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Given : sub H [C (graphite)] = 710 kJ mol-1 C-H H = 414 kJ mol-1 H-H H = 436 kJ mol-1 C=C H = 611 kJ mol-1 The f H for CH2=CH2 is kJ mol-1 (nearest integer value)
25
Formation reaction:
2C(graphite) + 2H2(g) CH2=CH2(g)
f H = (energy to atomise reactants) − (energy released on forming bonds of product)
= [2 sub H + 2 H-H H ] - [4 C-H H + C=C H ]
= [2(710) + 2(436)] - [4(414) + 611]
= 2292 - 2267
= 25 kJ mol-1
Answer: 25
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