JEE Advanced2021ChemistryHaloalkanes and HaloarenesActual
Paragraph: The amount of energy required to break a bond is same as the amount of energy released when the same bond is formed. In gaseous state, the energy required for homolytic cleavage of a bond is called Bond Dissociation Energy (BDE) or Bond Strength. BDE is affected by s -character of the bond and the stability of the radicals formed. Shorter bonds are typically stronger bonds. BDEs for some bonds are given be
Options
- AInitiation step is exothermic with ΔH ° = - 58   kcal   mol - 1 .
- BPropagation step involving CH 3 ∙ formation is exothermic with ΔH ° = - 2   k   cal
- CPropagation step involving CH 3 Cl formation is endothermic with ΔH ° = + 27   k   cal 
- DThe reaction is exothermic with ΔH ° = - 25   k   cal   mol - 1 .
Correct answer
D. The reaction is exothermic with ΔH ° = - 25   k   cal   mol - 1 .
Step-by-step solution
CH 4 g + Cl 2 g → light CH 3 Cl g + HCl g ΔH r ∘ = ∑ BDE reactans − ∑ BDE products = E C − H + E Cl − Cl − E C − Cl − E H − Cl = + 105 + 58 − 85 − 103 = − 25   k   cal   mol − 1 reaction is exothermic.