UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper I
Q1(d). Conductivity of phosphoric acid
Question
Although H₃PO₄ is viscous it exhibits large electrical conductivity. Why?
Answer in brief
An extended hydrogen-bond network permits structural proton transfer, so charge transport is not limited to slow movement of whole molecules.
Key points
- Viscosity hinders ordinary translational diffusion of ions.
- Phosphoric acid supports proton defects through proton-transfer/self-ionisation equilibria, represented by 2H₃PO₄ ⇌ H₄PO₄⁺ + H₂PO₄⁻.
- Successive proton exchanges along hydrogen bonds transfer charge through rearrangement of the network (Grotthuss-type structural diffusion).
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.
CoversConductivity of phosphoric acidProton transport
All parts of Question 1
- 1(a) Permanganate and iron(II)5 marks
- 1(b) Carbon isotopes and stability5 marks
- 1(c) Diagonal relationship5 marks
- 1(d) Conductivity of phosphoric acid5 marks
- 1(e) Oxidation of Fe(II) and Mn(II)5 marks
- 1(f) Melting points of bromine and iodine chloride5 marks
- 1(g) Small dipole moment of CO5 marks
- 1(h) Acidity of concentrated HF5 marks
- 1(i) Temperature and conductivity5 marks
- 1(j) Diborane with excess ammonia5 marks