QA (qualitative analysis) - cations Flashcards
How are cations distinguished?
Based on their reactions with aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH4)
Added both ______ (observe colour of precipitate formed) then in _____ (solubility) to observe the cation present in the solution
Added both dropwise (observe colour of precipitate formed) then in excess (solubility) to observe the cation present in the solution
Adding NaOH to salt solutions containing transition metal ions (Cu2+, Fe2+, Fe3+) usually form _____ precipitates while adding it to salt solutions containing other metal ions (e.g. Zn2+, Al3+, Pb2+, Ca2+) (except group I + ammonium) form _____ precipitates
Adding NaOH to salt solutions containing transition metal ions (Cu2+, Fe2+, Fe3+) usually form coloured precipitates while adding it to salt solutions containing other metal ions (e.g. Zn2+, Al3+, Pb2+, Ca2+) (EXCEPT group I + ammonium) form white precipitates
Metal hydroxides such as Al(OH)3, Zn(OH)3, Pb(OH)3 are ______ (ie they can react with ____________). In this case, they react with excess sodium hydroxide to form complex salts that are soluble to give a ______ solution.
Metal hydroxides such as Al(OH)3, Zn(OH)3, Pb(OH)3 are amphoteric (ie they can react with both acids and bases). In this case, they react with excess sodium hydroxide to form complex salts that are soluble to give a colourless solution.
Aqueous ammonia is a ____ alkali, hence it dissociates _____ in water.
Aqueous ammonia is a weak alkali, hence it dissociates partially in water.
Zn2+ and a few drops of NaOH: what do you observe?
White precipitate (Zn(OH)2) is formed
Zn2+ and NaOH in excess: what do you observe?
White precipitate soluble in excess NaOH (aq) to form a colourless solution.
Al3+ and a few drops of NaOH: what do you observe?
White precipitate (Al(OH)3) is formed
Al3+ and NaOH in excess: what do you observe?
White precipitate soluble in excess NaOH (aq) to form a colourless solution.
Pb2+ and a few drops of NaOH: what do you observe?
White precipitate (Pb(OH)2) formed
Pb2+ and NaOH in excess: what do you observe?
White precipitate soluble in excess NaOH (aq) to form a colourless solution.
Ca2+ and a few drops of NaOH: what do you observe?
White precipitate (Ca(OH)2) is formed
Ca2+ and NaOH in excess: what do you observe?
White precipitate is insoluble in excess NaOH (aq).
Cu2+ and a few drops of NaOH: what do you observe?
Light blue precipitate (Cu(OH)2) formed
Cu2+ and NaOH in excess: what do you observe?
Light blue precipitate is insoluble in excess NaOH (aq).
Fe2+ and a few drops of NaOH: what do you observe?
Green precipitate (Fe(OH)2) formed
Fe2+ and NaOH in excess: what do you observe?
Green precipitate is insoluble in excess NaOH (aq).
Fe3+ and a few drops of NaOH: what do you observe?
Reddish-brown precipitate (Fe(OH)3) formed
Fe3+ and NaOH in excess: what do you observe?
Reddish-brown precipitate is insoluble in excess NaOH (aq).
NH4+ and a few drops of NaOH: what do you observe? Is there any observations when NaOH added in excess?
No precipitate. On heating, ammonia gas is given off. Colourless and pungent gas evolved turned moist red litmus paper blue. No change is observed when NaOH added in excess.
What is the ionic equation that shows how aq ammonia dissociates partially in water?
NH3 (g) + H2O <=> NH4+ (aq) + OH- (aq)
Zn2+ and a few drops of NH3: what do you observe?
White precipitate formed
Zn2+ and NH3 in excess: what do you observe?
White precipitate soluble in excess NH3 (aq) to form a colourless solution (soluble complex salt).
Al3+ and a few drops of NH3: what do you observe?
White precipitate formed
Al3+ and NH3 in excess: what do you observe?
White precipitate is insoluble in excess NH3 (aq).
Pb2+ and a few drops of NH3: what do you observe?
White precipitate formed
Pb2+ and NH3 in excess: what do you observe?
White precipitate is insoluble in excess NH3 (aq).
Does adding NH3 to Ca2+ produce any precipitate?
No precipitate
Cu2+ and a few drops of NH3: what do you observe?
Light blue precipitate is formed
Cu2+ and NH3 in excess: what do you observe?
Light blue precipitate soluble in excess NH3 (aq) to form a deep blue solution.
Fe2+ and a few drops of NH3: what do you observe?
Green precipitate formed
Fe2+ and NH3 in excess: what do you observe?
Green precipitate is insoluble in excess NH3 (aq).
Fe3+ and a few drops of NH3: what do you observe?
Reddish-brown precipitate formed
Fe3+ and NH3 in excess: what do you observe?
Reddish-brown precipitate is insoluble in excess NH3 (aq).
How to differentiate between Al3+ and Pb2+ since both cations produce the same observation in NaOH and NH3?
Use H2SO4 (dilute sulfuric acid) / HCl (hydrochloric acid) - one salt formed is soluble the other is not
