Qualitative Analysis Flashcards
Carbonate Test
(1) In test tube add HNO3 (aq) to substance tested
(2) If bubbles present, unknown compound may be Carbonate
Test for CO2 produced w/ Carbonate test
Bubble gas through limewater to form white precipitate of CaCO3
: CO2(g) + Ca(OH)2 (aq) ==> CaCO3(s) + H2O(l)
Test for SULFATES
Adding Ba2+ (aq) as aqueous barium chloride to unknown solution
If it is a sulfate - forms white precipitate of BaSO4(s) :
Ba2+ (aq) + SO4 2- (aq) ==> BaSO4
Halide Test
Ag2+ (aq) forms precipitates of silver halides :
Ag+ (aq) + X- (aq) ==> AgX (s)
(1) Add AgNO3 (aq) to aqueous halide solution
(2) Precipitates are diff colours
AgCl - white, AgBr - cream, AgI - yellow
(3) Add aqueous ammonia to test solubility of precipitate
Sequence of Tests
(1) Carbonate Test => (2) Sulfate Test => (3) Halides
Why Carbonate Test First
1, Neither SO4 or halides produce bubbles w/ dilute acid so no possibility of incorrect conclusion
- If no bubbles no CO3 so go on to the next
Why Sulfate Test Second
Adding Ba2+ to a carbonate would form BaCO3 which is insoluble + white like BaSO4
Must do CO3 test first and only proceed to next when CO3 is present
Why Halide Test is Last
Ag2CO3 and AgSO4 is also insoluble in water and form a precipitate like AgX
Important to get rid of CO3 and SO4 ions to rule out possibilities
Add NH3 (aq) to confirm which halide to cahnge colour of precipitate
Test for Ammonium ion, NH4+
(1) NaOH (aq) is added to solution of an ammonium ion
(2) Ammonia gas is produced - won’t see bubbles as very soluble in water
(3) Mixture warmed => gas released
: NH4+ (aq) + OH- (aq) ==> NH3 (g) + H2O (l)
(4) May smell it but TEST W/ LITMUS PAPER - is alkaline so will turn blue
Reactivity in Group 7
Decreases down the group
- wants to attract an electron
- increased sheilding that outweighs the increased nuclear charge
- nuclear attraction decreases
Displacement
Chlorine => Bromine => Iodine
