Edexcel A Level Chemistry:复习笔记4.2.4 Qualitative Analysis

Core Practical 7: Analysis of Inorganic and Organic Unknowns

 

Test Tube Reactions

  • Simple test tube reactions can be done to identify the following ions:
    • Ammonium ions (NH4+)
    • Halide ions (X-) - covered in 2.3.4 Halide Ion Reactions
    • Carbonate ions (CO32-) and hydrogencarbonate ions (HCO3-)
    • Sulfate ions (SO42-)

     

  • If the sample to be tested is a solid, then it must be dissolved in deionised water and made into an aqueous solution

Testing for Ammonium Ions

  • About 10 drops of a solution containing ammonium ions, such as ammonium chloride, should be added to a clean test tube
  • About 10 drops of sodium hydroxide should be added using a pipette

Overall equation: NH4Cl (aq) + NaOH (aq) → NH3 (g) + H2O (l) + NaCl (aq)

Ionic equation: NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)

  • The test tube should be swirled carefully to ensure that it is mixed well
  • The test tube of the solution should then be placed in a beaker of water, and the beaker of water should be placed above a Bunsen burner, so that it can become a water bath
  • As the solution is heated gently, fumes will be produced
  • A pair of tongs should be used to hold a damp piece of red litmus paper near the mouth of the test tube, to test the fumes
  • The red litmus paper will change colour and become blue in the presence of ammonia gas

Litmus-paper-turns-red

Damp red litmus paper turning blue in the presence of ammonia gas

 

Testing for Carbonate Ions

  • A small amount (around 1 cm3) of dilute hydrochloric acid should be added to a test tube using a pipette
  • An equal amount of sodium carbonate solution should then be added to the test tube using a clean pipette

Overall equation: 2HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + CO2 (g) + H2O (l)

Ionic equation: 2H+ (aq) + CO32- (aq) → CO2 (g) + H2O (l)

  • As soon as the sodium carbonate solution is added, a bung with a delivery tube should be attached to the test tube
    • The delivery tube should transfer the gas which is formed into a different test tube which contains a small amount of limewater (calcium hydroxide solution)

     

  • Carbonate ions will react with hydrogen ions from the acid to produce carbon dioxide gas
  • Carbon dioxide gas will turn the limewater milky

Using-Limewater-to-test-for-Carbon-Dioxide_watermark-768x521-1

When carbon dioxide gas is bubbled into limewater it will turn cloudy as calcium carbonate is produced

 

  • A similar reaction is seen with sodium hydrogen carbonate, but the equations are:

Overall equation: HCl (aq) + NaHCO3 (aq) → NaCl (aq) + CO2 (g) + H2O (l)

Ionic equation: H+ (aq) + HCO3- (aq) → CO2 (g) + H2O (l)

Testing for Sulfate Ions

  • Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride
  • If a sulfate is present then a white precipitate of barium sulfate is formed:

Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

Sulfate-Ion-Test-1

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

 

Exam Tip

HCl is added first to remove any carbonates which may be present and would also produce a precipitate and interfere with the results.

 

转载自savemyexams

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