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The first ionisation potential of Na is 5.1 eV. The value of electron gain enthalpy of Na+ will be

1. -2.55 eV
2. -5.1 eV
3. -10.2 eV
4. +2.55 eV

Correct answer: -5.1 eV

Solution

The electron gain enthalpy of Na+ is equal in magnitude but opposite in sign to the first ionisation potential of Na, as it represents the energy change when an electron is added to a positively charged ion. Therefore, since the ionisation potential is 5.1 eV, the electron gain enthalpy for Na+ is -5.1 eV.

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