Question

Calculate bond energy of C - H bond if ΔHc of CH4 is -891 kJ, ΔHc of C () is – 394 kJ, ΔHc of H2(g) is -286 kJ, heat of sublimation of C() 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(*) + 1/2O2(g) → CO2(g) ;ΔH = -394 kJ

H2(g) + O2(g) → H2O(l) ; ΔH = -286 kJ

C(*) → 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-