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

💡 Solution & Explanation

# Reactivity of Halogens Towards Hydrogen **Step 1: Consider the reaction type** Halogens react with hydrogen to form hydrogen halides: $$X_2 + H_2 \rightarrow 2HX$$ **Step 2: Identify the controlling factor** Reactivity depends on the **bond dissociation energy** of the $X-X$ bond and the **H-X bond strength**. The reaction is thermodynamically favorable when the $H-X$ bond formed is very strong, and the $X-X$ bond is weak enough to break. **Step 3: Analyze bond strengths** | Halogen | X-X Bond Energy | H-X Bond Energy | |---------|-----------------|-----------------| | $F_2$ | 158 kJ/mol | 570 kJ/mol | | $Cl_2$ | 243 kJ/mol | 432 kJ/mol | | $Br_2$ | 193 kJ/mol | 366 kJ/mol | | $I_2$ | 151 kJ/mol | 298 kJ/mol | **Step 4: Order of reactivity** $$F_2 > Cl_2 > Br_2 > I_2$$ Fluorine is most reactive because it forms the strongest $H-F$ bond (570 kJ/mol), providing maximum thermodynamic driving force. Reactivity decreases down the group as $H-X$ bond strength decreases. **Answer A** represents this order: **$F_2 > Cl_2 > Br_2 > I_2$**