Use the following bond energies to calculate ΔH°f or the given reaction.(H - C = 414; C - C = 347; C - C = 619; H - H = 435).CH2 = CH2(g) +H2(g) → CH3 - CH3(g)
Correct answer: D. -121 kJ/mol
- A. -92 kJ/mol
- B. -55 kJ/mol
- C. -102 kJ/mol
- D. -121 kJ/mol
Explanation
The given reaction is CH2=CH2(g) + H2(g) → CH3-CH3(g). To find ΔH°f, calculate the total energy for breaking bonds (reactants) and forming bonds (products): Break 1 C=C bond and 1 H-H bond: (619 + 435) kJ/mol = 1054 kJ/mol. Form 1 C-C bond and 4 C-H bonds: (347 + 4×414) kJ/mol = 2003 kJ/mol. ΔH°f = Energy of bonds formed - Energy of bonds broken = 2003 - 1054 = -121 kJ/mol. Thus, the correct answer is -121 kJ/mol. The other options result from errors in accounting for bond energies or miscalculations.
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Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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