QA ( Test For Cations, Anions, Gases) Flashcards
List some acidic cations in QA and state how they release H3O+ ions (TLDR version)
Cr3+, NH4+, Al3+, Fe3+
Cr, Fe and Al form complex ions (Al(H2O)6)3+, for example
They undergo hydrolysis to release H3O+, giving an acidic solution
Given the acidic nature of Fe3+, Cr3+, Al3+ and NH4+, what is another way to distinguish these ions? Explain the reactions that occur
Add Na2CO3. [Al(H2O)6]3+ reacts with Na2CO3 to give Al(OH)3 and CO2 gas.
The complex ion of Al3+ will release H3O+ ions into the solution, which react with CO32- ions in an acid base reaction to give carbon dioxide + water.
H2O ligands surrounding Al3+ will keep releasing H+ until [Al(H2O)3(OH)3] is formed, which is essentially Al(OH)3
NOTE: the hydrolysis of NH4+ will NOT form any precipitate!!!!
Explain why Al3+, Cr3+ and Zn2+ are soluble in excess NaOH in the QA table
When NaOH added to solutions containing these ions, amphoteric metal hydroxides are formed, which can react further with NaOH to give soluble complex ions. (Metal hydroxide act as acid)
Zn2+ forms [Zn(OH)4]-
Cr3+ forms [Cr(OH)6]3-
Al3+ forms [Al(OH)4]-
These amphoteric metal hydroxides can also react with acids to give salt + water
Explain why Al3+, Cr3+ and Zn2+ are soluble in excess NaOH in the QA table
When NaOH added to solutions containing these ions, amphoteric metal hydroxides are formed, which can react further with NaOH to give soluble complex ions. (Metal hydroxide act as acid)
Zn2+ forms [Zn(OH)4]-
Cr3+ forms [Cr(OH)6]3-
Al3+ forms [Al(OH)4]-
These amphoteric metal hydroxides can also react with acids to give salt + water
Explain why Cu2+ and Zn2+ form precipitates that are soluble in excess NH3 in the QA table
LIGAND EXCHANGE REACTION. NH3 ligands replace weaker H2O ligands, forming soluble complex ions [Cu(NH3)4]2+ and [Zn(NH3)4]2+
What is another way to test for Cu2+ and Fe3+ ions?
Add excess aqueous potassium iodide, which acts as a reducing agent
With Cu2+: it forms CuI and I2
With Fe3+: it forms Fe2+ and I2
How do you distinguish between BaSO4 and BaSO3?
Add dilute acid. BaSO4 does not react but BaSO3 reacts to give SO2, Ba2+ and H2O
Why is NH3 liberated when aluminium and NaOH is added to a solution containing nitrate or nitrite ions?
Aluminium reduces Nitrate/nitrite ions to NH4+, which reacts with NaOH in an acid base reaction to form NH3
Why is it necessary to warm the mixture to get a positive test for test for nitrate ions??
Ammonia is soluble in water. Heating the mixture reduces the solubility of NH3 gas, thus causing it to be evolved
