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

Answer 1

Caleb Adams

Because you are bringing together two negative charges.

The enthalpy change for: is the definition of the first electron affinity.

#X_((g))+erarrX_((g))^-#

The process of bringing the negative electron up to the positive nucleus releases energy.

The enthalpy change for the following defines the second electron affinity:

#X_((g))^(-)+erarrX_((g))^(2-)#

The positive value indicates that the process is endothermic, as there will be repulsion between the electron and the ion due to the electron's negative charge.

It is possible because this energy shift is only one in a series of changes that could take place during a reaction.

It is the net sum of these which decides the overall enthalpy change (#DeltaH#) for a reaction.

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Answer 2

Jeremiah Adams

The second electron affinity for oxygen is positive because adding an electron to an oxygen ion (O^-) would require overcoming the repulsion between the negatively charged electron and the negatively charged ion, making the process energetically unfavorable.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.