Question

Calculate bond energy of C - H bond if ΔHc of CH₄ is -891 kJ, ΔHc of C (s) is – 394 kJ, ΔHc of H₂(g) is -286 kJ, heat of sublimation of C(s) is 717 kJ, heat of dissociation of H₂ is 436 kJ.

Answer 1

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ; ΔH = -891 kJ 

C(s) + 1/2O₂(g) → CO₂(g) ;ΔH = -394 kJ 

H₂(g) + O₂(g) → H₂O(l) ; ΔH = -286 kJ 

C(s) → C(g) ;ΔH = +717 kJ H₂(g) → 2H(g);Δ H = +436 kJ 

Target equation is CH₄ +4H(g) 

Reversing (ii),Multiply (iii) by 2 and reverse, Multiply eqn. (v) 2 and then adding all together we get: 

CH₄ → C(g) + 4H(g) ΔH = -891kJ + 394kJ + 575kJ + 717kJ + 872kJ ΔH = 1664 kJ 

Energy required to break 4(C – H) bond = 1664 kJ 

Energy required to break one (C – H) bond =1664/4 = 416 kJ mol-