The amount of energy required to break a bond is same as the amount of energy released…

Chemistry · JEE Advanced · NTA Exams — Chemical Thermodynamics

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 below.
\(\mathrm{H}_{3} \mathrm{C}=\mathrm{H}_{(g)} \longrightarrow \mathrm{H}_{3} \mathrm{C}_{(g)}^{\cdot}+\mathrm{H}_{(g)}^{\cdot}\) ; ΔH° = 105 kcal mol–1
Cl Cl(g) Cl•(g) + Cl•(g) ; ΔH° = 58 kcal mol–1
H3C Cl(g) H3C•(g) + Cl•(g) ; ΔH° = 85 kcal mol–1
H Cl(g) H•(g) + Cl•(g) ; ΔH° = 103 kcal mol–1

For the following reaction
CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)
the correct statement is
  1. initiation step is exothermic with
    DH° = –58 kcal mol–1.
  2. propagation step involving •CH3 formation is exothermic with DH° = –2 kcal mol–1.
  3. propagation step involving CH3Cl formation is endothermic with DH° = +27 kcal mol–1.
  4. the reaction is exothermic with
    DH° = –25 kcal mol–1.

Answer

(D) the reaction is exothermic with D H ° = –25 kcal mol –1 .

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