₂²⁺, react with dilute HCl to form insoluble white precipitates such as AgCl and Hg₂Cl₂.
• In another instance, sulfide ions ( ext{S}^{2-}) are used to form insoluble sulfides with certain cations, like Pb²⁺ and Cd²⁺, leading to the formation of PbS or CdS precipitates.

Recognizing the precipitate helps determine which metal cations were in the solution. This method is part of a systematic procedure to identify different groups of metal ions based on their chemical behavior and solubility properties.

Group Reagents

In qualitative analysis of cations, group reagents play a crucial role. These chemicals are specifically chosen to react with certain groups of cations in a solution, catalyzing the formation of precipitates.

Each group reagent is selected based on its ability to react with all ions in a particular group and not with ions in other groups, offering targeted specificity:

• The group 1 reagent, dilute hydrochloric acid (HCl), is used to precipitate chlorides of cations like Ag⁺ and Hg₂²⁺.
• Group 2 uses hydrogen sulfide in acidic conditions to precipitate sulfides of ions such as Pb²⁺ and Cd²⁺.
• A strong base like NaOH serves as the group 3 reagent, reacting with ions like Fe³⁺ and Al³⁺ to form hydroxides.
• In group 4, carbonates facilitate the precipitation of ions like Ca²⁺ and Ba²⁺ as carbonates.

Using these group reagents effectively separates and identifies ions present in a solution based on their group characteristics and the specific reactions they undergo.

Alkali Metal Flame Test

The alkali metal flame test is a simple yet effective method for detecting specific alkali metal ions. The flame test provides immediate visual clues as to what metal ions may be present in a sample by causing the ions to emit characteristic colors when heated.

During the test, a small amount of the sample is introduced to a flame. The heat excites the electrons in the metal ions, causing them to jump to higher energy levels. As the electrons return to their original energy levels, they release energy in the form of light. Each element emits a specific color based on its electron configuration.

• Sodium ions (Na⁺) give off a bright yellow flame.
• Potassium ions (K⁺) produce a lilac or light purple color.

To perform a flame test, dip a clean loop of wire into the sample solution, and then hold it in the hottest part of a Bunsen burner flame. Observe the color change, if any. This simple observational test is invaluable in rapidly identifying the presence of alkali metals and serves as an initial step in the broader scheme of ion analysis.