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

💡 Solution & Explanation

# Reactivity of Halogens Towards Hydrogen **Step 1: Understand the reaction type** Halogens react with hydrogen to form hydrogen halides: $$X_2 + H_2 \rightarrow 2HX$$ **Step 2: Identify the key factor** Reactivity depends on the **bond dissociation energy** of the $X-X$ bond and the **stability of the H-X bond** formed. **Step 3: Compare bond strengths** - **F–F bond**: ~158 kJ/mol (weakest X–X bond) - **Cl–Cl bond**: ~243 kJ/mol - **Br–Br bond**: ~193 kJ/mol - **I–I bond**: ~151 kJ/mol The **H–X bonds formed** follow: H–F (570) > H–Cl (432) > H–Br (366) > H–I (299) kJ/mol **Step 4: Apply thermodynamic analysis** Despite F–F being weak, the **very strong H–F bond** makes the overall reaction highly exothermic and favorable. **Step 5: Consider kinetic factors** F₂ reacts explosively with H₂ (even in dark), Cl₂ reacts readily (UV light helps), Br₂ reacts slowly, and I₂ barely reacts. **Correct order: $F_2 > Cl_2 > Br_2 > I_2$** This reflects both the thermodynamic driving force (strong H–F formation) and kinetic accessibility for the reaction.