3. The reactions of halide ions with silver nitrate. This reaction is used as a test to identify which halide ion is present. The test solution is made acidic with nitric acid, and then Silver nitrate solution is added dropwise. The role of nitric acid is to react with any carbonates present to prevent formation of the precipitate Ag2CO3 . This would mask the desired observations 2 HNO3 + Na2CO3 2 NaNO3 + H2O + CO2 Fluorides produce no precipitate Chlorides produce a white precipitate Ag+ (aq) + Cl- (aq) AgCl(s) Bromides produce a cream precipitate Ag+ (aq) + Br- (aq) AgBr(s) Iodides produce a pale yellow precipitate Ag+ (aq) + I- (aq) AgI(s) The silver halide precipitates can be treated with ammonia solution to help differentiate between them if the colours look similar: Silver chloride dissolves in dilute ammonia to form a complex ion AgCl(s) + 2NH3 (aq) [Ag(NH3 )2 ] + (aq) + Cl- (aq) Colourless solution Silver bromide dissolves in concentrated ammonia to form a complex ion AgBr(s) + 2NH3 (aq) [Ag(NH3 )2 ] + (aq) + Br – (aq) Colourless solution Silver iodide does not react with ammonia – it is too insoluble
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3.1.3 The halogens
Characteristic reactions of halide ions (g) the precipitation reactions, including ionic equations, of the aqueous anions Cl –, Br– and I– with aqueous silver ions, followed by aqueous ammonia, and their use as a test for different halide ions. Complexes with ammonia are not required other than observations. PAG4 (see also 3.1.4 a) HSW4 Qualitative analysis.4 Qualitative analysis of
ions
• Use of apparatus for qualitative tests for ions
• Make and record qualitative observations
Identification of the anions and cations
present in a mixture of Group 2 salts
3.1.3(g), 3.1.4(a), 5.3.2(a)
ions
• Use of apparatus for qualitative tests for ions
• Make and record qualitative observations
Identification of the anions and cations
present in a mixture of Group 2 salts
3.1.3(g), 3.1.4(a), 5.3.2(a)
3.1.3 halogens Page 2
Oxford Textbook Pages : 116, 118
CGP Revision Guide Pages : 63, 66
