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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II

Q1(g). Overall equation and the role of each species

Question

The mechanism of a chemical reaction is: A + B → C + D; 2C → F; F + B → 2A + G. Give the overall balanced equation and classify each species as reactant, product, intermediate or catalyst. Give reasons.

Answer in brief

Overall: 3B → 2D + G. Reactant B; products D, G; intermediates C, F; catalyst A.

Key points

  • Adding the steps (step 1 taken twice, because step 2 needs 2C) gives 3B → 2D + G.
  • B is the reactant: consumed; appears on the left of the overall equation.
  • D, G are products: formed and never used again; on the right of the overall equation.
  • C, F are intermediates: formed in one step, consumed in a later step; absent from the overall equation.
  • A is the catalyst: consumed in step 1, regenerated in step 3; absent from the overall equation.

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.

Model answer to Q1(g), 2025 Mains Paper II: solution pages 7 and 8
Solution pages 7 and 8 of 33
Read the whole 2025 solution2025 question paper

Topic notes: Arrhenius equation, collision theory and transition-state theory, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversreaction mechanismreaction intermediatecatalystoverall balanced equationelementary steps

All parts of Question 1