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Chemistry Question 123 – AP-EAMCET 2025

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From the following identify the change in which electron gain enthalpy is positive.

Electron gain enthalpy is the energy change that occurs when an electron is added to a neutral gaseous atom to form a gaseous anion.

Step 1: Define Electron Gain Enthalpy✦ Active

Electron gain enthalpy (ΔegH) is the energy change that occurs when an electron is added to a neutral gaseous atom to form a gaseous anion. This process can be either exothermic (energy released, ΔegH<0) or endothermic (energy absorbed, ΔegH>0). Most elements have negative electron gain enthalpies for the first electron, as the incoming electron is attracted to the positively charged nucleus.

Step 2: Identify Exceptions to the General Trend○ Expand

Elements with stable electron configurations, such as noble gases (Group 18), alkaline earth metals (Group 2), and nitrogen (Group 15), tend to have positive electron gain enthalpies. For these elements, adding an extra electron requires energy input because the electron must occupy a higher energy orbital or overcome significant electron-electron repulsion due to an already stable electron shell.

💡 Teacher's Secret Hint

Remember that the second electron gain enthalpy (e.g., O−→O2−) is always positive due to repulsion between the incoming electron and the already negatively charged ion.

Step 3: Analyze Each Option○ Expand

Let's examine each given reaction:

1. Li(g)⟶Li−(g): Lithium is an alkali metal. It readily accepts an electron to achieve a stable noble gas configuration (like Helium). This process is exothermic, so ΔegH<0.

2. O(g)⟶O−(g): Oxygen is a chalcogen. It readily accepts an electron to form a more stable anion. This process is exothermic, so ΔegH<0.

3. Xe(g)⟶Xe−(g): Xenon is a noble gas. It has a completely filled valence shell, making it highly stable. Adding an electron to Xenon requires significant energy input because the electron must enter a much higher energy level and overcome strong electron-electron repulsion. Therefore, this process is endothermic, and ΔegH>0.

4. S(g)⟶S−(g): Sulfur is also a chalcogen, similar to oxygen. It readily accepts an electron. This process is exothermic, so ΔegH<0.

💡 Teacher's Secret Hint

Focus on the group number and electron configuration of each element to predict its tendency to gain an electron.

Step 4: Conclusion○ Expand

Based on the analysis, the change in which electron gain enthalpy is positive is for Xenon, a noble gas.

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