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Q3.31(c):
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 : (c) the most reactive non-metal.

Solution :

Step 1: Establishing Theoretical Criteria for Non-Metallic Reactivity

To identify the "most reactive non-metal" among the given elements, we must analyze the periodic trends governing chemical behavior:

  • Ionization Enthalpy ($\Delta_{i}H$): Non-metals typically possess high first ionization enthalpies because they have a high effective nuclear charge ($Z_{eff}$) and a strong tendency to gain electrons rather than lose them.
  • Electron Gain Enthalpy ($\Delta_{eg}H$): This represents the energy released when an electron is added to a neutral gaseous atom. A large negative value indicates high electron affinity, which is a hallmark of highly reactive non-metals (especially those in Group 17, the halogens).
  • Selection Rule: The most reactive non-metal will exhibit the most negative value of $\Delta_{eg}H$ (most exothermic) combined with a high $\Delta_{i}H_1$.

Step 2: Comparative Analysis of Data

We tabulate the provided values to facilitate systematic comparison:

Element $\Delta_{i}H_{1}$ ($kJ \cdot mol^{-1}$) $\Delta_{i}H_{2}$ ($kJ \cdot mol^{-1}$) $\Delta_{eg}H$ ($kJ \cdot mol^{-1}$)
I 520 7300 -60
II 419 3051 -48
III 1681 3374 -328
IV 1008 1846 -295
V 2372 5251 +48
VI 738 1451 -40

Step 3: Analytical Deduction

Analyzing the values:

  • Elements I, II, and VI have low $\Delta_{i}H_{1}$ values, characteristic of alkali and alkaline earth metals.
  • Element V has a positive $\Delta_{eg}H$, implying a stable configuration (noble gas-like) that resists electron gain.
  • Element III and IV exhibit high $\Delta_{i}H_{1}$ and highly negative $\Delta_{eg}H$ values.
  • Comparing III and IV, Element III possesses a significantly higher $\Delta_{eg}H$ value of $-328 \ kJ \cdot mol^{-1}$. This magnitude is characteristic of a halogen (specifically Chlorine, $Cl$), which is highly reactive due to its strong ability to achieve an octet configuration [Per the Octet Rule].

Step 4: Graphical Representation of Trends

Elements -$\Delta_{eg}H$ (kJ/mol) Element III

Final Solution: Element III is the most reactive non-metal because it possesses the highest negative value of electron gain enthalpy ($-328 \ kJ \cdot mol^{-1}$), indicating the highest propensity to attract an electron to complete its valence shell.


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