Presentation on theme: "Testing for halide ions"— Presentation transcript:
1 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.
2 Testing for Halides Cl- Br- I- White precipitate Precipitate dissolves Observation on adding AgNO3Observation on adding dilute NH3Observation on adding concentrated NH3Cl-Br-I-White precipitatePrecipitatedissolvesCream precipitatePrecipitatewon’t dissolvePrecipitatedissolvesYellow precipitatePrecipitatewon’t dissolvePrecipitatewon’t dissolveLearn these – there’s no excuse for losing marks on this
3 Ionic equations Na Cl(aq) Ag NO3(aq) Cl(s) + We aren’t interested in the sodium and nitrateions so we simplify the equation:The state symbolsare 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)AgCl(s)+Br-(aq)Ag+(aq)AgBr(s)+I-(aq)Ag+(aq)AgI(s)+
4 Typical Exam Questions A question on testing for halides might be quite specific, asking you for:The reagent you would useWhat you would observeThe ionic equationSilver nitrate, AgNO3White precipitateAg+(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…
5 MisconceptionsLet’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.