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Q: The electron gain enthalpy of chlorine is 349 kJ mol⁻¹. How much energy (in kJ) is released when 1 g of chlorine is completely converted to Cl– ions in the gaseous state?
Solution:
Step 1: Write the definition
Cl (g) + e– (g) → Cl– (g) ΔH = −349 kJ/mol
This means that 349 kJ energy is released when 1 mole (35.5 g) of chlorine atoms gains electrons.
Step 2: Energy released for 1 g of chlorine
Energy released = 349/35.5 x 1 = 9.83 kJ
Step 3: Calculation Energy released = 9.83 kJ
Final Answer:
When 1 g of chlorine is completely converted to Cl– ions, the energy released is: 9.83 kJ
Q : The amount of energy released when one million atoms of iodine in vapour state are converted to I– ions is 4.9 × 10-13 J according to the reaction: I(g) + e– → I–(g). Express the electron gain enthalpy of iodine in: (i) kJ mol⁻¹ (ii) eV per atom
Solution:
Step 1: Given data
Energy released for one million (1 × 106) atoms of iodine = 4.9×10−13 J
Step 2: Energy released for 1 atom of iodine
Energy for 1 atom = 4.9 × 10−13/1 × 106 = 4.9 × 10−19J
Step 3: Energy released for 1 mole (6.02 × 10²³ atoms)
E = 4.9 × 10−19 × 6.02 × 1023 = 2.95 × 105 J
Thus, the electron gain enthalpy of iodine = –295 kJ mol⁻¹.
Step 4: Converting to eV per atom
We know: 1 eV/atom = 96.3 kJ/mol
Electron gain enthalpy = 295/96.3 =3.06 eV/atom
Final Answer:
ΔHeg = −295 kJ/mol or −3.06 eV/atom
FAQs on Electron Gain Enthalpy Numericals
Q1. What is electron gain enthalpy?
👉 It is the amount of energy released (or absorbed) when an isolated gaseous atom accepts an extra electron to form a gaseous anion.
Q2. Why is electron gain enthalpy usually negative?
👉 Because energy is released when an atom gains an electron (exothermic process). A more negative value means higher tendency to gain an electron.
Q3. How do we convert energy released per atom into energy released per mole?
👉 Multiply the energy released per atom by Avogadro’s number (6.022 × 10²³ atoms mol⁻¹).
Q4. How do we convert electron gain enthalpy from kJ mol⁻¹ to eV per atom?
👉 Use the relation: 1 eV atom−1 = 96.3 kJ mol−1
So, divide the value in kJ mol⁻¹ by 96.3.
Q5. Why is the electron gain enthalpy of halogens highly negative?
👉 Because halogens need only one electron to complete their octet, so they readily accept an electron and release large amounts of energy.
Q6. Why is the electron gain enthalpy of fluorine less negative than chlorine?
👉 Due to fluorine’s very small atomic size, the incoming electron faces strong electron-electron repulsion, reducing the amount of energy released.
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