Qualitative Analysis - CAPE Unit 1 Chemistry

Qualitative Analysis - CAPE Unit 1 Chemistry

• Qualitative analysis is a chemical technique used to identify different ions in a sample.
• Cation identification involves flame tests and reactions with specific reagents, like NaOH and NH3. Different cations produce unique colours in flame tests, and specific reactions with reagents can identify specific cations.
• Anion identification uses specific reactions of anions with certain reagents. Reactions with dilute acid produce different gases or forms different precipitates.
• In qualitative analysis, solubility rules are crucial for predicting what precipitates might form.
• Flame test colours for different metal ions are used for cation identification
  • Potassium: Lilac
  • Sodium: Bright orange-yellow
  • Calcium: Brick red
  • Barium: Pale/apple green
  • Copper II: Blue-green with non-halide, blue with halide

Cation Identification with NaOH and NH3

• Using NaOH (aq): Certain cations produce precipitates of varying colours with excess NaOH, helping distinguish them

Cation Identification with Na₂CO₃

• Reactions of different metals with Na₂CO₃ are key
• Observations of gas produced or absence thereof are crucial for determining the cation
• different tests for different metals.

Other Confirmatory Tests

• Different tests confirm specific ions, for instance confirming Pb²⁺ and distinguishing it from Al³⁺, or distinguishing between Fe²⁺ and Fe³⁺.
• Specific reagents distinguish specific ions.

Solubility Rules

• Group 1 metal compounds (e.g., Lithium, Sodium and Potassium) and ammonium compounds are soluble
• Most nitrates and ethanoates are soluble
• Most chlorides, bromides and iodides are soluble
• Most sulfates are soluble
• All hydrogencarbonates are soluble.

Identification of Anions

• Anions react with particular reagents. The resulting observation such as colour formation of precipitate allows identification of the anion.
  • Precipitate tests with silver nitrate distinguish halide anions (Cl⁻, Br⁻, I⁻) from their colours, and adding ammonia reveals further detail.
  • Precipitate tests with barium nitrate or chloride reveal sulfate anions from their precipitates.
  • Tests for carbonate and sulfite with certain reagents produce gas or precipitate which can be tested further.

Identification of Anions by Heating

• Carbonates and nitrates decompose when heated, producing specific gases for identification (CO₂ and NO₂ respectively). The resulting tests determine the anion present.

Identification of Anions using Acid

• Dilute acids (HCl or HNO₃) produce different gases or precipitates, aiding anion identification.

Testing for Gases

• Testing for gases (e.g. Hydrogen, Oxygen, Carbon Dioxide, Ammonia, Chlorine etc) involves specific tests to determine their presence, such as burning splint, litmus paper.

Testing for Carbonate Ions

• Carbonate ions react with acid to produce carbon dioxide, a gas that turns lime water milky.

Testing for Hydroxide Ions

• Hydroxide ions present in a solution causes a change in color when tested for with red litmus or universal indicator.

Testing for Sulfate lons

• Sulfate ions form a precipitate with certain aqueous solutions which can be test for further.