4.2.4Organic Chemistry
Qualitative Analysis — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 40 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 5Testing for Positive and Carbonate Ions
Always dissolve a solid sample in deionised water before testing.
Ammonium
- Ammonium Warm with sodium hydroxide solution;
- ammonia gas is given off and turns damp red litmus paper blue: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l).
Carbonate
- Carbonate Add dilute hydrochloric acid;
- carbon dioxide bubbles off and turns limewater milky: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l).
Hydrogencarbonate
- Hydrogencarbonate Also fizzes with acid and gives carbon dioxide: HCO₃⁻(aq) + H⁺(aq) → CO₂(g) + H₂O(l).
Limewater
- Limewater Calcium hydroxide solution reacts with CO₂ to form insoluble calcium carbonate, producing the milky appearance.
Part 2 of 5Testing for Sulfate and Halide Ions
Testing for Sulfate and Halide Ions
Sulfate
- Sulfate Add dilute hydrochloric acid then barium chloride solution;
- a white precipitate of BaSO₄ forms: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Why acid first
- Why acid first Hydrochloric acid removes carbonate ions that would also give a white barium precipitate and give a false positive.
Halides
- Halides Add dilute nitric acid then silver nitrate: chloride gives a white precipitate, bromide cream and iodide yellow.
Ammonia confirmation
- Ammonia confirmation AgCl dissolves in dilute ammonia, AgBr only in concentrated ammonia and AgI does not dissolve in either.
| Ion | Reagent | Positive result |
|---|---|---|
| NH₄⁺ | NaOH, warm | Gas turns damp red litmus blue |
| CO₃²⁻ | Dilute HCl | CO₂ turns limewater milky |
| SO₄²⁻ | HCl, then BaCl₂ | White precipitate |
| Cl⁻ / Br⁻ / I⁻ | HNO₃, then AgNO₃ | White / cream / yellow |
Part 3 of 5Organic Tests
Alkene
- Alkene Shake with bromine water; the orange solution turns colourless as the double bond reacts.
Primary or secondary alcohol
- Primary or secondary alcohol Warm with acidified potassium dichromate(VI); the orange solution turns green.
Carboxylic acid
- Carboxylic acid Add sodium carbonate solution; effervescence of carbon dioxide appears.
Halogenoalkane
- Halogenoalkane Warm with sodium hydroxide, acidify with nitric acid, then add silver nitrate to identify the halide released.
Part 4 of 5Working Systematically
A sensible order avoids false results when identifying an unknown.
1
Record the appearance of the sample, including colour, state and whether it dissolves in water.
2
Test for carbonate first with dilute acid because carbonate would interfere with later precipitation tests.
3
Then add hydrochloric acid and barium chloride to test for sulfate, using a fresh portion of the solution.
4
Test for halide ions using nitric acid and silver nitrate, confirming with ammonia if the colour is uncertain.
5
Test for ammonium last by warming with sodium hydroxide and holding damp red litmus paper at the mouth of the tube.
Part 5 of 5Exam Checklist & Tips
4.2.4 – Key Points for the Exam
- Precipitate An insoluble solid formed when two solutions react.
- Limewater Aqueous calcium hydroxide used as a test for carbon dioxide.
- Positive test A result showing that the species being tested for is present.
- Effervescence The release of bubbles of gas from a liquid.
Common Mistakes
- Skipping the hydrochloric acid in the sulfate test, so carbonate gives a false positive.
- Using sulfuric or hydrochloric acid to acidify the silver nitrate test, which adds sulfate or chloride ions.
- Testing ammonia with dry rather than damp red litmus paper.
- Writing the result of the limewater test as clear when it should be milky.
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