3.6.1Physical Chemistry
Identification of Functional Groups by Test-Tube Reactions — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 87 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5First Clues from Physical Properties
First Clues from Physical Properties
State
- State A gas suggests very short carbon chains, a liquid medium chains or polar molecules, and a solid long chains or strong forces.
Solubility
- Solubility Solubility in water suggests polar groups such as –OH or –COOH that can hydrogen bond with water.
Acidity
- Acidity A solution that tests acidic with indicator paper suggests a carboxylic acid is present.
Next step
- Next step Physical clues only narrow the options, so chemical tests are then used to confirm the functional group.
Part 2 of 5Tests for Common Functional Groups
Alkene
- Alkene Shake with bromine water; the orange solution turns colourless as bromine adds across C=C.
Haloalkane
- Haloalkane Warm with aqueous NaOH, acidify with dilute nitric acid, then add silver nitrate to see a silver halide precipitate.
Primary or secondary alcohol
- Primary or secondary alcohol Warm with acidified potassium dichromate(VI); the orange solution turns green.
Aldehyde
- Aldehyde Warm with Tollens' reagent for a silver mirror, or with Fehling's solution for a brick-red precipitate.
Carboxylic acid
- Carboxylic acid Add sodium carbonate or sodium hydrogencarbonate;
- effervescence is seen as carbon dioxide forms.
| Group | Reagent | Positive observation |
|---|---|---|
| Alkene | Bromine water | Orange to colourless |
| Aldehyde | Tollens' reagent, warm | Silver mirror |
| Carboxylic acid | Na₂CO₃(aq) | Fizzing, gas turns limewater milky |
| Primary/secondary alcohol | Acidified K₂Cr₂O₇, warm | Orange to green |
Part 3 of 5Testing a Haloalkane
1
Warm the haloalkane with aqueous sodium hydroxide so that hydrolysis releases the halide ion into solution.
2
Cool the mixture and add dilute nitric acid to remove excess hydroxide ions that would form a precipitate of their own.
3
Add a few drops of aqueous silver nitrate and observe the colour of any precipitate.
4
A white precipitate indicates chloride, a cream precipitate bromide and a yellow precipitate iodide.
5
Confirm with ammonia solution: silver chloride dissolves in dilute ammonia, silver bromide only in concentrated ammonia, and silver iodide not at all.
Part 4 of 5Reporting Observations
Reporting Observations
Observation not deduction
- Observation not deduction Write fizzing or effervescence rather than carbon dioxide is given off, because only what is seen scores.
Negative result
- Negative result Write no visible change or stays orange, because nothing happens earns no mark.
Colours
- Colours Always give both the starting and the final colour, using colourless rather than clear.
Conditions
- Conditions State warming or heating where required, for example a water bath for Tollens' and Fehling's reagents.
Part 5 of 5Exam Checklist & Tips
3.6.1 – Key Points for the Exam
- Functional group The atom or group of atoms in a molecule that determines its characteristic reactions.
- Tollens' reagent Ammoniacal silver nitrate solution that forms a silver mirror with aldehydes.
- Fehling's solution Alkaline copper(II) solution that gives a brick-red precipitate with aldehydes.
- Hydrolysis Breakdown of a compound by reaction with water or aqueous hydroxide.
Common Mistakes
- Adding silver nitrate to a haloalkane without first hydrolysing it and acidifying with nitric acid.
- Writing carbon dioxide is released instead of effervescence in an observation question.
- Using the word clear for colourless in a decolourisation test.
- Expecting a ketone to give a silver mirror or brick-red precipitate; only aldehydes do.
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