CPrac K - QA

Describe observation and deductions when addition of KI to unknown solution 

Describe observation and deductions when addition of CuSO4 and Cu2+

Describe deduction from decolourisation of KMnO4, Yellow FeCI3 turns pale green and heating

1. Decolourisation of KMnO4 -> reducing agent present

2. Yellow FeCI3 turns pale green -> reducing agent present 

3. Heating 

• Heating of a solid sample should be done last if its residue is not needed to continue in other tests

Describe the reaction between CO32- and dilute acid (equations)

• Acid-base reaction:

• CO32- + 2H+ → H2CO3 → CO2 + H2O

• CO2 + Ca(OH)2 → CaCO3 + H2O

• Excess CO2: CaCO₃(s) + CO₂(g) + H₂O(l)  -> Ca(HCO₃)₂(aq) (CaCO3 dissolves and colourless solution is formed)

Describe the reaction between CI- and AgNO3 followed by NH3 

• White ppt. is AgCl.

• Ag+(aq) + C/-(aq) = AgC/(s) -- (1)

• Ag+(aq) + 2NH3(aq) = [Ag(NH3)2]+ (aq)

• White ppt. dissolved in NH3(aq) due to formation of [Ag(NH3)2]+ which caused [Ag+] to decrease and hence equilibrium position of (1) will shift to the left

Describe the reaction between Br- and AgNO3 followed by NH3 

Pale cream ppt. is AgBr

Br ion has a more polarisable charge cloud as it has a larger size than Cl-. Hence AgBr is partially soluble in NH3(aq) due to its greater covalent character.

Describe test for I-

Describe 2 tests for NO2-

Describe test for NO3-

Describe test for SO42-

Describe test for SO32-

How to differentiate NO2− from NO3−

• Add dilute hydrochloric or sulfuric acid and warm

• If brown NO2 gas (or fumes) is evolved, then NO2− is present

How to differentiate SO32− from SO42−

• Add dilute hydrochloric or nitric acid to the solution and warm

• If pungent SO2 gas is evolved which decolourises acidified KMnO4(aq), then SO32− is present 

• BaSO4 is insoluble in aqueous acid 

How to differentiate Br- and I- (OTHER than AgNO3) 

• Add some dilute H2SO4 and an equal volume of KMnO4(aq) to separate samples of both ions (oxidised both anions) 

• Add an equal volume of hexane, shake well and observe the colour of the hexane layer

• Br2 gives an orange-red colour in hexane while I2 gives a purple colour in hexane

Precaution when warming

• Apply the heating, using a gentle flame, near the surface of the liquid -> reduces the risk of the solution spurting out of the tube

• Wear goggles