Question

Why is the second electron affinity for oxygen positive instead of negative?

Answer 1

Hint: The amount of energy released when an electron is connected to a neutral atom or molecule in the gaseous state to produce a negative ion is described as the electron affinity ( $ {{E}_{ea}} $ ) of an atom or molecule.
This is not to be confused with the enthalpy change of electron capture ionisation, which is negative when energy is released. In other words, there is a negative difference between this enthalpy change and the electron affinity.

Complete answer:
Electron affinity may be described in two ways that are both equal. First, as the energy released when an electron is added to a single gaseous atom. The energy required to remove an electron from a singly charged gaseous negative ion is defined as electron affinity in the second (reverse) formulation. Either convention is acceptable. Ionization energies are always concerned with the creation of positive ions, whereas electron affinities are concerned with the formation of negative ions.
The enthalpy change for the first electron affinity is described as: There is energy released in getting the negative electron up to the positive nucleus.
 $ O\text{ }+\text{ }{{e}^{}}\to \text{ }{{O}^{}}~~~~~~~\text{ }\Delta H\text{ }=\text{ }Affinity\text{ }=\text{ }141\text{ }kJ/mol $
The enthalpy change for the second electron affinity is defined as: Because the electron is negatively charged, there will be repulsion between the ion and the electron, making the reaction endothermic, thus the positive value.
It's possible since this is just one energy shift among a long sequence that might occur during a reaction.
The total enthalpy change ( $ \Delta H $ ) for a reaction is determined by the net sum of these.
It's critical to maintain note of sign while using electron affinities. Energy is released when an electron is added to a neutral atom. This is called the first electron affinity, and the energies involved are negative. The negative symbol, by tradition, denotes a discharge of energy. However, adding an electron to a negative ion requires more energy, which overwhelms any energy released during the electron attachment process. This is called the second electron affinity, and the energies involved are positive.

Note:
Negative electron affinities are useful in situations when electron capture needs energy, such as when the impinging electron has enough kinetic energy to trigger a resonance in the atom-plus-electron system. Electron removal from the anion produced in this fashion, on the other hand, releases energy, which is carried out as kinetic energy by the liberated electron. In these situations, negative ions are always unstable. They can have lives ranging from microseconds to milliseconds, and they always autodetach after a certain amount of time.