24.5 Redox And Qualitative Analysis

Question 1

What reaction occurs during oxidation of Fe2+ to Fe3+? Associated colour changes?

Answer 1

Redox titration between Fe2+ and MnO4- ions:
Fe2+ → Fe3+ + e-
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
Manganate (VII) is purple, Mn2+ is colourless

Question 2

How is Fe3+ reduced to Fe2+? Any associated colour changes?

Answer 2

2Fe3+ (aq) + 2I- (aq) → 2Fe2+ (aq) + I2 (aq)
Fe3+ is orange-brown
Fe2+ is pale-green (colour change is obscured by I2)
I2 is brown

Question 3

How is Cr2O72- reduced to Cr3+? Any associated colour changes? What happens when this reactant is added in excess?

Answer 3

Zn (s) → Zn2+ (aq) + 2e-
Cr2O72- (aq) + 14H+ (aq) + 6e- → Cr3+ + 7H2O
Cr2O72- is orange
Cr3+ is green

When Zn is added in excess:
Zn (s) + 2Cr3+ (aq) → Zn2+ (aq) + 2Cr2+ (pale blue)
Zn2+ is pale blue

Question 4

How is Cr3+ oxidised to (CrO4)2-?

Answer 4

Use of hot alkaline hydrogen peroxide as an oxidising agent:
3H2O2 + 2Cr3+ + 10OH- → 2CrO42- + 8H2O
Cr3+ is green
Cr (VI) is orange

Question 5

How is Cu2+ reduced to Cu+? Associated colour changes?

Answer 5

2Cu2+ (aq) + 4I- (aq) → 2CuI (aq) + I2 (s)
Cu2+ is pale blue
CuI is a white precipitate (somewhat masked by brown)
I2 is brown

Question 6

How is Cu disproportionated in the reaction between solid Cu2O and hot dilute H2SO4?

Answer 6

Cu2O (s) + H2SO4 (aq) → Cu (s) + CuSO4 (aq) + H2O (l)
Cu is a brown solid
CuSO4 is a blue solution