Identifying Cations.

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Presentation transcript:

Identifying Cations

Introduction Cations are The ions of metals They have a positive charge e.g. Na+ Ca2+ Cr3+ They are called cations because they are attracted to the opposite charge of the cathode. NH4+ [ammonium] is also a cation.

Apparatus Test tubes Solutions of salts containing various cations [Cu2+, Fe2+, Fe3+, Zn2+, Ca2+, Al3+.] sodium hydroxide solution [NaOH] ammonium hydroxide solution [NH4OH]

Method Look at the solution Colour will often give a clue to the ions present. Take a small sample Add a small amount of NaOH solution. Add NaOH till it is in excess Note what happens in each case. Take a fresh sample of cation and repeat using ammonium hydroxide solution [NH4OH]

Cu2+ - Copper(II) The solution is blue this suggests that it contains copper ions When NaOH was added a pale blue precipitate of copper hydroxide formed Cu2+(aq) + 2 OH-(aq) = Cu(OH)2(s) The precipitate remains when excess sodium hydroxide is added

Cu2+ - Copper(II) Take a fresh sample When ammonia solution is added a pale blue precipitate of copper hydroxide is also formed. When excess ammonia solution is added the blue precipitate re-dissolves and forms a royal blue solution This if final confirmation that it is copper(II)

Fe2+ - Iron(II) The solution is pale green This suggest that it contains iron(II) When NaOH was added a dirty green precipitate of iron(II) hydroxide formed Fe2+(aq) + 2 OH-(aq) = Fe(OH)2(s) The precipitate remains when excess sodium hydroxide is added. This confirms it is iron(II)

Fe2+ - Iron(II) Take a fresh sample When ammonia solution is added a dirty green precipitate of Iron(II)hydroxide is also formed. It does not re-dissolve in excess ammonia solution. This confirms it is iron(II)

Fe3+ - Iron(III) The solution is brown/ yellow This suggest that it contains iron(III) When NaOH was added a dirty brown precipitate of iron(III) hydroxide formed Fe3+(aq) + 3 OH-(aq) = Fe(OH)3(s) The precipitate remains when excess sodium hydroxide is added. This confirms it is iron(III)

Fe3+ - Iron(III) Take a fresh sample When ammonia solution is added a dirty brown precipitate of Iron(III)hydroxide is also formed. It does not re-dissolve in excess ammonia solution. This confirms it is iron(III)

Aluminium Al3+ Colourless no inference Add sodium hydroxide White precipitate forms Al3+ (aq) + 3 OH- (aq) = Al(OH)3(s) precipitate does re-dissolve in excess NaOH

Aluminium Al3+ Take a fresh sample Add ammonium hydroxide White precipitate forms precipitate does not re-dissolve in excess NH4OH

Zinc Zn2+ Colourless no inference Add sodium hydroxide White precipitate forms Zn2+ (aq) + 2 OH- (aq) = Zn(OH)2(s) precipitate does re-dissolve in excess NaOH

Zinc Zn2+ Take a fresh sample Add ammonium hydroxide White precipitate forms precipitate does re-dissolve in excess NH4OH

Calcium Ca2+ Colourless no inference Add sodium hydroxide White precipitate forms Ca2+(aq) + 2 OH- (aq) = Ca(OH)2(s) Does not re-dissolve in excess

Calcium Ca2+ Take a fresh sample Add ammonium hydroxide No precipitate forms