Testing for halide ions

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Presentation transcript:

Testing for halide ions You need to know simple chemical tests for the presence of halide ions in solution. Do you know what we mean by halide ions? Halide ions are different from halogens! Halogens are the gp 7 elements in an uncombined state. They exist as diatomic molecules (e.g.Cl2, Br2) & are neutral. Halides (e.g. Cl-) are negative ions which are made when halogen atoms gain an electron. This happens when the halogens make compounds (e.g. NaCl, KBr). Halide formation is a reduction process, because the halogen atoms gain electrons to make negative ions.

Testing for Halides Cl- Br- I- White precipitate Precipitate dissolves Observation on adding AgNO3 Observation on adding dilute NH3 Observation on adding concentrated NH3 Cl- Br- I- White precipitate Precipitate dissolves Cream precipitate Precipitate won’t dissolve Precipitate dissolves Yellow precipitate Precipitate won’t dissolve Precipitate won’t dissolve Learn these – there’s no excuse for losing marks on this

Ionic equations Na Cl(aq) Ag NO3(aq) Cl(s) + We aren’t interested in the sodium and nitrate ions so we simplify the equation: The state symbols are crucial, as you need to show that the silver halides (AgCl etc) are formed as solids i.e. (s). This is why you see precipitates in the reactions! Cl-(aq) Ag+(aq) Ag Cl(s) + Br-(aq) Ag+(aq) Ag Br(s) + I-(aq) Ag+(aq) Ag I(s) +

Typical Exam Questions A question on testing for halides might be quite specific, asking you for: The reagent you would use What you would observe The ionic equation Silver nitrate, AgNO3 White precipitate Ag+(aq) + Cl-(aq)  AgCl(s) Another question might be a bit more open ended: “It is sometimes difficult to distinguish between chloride, bromide and iodide ions using silver nitrate. How can aqueous ammonia be used to distinguish the ions?” You can use the information on the first slide to work this one out for yourself. 3 marks are available…

Misconceptions Let’s not kid ourselves here; this is pretty simple stuff. All you’ve got to do is learn it. There’s no excuse for losing marks on this… HOWEVER! Lots of students get these wrong because they get mixed up with displacement reactions of halides. IF YOU ARE ASKED ABOUT ‘Testing for halide ions’, DISCUSS SILVER NITRATE! IF YOU ARE ASKED ABOUT THE ‘Reactivity’ OF HALOGENS, DISCUSS DISPLACEMENT REACTIONS.