Question

Question 3.21 Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.

Class XI Classification of Elements and Periodicity in Properties Page 93

Answer 1

$O(g) + e ------> O^-(g);\triangle{H}=-141KJ/mol\\O^-(g)+e----->O^{2-}(g);\triangle{H}=+780KJ/mol$

see above equations and electron gain enthalpy of O and O- . you observed that ,
electron gain enthalpy of O is negative while electron gain enthalpy of O- is positive .

when an electron is added to Oxygen atom to form O- ion , energy is released . hence, first electron gain enthalpy of oxygen is negative .
But when another electron is add to O- ion to form O2- ion , it feels stronger electrostatic repulsion. hence, when we add 2nd electron , then energy absorb by it. that's why the 2nd electron gain enthalpy of oxygen is positive.

Answer 2

Answer:

When an electron is added to Oxygen atom to form O- ion, energy is released. Hence, first electron gain enthalpy of oxygen is negative.

But when another electron is add to O- ion to form O2- ion, it feels stronger electrostatic repulsion. Hence, when we add 2nd electron, then energy absorb by it. That's why the 2nd electron gain enthalpy of oxygen is positive.