Question

When the first electron gain enthalpy $( \Delta_{eg}H) $ of oxygen is $-141\, kJ/mol$, its second electron gain enthalpy is :

• A. almost the same as that of the first
• B. negative , but less negative than the first
• C. a positive value
• D. a more negative value than the first

Answer 1

The Correct Option is C

To solve this question, we need to understand the concept of electron gain enthalpy, also known as electron affinity.

Electron gain enthalpy is the enthalpy change when an electron is added to an isolated gaseous atom to form a gaseous anion. It can be either negative or positive:

• A negative electron gain enthalpy means that energy is released when the electron is added.
• A positive electron gain enthalpy indicates that energy must be supplied to add an electron.

For oxygen, the first electron gain enthalpy (\Delta_{eg}H) is -141\, \text{kJ/mol}, which means energy is released when the first electron is added to an oxygen atom, forming \text{O}^-\).

However, when adding a second electron, the second electron gain enthalpy becomes positive. This is because the added electron must overcome the electron-electron repulsion of the already negatively charged ion \text{O}^-\)..

Thus, energy must be supplied to add the second electron, making the second electron gain enthalpy a positive value.

The correct option is:

• ***a positive value***

Now, let's rule out the other options:

• Almost the same as that of the first: This is incorrect because adding the first electron releases energy, while adding the second requires energy.
• Negative, but less negative than the first: This option is incorrect because it suggests that some energy is released, whereas energy is actually needed for the second electron.
• A more negative value than the first: This option is incorrect because it implies even more energy is released for the second electron, which contradicts the fact that energy input is required.