Indian Forest Service (Mains), Chemistry optional · 2025 · Paper II
Q8(a)(ii). Isotopic change in rotational constant
Question
Calculate the change in rotational constant when hydrogen is replaced by deuterium in the hydrogen molecule.
Answer in brief
For complete H₂ → D₂ substitution at unchanged bond length, B(D₂) = B(H₂)/2: a 50% decrease.
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Coversisotopic substitutionrotational constantreduced massH₂ and D₂HD
All parts of Question 8
- 8(a)(i) CO rotational constant and bond length10 marks
- 8(a)(ii) Isotopic change in rotational constant5 marks
- 8(b)(i) Isotope exchange and zero-point energy10 marks
- 8(b)(ii) Hot-band population in iodine10 marks
- 8(c) Metalation of thiophene5 marks