24.5 Redox and qualitative anaylsis

describe the oxidation of Fe²⁺ to Fe³⁺

MnO₄^-(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(aq)

Fe²⁺ is oxidised to Fe³⁺

MnO₄^- is reduced Mn²⁺

colour change from purple to colourless solution

describe the reduction of Fe³⁺ to Fe²⁺

2Fe³⁺(aq) + 2I^-(aq) → 2Fe²⁺(aq) + I₂(aq)

Fe³⁺ is reduced to Fe²⁺

I^- is oxidised to I₂

colour change from orange-brown to pale-green Fe²⁺(aq) ions and brown I₂(aq)

describe the reduction of Cr₂O₇^2- to Cr³⁺

Cr₂O₇^2-(aq) + 14H⁺(aq) + 3Zn(s) → 2Cr³⁺(aq) + 7H₂O(l) + 3Zn²⁺(aq)

colour change from orange to green

with excess Zn(s), Cr³⁺ ions are reduced further to Cr²⁺ ions

Zn (s) + 2Cr³⁺ (aq) → Zn²⁺ (aq) + 2Cr²⁺(aq)

colour change from green to pale blue

describe the oxidation of Cr³⁺ to CrO₄^2-

3H₂O₂ + 2Cr³⁺+ 10OH^- → 2CrO₄^2- + 8H₂O

chromium is oxidised from +3 in Cr³⁺ to +6 in CrO₄^2-

oxygen is reduced from -1 in H₂O₂ to -2 in CrO₄^2-

describe the reduction of Cu²⁺ to Cu⁺

2Cu²⁺(aq) + 4I^-(aq) → 2CuI(aq) + I₂(s)

I^- is oxidised to brown I₂

Cu²⁺ is reduced to Cu⁺

white precipitate of CuI(aq) formed

describe the disproportionation of Cu⁺ ions

Cu₂O(s) + H₂SO₄(aq) → Cu(s) + CuSO₄(aq) + H₂O(l)

brown precipitate of Cu(s) forms, and blue solution of CuSO₄(aq) forms

Cu has been reduced and oxidised