Practical Notes As

Addition of acidified silver nitrate solution: the halide test A few drops of dilute nitric acid are added to the unknown solution, followed by a few drops of silver nitrate solution. The solubility of the precipitate formed is then tested in dilute and concentrated ammonia. Test Add dilute nitric acid to a solution of the unknown and then silver nitrate* Test the precipitate: first with dilute ammonia and then (if no change) with concentrated ammonia.

Observations  White precipitate turning purple on standing; soluble in dilute ammonia.  Cream precipitate insoluble in dilute but soluble in concentrated ammonia.  Yellow precipitate insoluble in concentrated ammonia

Inferences  It is a chloride

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It is a bromide

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It is an iodide

*Sometimes the silver nitrate is added first, followed by the nitric acid. The observations for a chloride then are white precipitate which stays on the addition of nitric acid. The ionic equation for the precipitation of a silver halide (X stands for Cl, Br or I) is: Ag+ (aq) + X-(aq)

AgX (s)

Test for halogenoalkanes Halogenoalkanes are insoluble and so forms two layers. May be asked to hydrolyse the halogenoalkane to an alcohol, and then test for the halide ion produced. May be asked to carry out an elimination reaction (only with secondary or tertiary halogenoalkanes) Test Shake a few drops of the unknown with water Add a few drops of ethanol and aqueous silver nitrate and dilute nitric acid- stand the test tube in a beaker of hot water Add 4 drops of ethanol and then 2cm3 of dilute sodium hydroxide- stand the test tube in a beaker of hot water for 5 mins. Then add excess nitric acid followed by aqueous silver nitrate

Observations  Two layers form

Inferences

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White precipitate Cream Precipitate Yellow Precipitate

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Contains a C-CI group Contains a C-Br group Contains a C-I group

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White precipitate Cream Precipitate Yellow Precipitate

  

Contains a C-CI group Contains a C-Br group Contains a C-I group

Add concentrated sodium hydroxide in ethanol- fit the test tube with a delivery tube and warm gently, passing any gas evolved through a little bromine water

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Brown bromine goes colourless

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H-halogen eliminated forming an alkene.

The identity of the halogen in the precipitate formed with the silver nitrate can be confirmed by adding ammonia solution. Observations Precipitate dissolves in dilute ammonia Precipitate insoluble in dilute but soluble in concentrated ammonia Precipitate insoluble in concentrated ammonia

Inferences AgCI- the unknown is a chloroalkane AgBr- the unknown is a bromoalkane AgI- the unknown is an iodoalkane