UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper I
Q1(e). Interpreting the redox data
Question
Which of the two ions Fe(II) and Cr(III) would be easily oxidized? Fe³⁺ + e⁻ ⇌ Fe²⁺ E*=+0.77 volt Cr³⁺ + e⁻ ⇌ Cr²⁺ E*=0.41 volt E* is standard reduction potential.
Answer in brief
The supplied couples compare oxidation of Fe(II) and Cr(II), not Fe(II) and Cr(III). Under the intended comparison, Cr(II) is oxidised more readily.
Key points
- Oxidation is the reverse of each reduction half-reaction. On the printed numbers, Eₒₓ(Fe²⁺/Fe³⁺)=-0.77 V and Eₒₓ(Cr²⁺/Cr³⁺)=-0.41 V.
- Thus Cr²⁺ is the stronger reducing agent: its oxidation potential is the more positive of the two.
- The conventional aqueous standard value for Cr³⁺/Cr²⁺ is approximately -0.41 V, giving E°_(cell)=1.18 V. This sign correction does not change the intended conclusion.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversInterpreting the redox dataElectrode potentials
All parts of Question 1
- 1(a) Occurrence of lithium5 marks
- 1(b) Bond order and ionisation5 marks
- 1(c) Solvent levelling5 marks
- 1(d) Disproportionation and its reverse5 marks
- 1(e) Interpreting the redox data5 marks
- 1(f) Existence of hydrogen and helium dimers5 marks
- 1(g) Packing fraction5 marks
- 1(h) Square-planar geometrical isomers5 marks
- 1(i) Ground term and angular momenta5 marks
- 1(j) Effective atomic number5 marks