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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2018 · Paper I

Q8(b)(i). Einstein's law of photochemical equivalence

  • 5 marks
  • Answered
Question

Define and explain Einstein's law of photochemical equivalence.

Answer in brief

Each molecule taking part in a photochemical reaction absorbs one quantum of radiation, hν, in the primary (light-absorbing) step. One mole of reacting molecules therefore absorbs one einstein, N_Ahν=N_Ahc/λ.

Key points

  • in the primary process each molecule that reacts is activated by absorbing one photon; one quantum is absorbed per molecule activated.
  • the energy absorbed per mole is E=N_Ahν=(N_Ahc)/(λ), called one einstein. At 494 nm this equals the Cl–Cl bond enthalpy (242 kJ mol⁻¹), so light of shorter wavelength can dissociate Cl₂.
  • Φ=(number of molecules reacting)/(number of photons absorbed). The law strictly predicts Φ=1 for the primary step.
  • Apparent deviations come from secondary (dark) reactions: Φ≫1 for chain reactions (H₂ + Cl₂, 10⁴ to 10⁶); Φ<1 when excited molecules lose energy by fluorescence, collision or recombination (HI decomposition gives Φ=2 because…

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Statement (Stark–Einstein law): in the primary process each molecule that reacts is activated by absorbing one photon; one quantum is absorbed per molecule activated.
  2. Einstein: the energy absorbed per mole is E=N_Ahν=(N_Ahc)/(λ), called one einstein. At 494 nm this equals the Cl–Cl bond enthalpy (242 kJ mol⁻¹), so light of shorter wavelength can dissociate Cl₂.
  3. Quantum yield: Φ=(number of molecules reacting)/(number of photons absorbed). The law strictly predicts Φ=1 for the primary step.
  4. Apparent deviations come from secondary (dark) reactions: Φ≫1 for chain reactions (H₂ + Cl₂, 10⁴ to 10⁶); Φ<1 when excited molecules lose energy by fluorescence, collision or recombination (HI decomposition gives Φ=2 because the H atom attacks a second HI).
  5. The law holds for ordinary light intensities; with lasers two photons can be absorbed together.

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Model answer to Q8(b)(i), 2018 Paper I: solution page 50
Solution page 50 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 8