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Current Question (ID: 7927)

Question:

$\text{The formation of the oxide ion O}^{2-}\text{(g), from the oxygen atom requires first an exothermic and then an endothermic step as shown below,}$ $\text{O(g) + e}^- \rightarrow \text{O}^-; \Delta_f H^0 = -141 \text{ kJ mol}^{-1}$ $\text{O}^-\text{(g) + e}^- \rightarrow \text{O}^{2-}\text{(g)}; \Delta_f H^0 = +780 \text{ kJ mol}^{-1}$ $\text{Thus, the process of formation of O}^{2-}\text{ in the gas phase is unfavorable even though O}^{2-}\text{ is isoelectronic with neon. It is due to the fact that:}$

Options:

1. $\text{Electron repulsion outweighs the stability gained by achieving noble gas configuration.}$ (Correct)
2. $\text{O}^-\text{ ion has a comparatively smaller size than the oxygen atom.}$
3. $\text{Oxygen is more electronegative.}$
4. $\text{Addition of electrons in oxygen results in a large size of the ion.}$

Solution:

$\text{HINT: Electronic repulsions dominates the stability.}$ $\text{Explanation:}$ $\text{O(g) + e}^- \rightarrow \text{O}^-; \Delta_f H^0 = -141 \text{ KJ mol}^{-1}$ $\text{O}^-\text{(g) + e}^- \rightarrow \text{O}^{2-}\text{(g)}; \Delta_f H^0 = +780 \text{ KJ mol}^{-1}$ $\text{Formation of O}^{2-}\text{ in gaseous phase is unfavorable because electronic repulsions predominate over the stability gained by achieving noble gas configuration.}$ $\text{Thus, option 1 is the correct answer.}$ $\text{Detailed explanation:}$ $\text{The first electron addition to oxygen is exothermic (-141 kJ/mol) because:}$ $\text{1. The electron is attracted to the positively charged nucleus}$ $\text{2. There is some electron-electron repulsion, but nuclear attraction dominates}$ $\text{The second electron addition is highly endothermic (+780 kJ/mol) because:}$ $\text{1. The incoming electron must be added to an already negatively charged O}^-\text{ ion}$ $\text{2. Strong electron-electron repulsion occurs in the compact 2p orbitals}$ $\text{3. The repulsion energy outweighs the stability gained from achieving neon's electronic configuration}$ $\text{Even though O}^{2-}\text{ achieves the stable noble gas configuration of neon (1s}^2\text{2s}^2\text{2p}^6\text{), the enormous electron-electron repulsion makes the overall process energetically unfavorable in the gas phase.}$ $\text{This is why O}^{2-}\text{ ions are stable only in ionic solids where the lattice energy compensates for the high electron gain enthalpy.}$

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Expected JSON Format:

{
  "question": "The mass of carbon present in 0.5 mole of $\\mathrm{K}_4[\\mathrm{Fe(CN)}_6]$ is:",
  "options": [
    {
      "id": 1,
      "text": "1.8 g"
    },
    {
      "id": 2,
      "text": "18 g"
    },
    {
      "id": 3,
      "text": "3.6 g"
    },
    {
      "id": 4,
      "text": "36 g"
    }
  ],
  "solution": "\\begin{align}\n&\\text{Hint: Mole concept}\\\\\n&1 \\text{ mole of } \\mathrm{K}_4[\\mathrm{Fe(CN)}_6] = 6 \\text{ moles of carbon atom}\\\\\n&0.5 \\text{ mole of } \\mathrm{K}_4[\\mathrm{Fe(CN)}_6] = 6 \\times 0.5 \\text{ mol} = 3 \\text{ mol}\\\\\n&1 \\text{ mol of carbon} = 12 \\text{ g}\\\\\n&3 \\text{ mol carbon} = 12 \\times 3 = 36 \\text{ g}\\\\\n&\\text{Hence, 36 g mass of carbon present in 0.5 mole of } \\mathrm{K}_4[\\mathrm{Fe(CN)}_6].\n\\end{align}",
  "correct_answer": 4
}