[24.5] redox and qualitative analysis

Question 1

describe the oxidation of Fe²⁺ to Fe³⁺

Answer 1

• in the reaction between Fe²⁺ and MnO₄⁻, Fe²⁺ is oxidised to Fe³⁺ and MnO₄⁻ is reduced to Mn²⁺
• MnO₄⁻ (aq) + 8H⁺ (aq) + 5Fe²⁺ (aq) -> Mn²⁺ (aq) + Fe³⁺ (aq) + 4H₂O (l)
• purple -> colourless

Question 2

describe the reduction of Fe³⁺ to Fe²⁺

Answer 2

• in the reaction between Fe³⁺ ions and I⁻ ions, Fe³⁺ is reduced to Fe²⁺ and I⁻ is oxidised to I₂
• 2Fe³⁺ (aq) + 2I⁻ (aq) -> 2Fe²⁺ (aq) + I₂ (aq)
• 2Fe³⁺ = orange-brown, 2Fe²⁺ is page-green, but colour change is obscured by brown I₂

Question 3

describe the reduction of Cr₂O₇²⁻ to Cr³⁺

Answer 3

• Cr₂O₇²⁻ can be reduced to Cr³⁺ by the addition of zinc
• Cr₂O₇²⁻ (aq) + 14H⁺ (aq) + 3Zn (s) -> 2Cr³⁺ (aq) + 7H₂O (l) + 3Zn²⁺ (aq)
• Cr₂O₇²⁻ = organce, Cr³⁺ = green

Question 4

how can chromium (iii) ions be further reduced to chromium (ii)?

Answer 4

• by adding excess zinc
• Zn (s) + 2Cr³⁺ (aq) -> Zn²⁺ (aq) + 2Cr²⁺ (aq)
• Cr³⁺ = green, Cr²⁺ = pale blue

Question 5

describe the oxidation of Cr³⁺ to CrO₄²⁻

Answer 5

• oxidised by hot alkaline hydrogen perioxide, H₂O₂
• 3H₂O₂ + 2Cr³⁺ + 10OH⁻ -> 2CrO₄²⁻ + 8H₂O
• Cr is oxidised from +3 in Cr³⁺ to +6 in CrO₄²⁻
• O is reduced from -1 in H₂O₂ to -2 in CrO₄²⁻

Question 6

describe the reduction of Cu²⁺ to Cu⁺

Answer 6

• when Cu²⁺ reacts with excess I⁻ a redox reaction occurs
• I⁻ is oxidised to brown I₂, Cu²⁺ is reduced to Cu⁺
• 2Cu²⁺ (aq) + 4I⁻ (aq) -> 2CuI (aq) + I₂ (s)
• Cu²⁺ = pale blue, CuI = white precipitate, I₂ = brown

Question 7

describe the disproprtionation of Cu⁺ ions

Answer 7

• Cu₂O (s) + H₂SO₄ (aq) -> Cu (s) + CuSO₄ (aq) + H₂O (l)
• Cu = brown solid, CuSO₄ = blue solution
• Cu is reduced from +1 in Cu₂O to 0 in Cu
• Cu is oxidised from +1 in Cu₂O to +2 in CuSO₄