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

Correct answer: -5.1 eV

Solution

The electron gain enthalpy of Na+ is equal in magnitude but opposite in sign to the ionization potential of Na, as it reflects the energy change when an electron is added to Na+. Therefore, since the ionization potential is 5.1 eV, the electron gain enthalpy for Na+ is -5.1 eV.

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