Which halogen reacts very slowly with hydrogen?

Halogens react with hydrogen to form hydrogen halides (HX). The reactivity of halogens with hydrogen decreases down the group: fluorine > chlorine > bromine > iodine.

Fluorine (F₂) reacts explosively with hydrogen at room temperature, forming HF instantly. Chlorine (Cl₂) reacts quickly with hydrogen in light, forming HCl. Bromine (Br₂) reacts more slowly than chlorine, requiring heating to react efficiently. Iodine (I₂) reacts very slowly with hydrogen, even at higher temperatures, forming HI only under special conditions. This is because iodine has a larger atomic size and weaker bond energy compared to other halogens, reducing its reactivity with hydrogen.

The reaction can be represented as:

H2+I2→2HIH_2 + I_2 → 2HIH2+I2→2HI

Understanding halogen reactivity with hydrogen is important in inorganic chemistry. It explains the trends in bond strength, electronegativity, and reaction kinetics of halogens. Students can predict which halogens will react violently, moderately, or slowly with hydrogen, which is important for laboratory safety and industrial applications.

This question emphasizes halogen reactivity trends and the special behavior of iodine as the least reactive halogen with hydrogen. It helps students understand why iodine is handled differently compared to fluorine or chlorine in chemical reactions.