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Indian Forest Service (Mains)

Chemistry optional

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

Paper I covers physical and inorganic chemistry.

Indian Forest Service (Mains), Chemistry optional · 2023 · Paper I

Q6(b). Threshold wavelength for excitation

Question

In a photochemical reaction, it usually takes 1.5 eV to 2.0 eV to put a molecule into an excited state. What is the maximum wavelength of light capable of exciting the molecule? Which part of electromagnetic spectrum it belongs? Also, calculate the energy per mole (in kJ/mole) associated with photon of that light.

Answer in brief

For the lowest stated threshold, 1.5 eV, λmax ≃ 827 nm in the near infrared and the photon energy is 144.7 kJ/mol; a 2.0 eV threshold instead requires λ ≤ 620 nm.

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Coversphoton energythreshold wavelengtheinstein of energyelectronvolt conversionphotochemistry

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