At 298.2 K the relationship between enthalpy of bond dissociation (in kJ mol-1) for hydrogen (EH) and its isotope,…
Thermodynamics · Class 11 · JEE Main Previous Year Question
At 298.2 K the relationship between enthalpy of bond dissociation (in ) for hydrogen () and its isotope, deuterium (), is best described by:
- a
- b
- c✓
- d
🧠 Heavier isotope → lower zero-point energy → stronger bond Vibrational frequency scales as where is reduced mass. Deuterium is heavier, so DD vibrates more slowly, has lower zero-point energy, sits deeper in the potential well, and needs more energy to dissociate than HH.
The experimental difference in bond dissociation enthalpies is approximately 7.5 kJ mol, giving:
⚠️ Trap: Students often assume isotopes have identical chemical bond strengths. The kinetic isotope effect is small but measurable — deuterium bonds are genuinely stronger.
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