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If the first ionisation energy of sodium is 5.1 eV, what is the electron gain enthalpy of Na+?

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

Correct answer: −5.1 eV

Solution

Adding an electron to Na+ to reform Na is the exact reverse of the first ionization, so the electron gain enthalpy of Na+ = -5.1 eV, not -2.55 eV.

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