The order of reactivity of halogens towards hydrogen is — Hydrogen Chemistry Question

💡 Solution & Explanation

# Reactivity of Halogens Towards Hydrogen **Step 1: Understand the reaction** Halogens react with hydrogen to form hydrogen halides: $$X_2 + H_2 \rightarrow 2HX$$ **Step 2: Key factors determining reactivity** Reactivity depends on: - **Bond dissociation energy** of $X_2$ (lower = more reactive) - **Bond strength** of $H-X$ (stronger = more exothermic, more favorable) - **Electronegativity** of halogen (higher = stronger bonding with H) **Step 3: Compare halogens down the group** $$\text{Reactivity: } F_2 > Cl_2 > Br_2 > I_2$$ - **Fluorine ($F_2$):** Highest electronegativity, forms strongest $H-F$ bond (570 kJ/mol), most exothermic reaction - **Chlorine ($Cl_2$):** High electronegativity, very reactive (even explosive with $H_2$) - **Bromine ($Br_2$):** Moderate reactivity, requires heating - **Iodine ($I_2$):** Lowest reactivity, weakest $H-I$ bond, least exothermic **Step 4: Conclusion** The order follows electronegativity and is: $$\boxed{F_2 > Cl_2 > Br_2 > I_2}$$ This is option **A** — fluorine is most reactive because it forms the strongest H-X bonds and releases maximum energy.