Question

Observe the following data (\(\Delta_i H_1\), \(\Delta_i H_2\) and \(\Delta_{eg}H\) represent the first, second ionisation enthalpies and electron gain enthalpy respectively).

Using the data identify the most reactive metal.

• A. II
• B. I
• C. IV
• D. III

Hint:

For identifying the most reactive metal, the primary factor to look for is the lowest first ionization enthalpy (\(\Delta_i H_1\)). For the most reactive non-metal, look for the most negative electron gain enthalpy (\(\Delta_{eg}H\)).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Metallic character is associated with the tendency of an element to lose electrons and form positive ions. The most reactive metal will be the one that can lose its valence electron(s) most easily. This property is primarily indicated by a low first ionization enthalpy (\(\Delta_i H_1\)).
Step 2: Analyzing the Data:
Let's examine the first ionization enthalpy (\(\Delta_i H_1\)) for each element, as this is the energy required to remove the outermost electron, a key indicator of metallic reactivity.
- Element I: \(\Delta_i H_1 = 520\) kJ mol⁻¹
- Element II: \(\Delta_i H_1 = 490\) kJ mol⁻¹
- Element III: \(\Delta_i H_1 = 1681\) kJ mol⁻¹
- Element IV: \(\Delta_i H_1 = 2372\) kJ mol⁻¹
Comparing these values, Element II has the lowest first ionization enthalpy (\(490\) kJ mol⁻¹). This means it requires the least amount of energy to lose one electron, making it the most electropositive and thus the most reactive metal among the choices.
Step 3: Further Analysis for Confirmation:
- Element I: Has a low \(\Delta_i H_1\) but an extremely high \(\Delta_i H_2\) (7300 kJ mol⁻¹). This huge jump indicates that after losing one electron, it achieves a stable noble gas configuration. This is characteristic of an alkali metal (like Li). \(\Delta_i H_1\) is 520 kJ/mol, which is the value for Lithium.
- Element II: Has the lowest \(\Delta_i H_1\) (490 kJ/mol). The second ionization energy is also relatively high, but the jump is not as drastic as for Element I. These values are characteristic of a larger alkali metal like Sodium (Na: 496 kJ/mol) or Potassium (K: 419 kJ/mol). In any case, it is a very reactive alkali metal. - Element III: Has a very high \(\Delta_i H_1\) and a very negative \(\Delta_{eg}H\) (-328 kJ/mol). These are characteristics of a non-metal, specifically a halogen (like Fluorine).
- Element IV: Has the highest \(\Delta_i H_1\) and a positive \(\Delta_{eg}H\). These are characteristics of a noble gas (like Neon).
Based on this analysis, Elements I and II are reactive metals. Element II has a lower first ionization enthalpy than Element I, making it the more reactive metal.
Step 4: Final Answer:
The element with the lowest first ionization enthalpy is the most reactive metal. Element II has the lowest \(\Delta_i H_1\) value of 490 kJ mol⁻¹. Therefore, Element II is the most reactive metal. Option (A) is correct.