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Q3.31(d):
The first ($\Delta_{i}H_{1}$) and the second ($\Delta_{i}H_{2}$) ionization enthalpies (in kJ $mol^{-1}$) and the ($\Delta_{eg}H$) electron gain enthalpy (in kJ $mol^{-1}$) of a few elements are given below: Elements $\Delta H_{1}$ $\Delta H_{2}$ $\Delta_{eg}H$ I 520 7300 –60 II 419 3051 –48 III 1681 3374 –328 IV 1008 1846 –295 V 2372 5251 +48 VI 738 1451 –40 Which of the above elements is likely to be : (d) the least reactive non-metal.

Solution :

Step 1: Theoretical Foundation of Non-Metallic Reactivity

Reactivity in non-metals is governed by two primary thermodynamic parameters:

  • Electron Gain Enthalpy ($\Delta_{eg}H$): A highly negative value indicates a high tendency to gain electrons and form anions, implying high chemical reactivity (e.g., Halogens).
  • Ionization Enthalpy ($\Delta_{i}H$): High values suggest a strong resistance to losing electrons, which is characteristic of stable non-metals.

A "least reactive" non-metal is characterized by a low, near-zero, or positive electron gain enthalpy and high ionization enthalpy. This indicates an element that neither readily gains nor loses electrons, approaching the stability of noble gas configurations [Per Periodic Trends of Inert Gases].

Step 2: Comparative Analysis of Provided Data

Element $\Delta_{i}H_{1}$ ($kJ/mol$) $\Delta_{i}H_{2}$ ($kJ/mol$) $\Delta_{eg}H$ ($kJ/mol$) Assessment
I 520 7300 -60 Active Metal (Low $\Delta_{i}H_{1}$)
II 419 3051 -48 Active Metal (Low $\Delta_{i}H_{1}$)
III 1681 3374 -328 Highly Reactive Non-Metal (High negative $\Delta_{eg}H$)
IV 1008 1846 -295 Reactive Non-Metal (High negative $\Delta_{eg}H$)
V 2372 5251 +48 Inert/Least Reactive
VI 738 1451 -40 Alkaline Earth Metal

Step 3: Justification for Element V

Element V exhibits:

  1. Extremely high $\Delta_{i}H_{1}$ ($2372\ kJ/mol$): This indicates a closed-shell configuration or a very stable electronic structure, making the removal of an electron energetically unfavorable [By Ionization Energy Periodicity].
  2. Positive $\Delta_{eg}H$ ($+48\ kJ/mol$): A positive value implies that energy must be supplied to force an electron onto the atom. This signifies that the addition of an electron is non-spontaneous, which is the hallmark of noble gases or highly stable non-reactive systems [Per The Second Law of Thermodynamics regarding spontaneity].

Visual Representation of Reactivity Trends

Ionization Enthalpy Stability Element V

Final Solution: Element V is the least reactive non-metal due to its exceptionally high first ionization enthalpy and positive electron gain enthalpy, indicating a highly stable electronic configuration that resists chemical interaction.


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