A 'p'-block element (E) and hydrogen form a binary cation (EHx)+, while EH3 on treatment with K2HgI4 in alkaline medium gives a precipitate of basic mercury(II)amido-iodine. Given below are first ionisation enthalpy values (kJ mol−1) for first element each from group 13, 14, 15 and 16. Identify the correct first ionisation enthalpy value for element E.

First Ionisation Enthalpy of p-block Element | JEE Main 2026 Chemistry

A 'p'-block element (E) and hydrogen form a binary cation (EH_x)⁺, while EH₃ on treatment with K₂HgI₄ in alkaline medium gives a precipitate of basic mercury(II)amido-iodine. Given below are first ionisation enthalpy values (kJ mol⁻¹) for first element each from group 13, 14, 15 and 16. Identify the correct first ionisation enthalpy value for element E.

A. 1402

B. 1086

C. 801

D. 1312

Correct Answer: 1402

Detailed Explanation

The compound EH₃ gives a precipitate of basic mercury(II)amido-iodine with K₂HgI₄ in alkaline medium.

This reaction is a characteristic test for ammonia (NH₃) and amines.

Hence, element E must be nitrogen, because:

• Nitrogen forms NH₃
• NH₃ gives brown precipitate of basic mercury(II)amido-iodine

Now verify with the first condition:

Nitrogen and hydrogen form a binary cation:

NH₄⁺ (ammonium ion)

So both given conditions are satisfied only by nitrogen.

Nitrogen belongs to group 15.

First ionisation enthalpy of nitrogen is:

1402 kJ mol⁻¹

Hence, the correct answer is 1402.

This type of concept-based identification question is very important for JEE Main, JEE Advanced and IIT JEE.

Detailed Explanation

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