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Which of the following statements about electron gain enthalpy is correct?

1. Electron gain enthalpy may be positive for some elements
2. The second electron gain enthalpy is always positive for all elements
3. The electron gain enthalpy of K⁺ equals the negative of the ionisation enthalpy of K
4. All of these statements are correct

Correct answer: All of these statements are correct

Solution

A: Some elements (e.g. N with half-filled 2p, noble gases, Be with filled 2s) have positive (endothermic) electron gain enthalpies because adding an electron is unfavourable. True. B: Adding a second electron to an already-negative ion requires overcoming strong electrostatic repulsion, so second electron gain enthalpy is always positive. True. C: K⁺ + e⁻ -> K: this is the reverse of K's first ionisation (K -> K⁺ + e⁻), so delta_eg(K⁺) = -IE1(K). True. All three are correct, so the answer is D.

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