Coloured compounds

Question 1

Iron(ii)

Answer 1

• aqueous ion = [Fe(H2O)6]2+, green solution
• adding NaOH = Fe(H2O)4(OH)2, green precipitate that goes brown in air
• adding excess of NaOH = No further change
• adding NH3 = Fe(H2O)4(OH)2, green precipitate that goes brown in air
• adding excess of NH3 = no further change
• adding Na2CO3 = FeCO3, green precipitate

Question 2

Copper(ii)

Answer 2

• aqueous ion = [Cu(H2O)6]2+, blue solution
• adding NaOH = Cu(H2O)4(OH)2, blue precipitate
• adding excess of NaOH = No further change
• adding NH3 = Cu(H2O)4(OH)2, blue precipitate
• adding excess of NH3 = [Cu(H2O)2(NH3)4]2+, deep-blue ppt
• adding Na2CO3 = CuCO3, blue/green precipitate

Question 3

Iron(iii)

Answer 3

• aqueous ion = [Fe(H2O)6]3+, purple solution, may look slightly yellow/brown due to some [Fe(H2O)5(OH)]2+
• adding NaOH = Fe(H2O)3(OH)3, brown precipitate, (might look orange/brown)
• adding excess of NaOH = No further change
• adding NH3 = Fe(H2O)3(OH)3, blue precipitate
• adding excess of NH3 = No further change
• adding Na2CO3 = Fe(H2O)3(OH)3, brown precipitate, also produces CO2 gas

Question 4

Aluminium(iii)

Answer 4

• aqueous ion = [Al(H2O)6]3+,colourless solution
• adding NaOH = Al(H2O)3(OH)3, white precipitate
• adding excess of NaOH = [Al(OH)4]-, colourless solution
• adding NH3 = Al(H2O)3(OH)3, white precipitate
• adding excess of NH3 = No further change
• adding Na2CO3 = Al(H2O)3(OH)3, white precipitate, CO2 gas produced

Question 5

Vanadium oxidation states

Answer 5

• Vanadium (v) = [VO2(H2O)4]+ in acidic solution, yellow
• Vanadium (iv) = [VO(H2O)5)2+ in acidic solution, blue
• Vanadium (iii) = [V(H2O)6]3+ in acidic solution, green
• Vanadium (ii) = [V(H2O)6]2+ in acidic solution, purple